The effect of adjacent buildings and topographical

HSE
Health & Safety
Executive
The effect of adjacent buildings and topographical
features on the wind pressure field around buildings
Prepared by BRE for the
Health and Safety Executive 2003
RESEARCH REPORT 157
HSE
Health & Safety
Executive
The effect of adjacent buildings and topographical
features on the wind pressure field around buildings
Dr Philippa Westbury, Mr Liam Roche,
Dr Bridget Pierce & Mr James Smith
BRE
Environment Division
Bucknalls Lane
Garston
Watford
WD25 9XX
This work has been carried out to help develop guidance on siting open flue terminals for natural
draught domestic gas boilers. It aims to investigate the effects of adjacent structures and topographical
features on wind pressure fields around dwellings, and to identify scenarios where the likelihood of flue
flow reversal is high. A combination of wind tunnel testing and computational modelling has been used.
Ridge vent pressures on isolated dwellings have also been investigated. For the cases examined, the
maximum pressure experienced by a flue terminal above a 45° pitch roof was found to exceed that for
an isolated dwelling by up to 60% when very tall or wide structures (e.g. embankments) were located
downwind of the dwelling. The percentage increase was higher for the 30° pitch roof, although absolute
pressures for the 45° pitch roof were greater.
The greatest increased risk of flue flow reversal was found to occur with a background ventilation
strategy rather than when a dominant opening was present, and when the adjacent structure was to
one side of the dwelling, or offset or rotated relative to the dwelling. For these situations, large positive
roof pressures occurred concurrently with negative wind-induced interior pressures.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its
contents, including any opinions and/or conclusions expressed, are those of the authors alone and do
not necessarily reflect HSE policy.
HSE BOOKS
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First published 2003
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ii
EXECUTIVE SUMMARY This report describes a programme of wind tunnel testing and computer modelling which has
been carried out to investigate the impact of adjacent structures and topographical features on
the wind pressure field around dwellings, in order to determine building configurations that
could increase the susceptibility of flues to flow reversal. The work is part of a wider
programme to investigate the flue performance of open flued, natural draught domestic gas
boilers, and to identify any safety implications arising from their increased efficiency which
tends to cause greater sensitivity to adverse wind effects. The main objectives of the work are to
investigate the effects of height and separation distance of adjacent structures and topographical
features on wind pressure fields around dwellings, and to determine the sensitivity of the wind
pressure field to additional factors such as dwelling geometry and orientation/geometry of the
adjacent building. Furthermore, the work aims to identify scenarios where the likelihood of flue
flow reversal is high due to positive pressures acting on the flue terminals and/or negative wind­
induced interior pressures. For this part of the work, a combination of wind tunnel testing and
computational modelling has been used. The results from this work will help to develop
guidance in BS5440 on siting open flue terminals.
The results show that, for a ‘standard’ domestic dwelling with a 45° pitch roof, the maximum
positive pressure on the dwelling roof occurs when an adjacent structure is directly downwind
of the dwelling, and the dwelling is immersed in the positive pressure field produced by the
downwind structure. The magnitude of the pressure depends on the height and width of the
adjacent structure, and its distance from the dwelling. The maximum pressure experienced by a
flue terminal located above a 45° pitch roof may exceed that for the isolated dwelling by up to
60% when very tall or wide structures (e.g. embankments) are located close to the dwelling.
Similarly to Upton et al. (1999b), increases in flue height up to about 2.5m (full scale) have not
been found to significantly reduce the risk of flue flow reversal.
The percentage increase in maximum roof pressure with an adjacent structure present, relative
to the isolated dwelling, is considerably greater for the 30° pitch roof than the 45° pitch roof. In
addition, the eaves pressure for an isolated 30° pitch roof dwelling is negative, whereas it is
positive with an adjacent structure close by. However, magnitudes of the maximum pressures
remain lower for the 30° pitch roof than those for the 45° pitch roof. This indicates that the
increased risk of flue flow reversal due to the presence of an adjacent structure is greater for the
30° pitch roof relative to the isolated dwelling case. However, the absolute risk for the 45° pitch
roof is still greater.
For the range of additional factors investigated for the 45° pitch roof, maximum roof pressures
have not been found to increase significantly when different factors are introduced. Factors that
have been investigated include adjacent structure geometry, offset or rotation of the adjacent
structure relative to the dwelling, location of the dwelling within a terrace, location of a
dwelling adjacent to similar neighbours, geometry of topographical feature (hill or
embankment) and terrain type.
The zone of influence over which a downwind adjacent structure of height H causes an increase
in positive roof pressure relative to the isolated case has been found to extend to a distance of
~1.5H when the adjacent structure width, W, is relatively small in comparison to the dwelling
eaves height, h (W ~ 2h). For wider adjacent structures (W ≥ 4h), the zone of influence extends
over a distance of at least 2H. For all cases, the maximum roof pressure tends to decrease with
increasing separation distance.
iii
The impact of the wind pressure field on the interior pressure has been investigated by
considering two ventilation scenarios. The first is a worst-case ventilation scenario which occurs
when an open window or ‘dominant opening’ is situated on the façade where the minimum
external façade pressure acts. This results in the maximum possible difference between the roof
and interior pressures. However, the maximum difference between the roof pressure and the
interior pressure has been found to occur for the isolated dwelling rather than obstructed
dwellings. This is because the adjacent structure tends to smooth out the variation in pressure
over the dwelling, resulting in a smaller difference between maximum roof and minimum
façade pressures.
The second ventilation scenario is for a dwelling where background ventilation only is
provided. When the dwelling is fully enclosed by the positive pressure field, the interior
pressure is positive and the net pressure (difference between the maximum roof pressure and the
interior pressure) is smaller with the obstruction present than in the isolated case. This indicates
that in a steady-state situation, the risk of flue flow reversal could be lower with the obstruction
present.
When the adjacent structure is to one side of the dwelling, or is rotated or offset relative to the
dwelling, the dwelling is only partially enclosed by the positive pressure field created by the
adjacent structure. In this case, large positive roof pressures can occur concurrently with
relatively large negative façade pressures and net pressures tend to be higher than for the
isolated case. Thus the risk of flue flow reversal increases relative to the isolated case when
background ventilation only is provided.
For the moderate wind speeds at which Upton et al. (1999a) identified high flue flow reversal
risk, the influence of stack pressure on the interior pressure has been found to be significant
when internal/external temperature differences are large. Stack-induced pressures have a larger
relative effect when background ventilation only is provided than when a dominant opening is
present. Negative stack-induced interior pressures occur when the external temperature is lower
than the internal temperature, which will occur during winter months. These will tend to
increase the risk of flue flow reversal, in addition to any adverse wind effects.
Thus the greatest increase in the risk of flue flow reversal with an adjacent structure present
relative to the isolated case has been found to occur when the ventilation strategy is background
ventilation, with no dominant opening provided by a window. The former ventilation strategy is
more likely to be used when internal/external temperature differences are large (i.e. in winter),
when the risk is further increased by the influence of the stack effect on interior pressure.
It is recommended that fanned draught systems should be considered for dwellings with roof
pitches greater than or equal to 45°, both with and without an adjacent structure present. For
dwellings with less steep roof pitches (30° or lower), it is recommended that alternatives to
natural draught systems should be considered for the 30° roof pitch when an adjacent structure
is at a sufficiently close distance to the dwelling.
Additional wind tunnel experiments could be considered for dwellings with lower roof pitches
(for example, 30° pitch roof or flat roof), in order to provide a more detailed assessment of the
impact of adjacent structures. In particular, the effect of size and separation distance of an
adjacent structure on the roof pressure in the eaves region (which is negative for the isolated
dwellings) could be investigated further.
Ridge vent pressures have been investigated for both 45° and 30° roof pitch dwellings located in
3- and 9-house terraces. For both roof pitches, ridge vent pressures are found to be negative for
most wind directions. Furthermore, for the 45° case, when positive pressures occur on the
upwind side of the ridge (0° and 30° wind directions), much greater negative pressures occur on
iv
the downwind side, and thus the net wind effect will tend to enhance ventilation from the
terminal. For both roof pitches, the most problematic wind direction occurs when the wind is
along the ridge. In this case, although the magnitude of the pressure is found to be small,
sustained positive pressure 'events' can occur for a significant proportion of the time. The
susceptibility of the terminal to adverse wind effects is found to increase slightly with
increasing terrace length, when the wind is parallel to the ridge.
It is suggested that the use of ridge vents is preferable to flue terminals located close to the
eaves, particularly in the case of the 45° pitch roof.
This report and the work it describes were funded by the Health and Safety Executive. Its
contents, including any opinions and/or conclusions expressed, are those of the authors alone
and do not necessarily reflect HSE policy.
v
vi
CONTENTS
EXECUTIVE SUMMARY
1. INTRODUCTION
1
2. SELECTION OF TEST SCENARIOS
2.1 Overall scope of the investigation
2.2 Selection of dwellings
2.3 Selection of adjacent buildings
2.4 Arrangements of dwellings relative to adjacent buildings
2.5 Selection of topographical features
2.6 Selection of wind directions
2.7 Additional sensitivity analyses
2.8 Selection of ridge vent cases
2.9 Inventory of tests carried out
3
3
3
5
6
9
10
10
11
12
3. EXPERIMENTAL METHODOLOGY
3.1 Wind tunnel modelling
3.2 Model construction
3.3 Measurement of pressures
3.4 Data analysis
17
17
17
20
21
4. COMPUTATIONAL MODELLING
23
5. RESULTS
27
5.1 Impact of adjacent structures and topographical features on the external pressure field 27
over dwelling roofs
5.2 Effect of flue height on the external pressure at the flue terminal
43
5.3 Impact of adjacent structures and topographical features on maximum internal/external 45
pressure differences in the case of a dominant opening
5.4 BREEZE modelling
50
5.5 Ridge vent investigation
60
6. DISCUSSION
6.1 Impact of adjacent structures and topographical features
6.2 Ridge vents
67
67
69
7. CONCLUSIONS
7.1 Impact of adjacent buildings and topographical features
7.2 Ridge vents
71
71
72
8. RECOMMENDATIONS
8.1 Impact of adjacent buildings and topographical features
8.2 Ridge vents
73
73
73
9. ACKNOWLEDGEMENTS
75
10. REFERENCES
77
11. Appendix A
79
12. Appendix B
87
vii
viii
1. INTRODUCTION Since April 1997, BRE has carried out a number of studies on behalf of HSE to investigate the
flue performance of domestic gas boilers, and to identify any safety implications arising from
the increased efficiency of open flued, natural draught appliances. These studies have
investigated the susceptibility of flues to flow reversal, using a combination of full scale tests in
BRE’s test houses, wind tunnel testing and computational modelling. From the full scale tests,
climate and other factors which tend to increase the risk of flue flow reversals were identified
for external flues discharging near to the building eaves (Upton et al. (1999(a)), and for
internally run flues discharging to near the building eaves or to ridge vents (Upton et al. 2000).
Later full-scale studies in combination with computational modelling investigated the effects of
ventilation strategy on flue performance (Upton et al. 2001, Plathner & Stephen 2001).
The studies found that factors contributing to the risk of flue flow reversal included the wind
speed and direction, the interior dwelling pressure resulting from the ventilation strategy, the
type of flue terminal, and its location on the roof. The risk was found to be greater for high
efficiency boilers than lower efficiency boilers, due to the reduction in flue gas temperatures
causing a reduction in the available buoyancy forces for exhausting flue gases. Flues running
predominantly externally were also more prone to flow reversal than those running
predominantly internally, and the risk of flue flow reversal was greater when the boiler was
starting up (cold flue) than when it had been firing for a period of time. High risk conditions
tended to be initiated by external wind conditions producing positive pressures around the flue
terminal; however, once flue flow reversals were initiated, it was found that maintenance of the
reversed flow depended not only on externally induced wind pressures, but also on temperature
conditions prevailing both within the flue and between the house and ambient environment.
A programme of wind tunnel testing was carried out in order to identify building shapes and
configurations that would tend to cause flue flow reversal to occur due to the formation of
positive wind pressure fields around the dwelling (Upton et al. (1999(b)). This work identified a
number of building shapes for which flues might be subjected to positive pressures, including
roofs with pitch angles of 45°, single storey flat roof extensions and 'L' shaped buildings. The
tests also investigated wind pressure fields for a limited number of groups of buildings, and
found that positive pressure fields were caused by taller adjacent buildings. It was recommended
that further work was required to investigate the effect of adjacent buildings and topography on
the wind pressure field around dwellings.
Previous investigations by other authors into the effects of adjacent objects (buildings or
topography) on the pressure field over a dwelling include those by De Gids et al. (1974) and
Lilly et al. (1996). Lilly et al. investigated the effects of adjacent embankments (of lower, equal
height and higher than the building) and adjacent houses of heights h and 2h, where h is the
dwelling height. De Gids et al. investigated the effects of taller buildings, ranging from 2h to 8h,
and found that the effect of the taller buildings was strongly dependent on the ratio of the
heights of the respective buildings and the separation between them.
This report describes a programme of wind tunnel testing and computational modelling which
has been carried out to investigate further the effect of adjacent buildings and topographical
features on the wind pressure field around buildings, in order to determine building
configurations that could increase the susceptibility of flues to flow reversal. This work is
required in order to extend the guidance in BS5440 on siting open flue terminals, particularly
given the increased sensitivity of high efficiency open flued, natural draught boilers to adverse
wind pressure effects. BS5440 currently recommends that 'Additional precautions may be
needed in siting a terminal in certain circumstances, such as on a sloping site or near to a very
1
large structure' (Section 5.1.6.1, Commentary and Recommendations). More specific guidelines
are not currently given. Thus there is a need to investigate a wider range of configurations of
dwellings with adjacent buildings and topographical features than has previously been
investigated. This work follows on from the wind tunnel testing carried out previously by Upton
et al. (1999(b)).
The main objectives of this work are as follows:
· To determine the impact of adjacent buildings or topography on the wind pressure field
over the roof of a building, as a function of the type of object, its height, its distance
from the building and the wind direction;
· To determine the implications of these findings for safe siting of open flue terminals, to
minimise the risk of flue flow reversal occurring due to the presence of positive
pressure fields or strong negative interior pressures;
· To extend the design guidance given in BS5440 on locating open flue terminals next to
adjacent objects, in the light of these results.
The work uses a combination of wind tunnel testing and computational modelling to determine
the impact of adjacent buildings and topographical features on the wind pressure field around a
dwelling, and the interior dwelling pressures resulting from the wind pressure field.
In addition to the work described above, a study has been carried out to determine the wind
pressures experienced by ridge vents located on isolated buildings, in order to determine the
optimum siting for ridge vents.
2
2. SELECTION OF TEST SCENARIOS
2.1 OVERALL SCOPE OF THE INVESTIGATION
2.1.1 Effect of adjacent buildings and topographical features
The arrangements of dwellings, adjacent buildings and topographical features were selected in
order to achieve the following aims:
· To investigate the effects of height, separation distance and wind direction on the
pressure field around the dwelling for different adjacent buildings and topographical
features for one ‘standard’ domestic dwelling;
· To investigate the sensitivity of the pressure field to additional factors such as the
dwelling geometry, the geometry of the adjacent building or topographical feature and
the orientation of the adjacent building relative to the dwelling;
· To identify scenarios where flues could be particularly sensitive to flow reversals due
to:
o Positive pressures acting on the flue terminals;
o Negative interior pressures resulting from the wind pressure field around the
building.
2.1.2 Ridge vent investigation
Arrangements of dwellings were selected in order to achieve the following aims:
· To investigate the effect of roof geometry on the pressure field at dwelling ridges;
· To examine the sensitivity of the pressure field to the location of a dwelling in a terrace,
and the length of the terrace.
2.2 SELECTION OF DWELLINGS
The ‘standard’ domestic dwelling was chosen as a detached house with a 45° pitch roof. 45°
represents a commonly occurring roof pitch and a ‘worst case’ scenario, since a steeper roof
pitch tends to cause higher positive pressures on the upwind side of the roof. The choice of a
detached house, rather than a terraced house, allows a wider range of relative widths between
the dwelling and adjacent buildings to be tested. The dimensions of the detached house were
based on the dimensions for standard dwellings defined previously in Iles (1998). The roof
shape was chosen to be a dual-pitch, rather than a hip roof. Most of the investigations were
carried out using the ‘standard’ dwelling described above.
In order to determine the sensitivity of the wind pressure field to the geometry of the dwelling,
the following dwelling geometries were additionally tested:
· Detached dwelling with 30° roof;
· Detached dwelling with 45° roof and flat roof extension;
· Dwelling within a three-house terrace.
3
Dwellings with a roof pitch of 30° were selected as they are likely to be built more commonly in
the future, although will tend to be less susceptible than 45° roof pitches to adverse pressure
effects. The flat roof extension was identified as being particularly vulnerable to adverse
pressure effects in Upton et al. (1999(b)). In the case of the terraced dwelling, the dwelling
under investigation was tested both in the centre of the terrace and at one end.
Photographs of the dwellings are shown in Figures 1-4.
Figure 1
Pressure tapped model of 45° pitch roof dwelling
Figure 2
Pressure tapped model of 30° pitch roof dwelling
4
Figure 3
Pressure tapped model of dwelling with flat roof extension
Figure 4
Pressure tapped model of 45° pitch roof dwelling in mid terrace location
2.3 SELECTION OF ADJACENT BUILDINGS
To investigate the effect of adjacent buildings, a number of geometries were selected, with
dimensions defined as multiples of the eaves height, h, of the dwelling. The width is the cross­
wind dimension, while the breadth is the along-wind dimension when the wind direction is 0° or
180°. Parameters defining the height, width and breadth are shown in Figure 5.
5
Figure 5
Definition of dimensions and wind directions
The following geometries were tested in order to determine the impact of adjacent building
height and width on the wind pressure field over the dwelling:
· Flat-roof adjacent buildings of width 2h, with heights 2h, 4h and 8h;
· Flat-roof adjacent buildings of height 2h, with widths 2h, 4h and 8h;
· Flat-roof adjacent buildings of height 4h, with widths 2h and 4h;
All adjacent building breadths were h. The following geometries were tested in order to determine the impact of adjacent building
geometry on the wind pressure field:
· 30° pitch roof adjacent building of height 4h and width 2h;
· 45° pitch roof adjacent building of height 4h and width 2h.
2.4 ARRANGEMENTS OF DWELLINGS RELATIVE TO ADJACENT BUILDINGS
The adjacent buildings and topographical features were arranged in different positions relative
to the dwelling in order to test the following:
· The effect of separation distance between the dwelling and the adjacent building or
topographical feature;
· The effect of offset of the adjacent building relative to the dwelling;
6
·
The effect of angle of rotation of the adjacent building relative to the dwelling.
The parameters defining the arrangements (separation distance, offset, rotation) are shown in
Figure 6, along with the definition of wind direction in relation to the arrangement. Examples of
the arrangements are shown in photographs (Figures 7 and 8).
Figure 6
Plan view offset and rotated building configurations
Wind pressures were also investigated for isolated dwellings in the absence of adjacent
buildings or topographical features.
7
Figure 7
4h x 2h adjacent structure at 2h distance Figure 8
2h x 8h adjacent structure at 4h
8
2.5 SELECTION OF TOPOGRAPHICAL FEATURES
Embankments and hills were selected to represent topographical features. The following
geometries were tested in order to determine the effect of the topographical feature on the wind
pressure field (the width is the long dimension of the feature and the breadth is the shorter
dimension of the flat top as in figure 5):
·
Embankment of height 2h, width 16h and breadth h, slope 90°;
·
Hills of height h, 2h and 3h, width 16h and breadth 4h.
The following additional embankment geometries were tested in order to determine the effect of
embankment breadth and slope on the wind pressure field:
·
Embankment of height 2h, width 16h and breadth 4h;
·
Embankment of height 2h, width 16h, breadth h, with slopes 30° and 60°.
Plans of embankments and hills, and definitions of separation distances, are shown in Figure 9a.
A photograph of a large hill is shown in figure 9b. Remote slope was omitted from large hills.
Figure 9a
Configurations of hills and embankments.
(Tops are horizontal and slopes reach ground level) 9
Figure 9b
Upwind hill of height 3h with base 2h from dwelling
2.6 SELECTION OF WIND DIRECTIONS
Preliminary studies identified that the worst case wind direction was with the adjacent building
or topographical feature directly downwind of the dwelling (wind direction 180°). Thus the
majority of cases were tested with this wind direction. In addition, a number of cases were
tested with the adjacent building or topographical feature directly upwind of the dwelling (wind
direction 0°). Where other wind directions could have a significant impact (for example, for
wider adjacent buildings or topographical features, or where the adjacent building was offset or
rotated relative to the dwelling), additional wind directions were tested.
2.7 ADDITIONAL SENSITIVITY ANALYSES
Additional sensitivity analyses were carried out to determine the following:
·
The effect of surrounding terrain (suburban or urban) on the wind pressure field
measured over the dwelling;
·
The effect of similar dwellings neighbouring the dwelling under investigation
(Figure 10).
10
Figure 10
Detached dwelling with two identical neighbours
2.8 SELECTION OF RIDGE VENT CASES
Wind pressures were measured at positions on the roof coinciding with the locations of ridge
vents. Tests were carried out for the following scenarios, in the absence of adjacent structures or
topographical features:
·
3 house terrace with 30° dual pitch roof
·
3 house terrace with 45° dual pitch roof
In addition, a 9 house terrace with 45° dual pitch roof was tested in order to determine the effect
of terrace length on the results. Each terrace was tested with the terrace perpendicular and
parallel to the adjacent building (see Figure 11).
11
Figure 11
Parallel and perpendicular terrace configurations with an adjacent building
2.9 INVENTORY OF TESTS CARRIED OUT
2.9.1 Adjacent buildings
The wind tunnel tests carried out have been grouped according to the following objectives, and
are summarised in Tables 1(a) to 1(g). Note that some wind tunnel tests are common to more
than one objective.
·
Objective 1(a) - Effect of height and separation distance
·
Objective 1(b) - Effect of width of adjacent building
·
Objective 1(c) - Effect of dwelling geometry
·
Objective 1(d) - Effect of adjacent building geometry
12
·
Objective 1(e) - Effect of adjacent building offset
·
Objective 1(f) - Effect of rotation of adjacent building relative to dwelling
·
Objective 1(g) - Effect of neighbouring dwellings
·
Objective 1(h) - Effect of upwind terrain
2.9.2 Topographical features
·
Objective 2(a) - Effect of embankment slope and breadth ·
Objective 2(b) - Effect of hill
Table 1(a)
Effect of adjacent building height and separation
Dwelling
roof pitch
45°
45°
45°
H
W
B
2h
2h
2h
2h
2h
2h
h
h
h
Separation
distance
h
2h
3h
45°
45°
45°
4h
4h
4h
2h
2h
2h
h
h
h
2h
4h
6h
45°
45°
45°
8h
8h
8h
2h
2h
2h
h
h
h
4h
8h
12h
13
Table 1(b)
Effect of aspect ratio
Dwelling
roof
pitch
45°
45°
45°
H
W
B
Separation
distance
2h
2h
2h
2h
2h
2h
h
h
h
h
2h
3h
45°
45°
45°
2h
2h
2h
4h
4h
4h
h
h
h
2h
4h
6h
45°
45°
45°
45°
2h
2h
2h
2h
8h
8h
8h
8h
h
h
h
h
2h
4h
6h
8h
45°
45°
45°
45°
4h
4h
4h
4h
4h
4h
4h
4h
h
h
h
h
2h
4h
6h
8h
Table 1(c)
Effect of dwelling geometry
H
W
B
Separation
distance
45°
45°
Location
within
terrace
-
4h
4h
2h
2h
h
h
2h
4h
30°
30°
-
4h
4h
2h
2h
h
h
2h
4h
45° + flat roof
45° + flat roof
-
4h
4h
2h
2h
h
h
2h
4h
45° terrace
45° terrace
centre
centre
4h
4h
2h
2h
h
h
2h
4h
45° terrace
45° terrace
centre
centre
4h
4h
4h
4h
h
h
2h
4h
45° terrace
45° terrace
end
end
4h
4h
2h
2h
h
h
2h
4h
45° terrace
45° terrace
end
end
4h
4h
4h
4h
h
h
2h
4h
Dwelling roof
pitch / geometry
14
Table 1(d)
Effect of adjacent building geometry
H
W
B
Separation
distance
45°
45°
Adjacent
building roof
pitch
30°
30°
4h
4h
2h
2h
h
h
2h
4h
45°
45°
45°
45°
4h
4h
2h
2h
h
h
2h
4h
Dwelling roof
pitch
Table 1(e)
Effect of adjacent building offset
Dwelling roof
pitch
45°
45°
Offset of
adjacent
building
h
2h
H
W
B
Separation
distance
2h
2h
4h
4h
h
h
2h
4h
Table 1(f) Effect of rotation of adjacent building relative to dwelling Dwelling roof
pitch
45°
45°
Adjacent
building
rotation
30°
60°
H
W
B
Separation
distance
2h
2h
4h
4h
h
h
2h
2h
Table 1(g)
Effect of neighbouring dwellings H
W
B
Separation
distance
45°
45°
Neighbouring
dwellings
present
Yes
Yes
2h
2h
4h
4h
h
h
2h
4h
45°
45°
Yes
Yes
4h
4h
2h
2h
h
h
2h
4h
Dwelling
roof pitch
15
Table 1(h)
Effect of upwind terrain Dwelling roof
pitch
Upwind
terrain
H
W
B
Separation
distance
Suburban
Suburban
Neighbouring
dwellings
present
Y/N
Y/N
45°
45°
4h
4h
2h
2h
h
h
2h
4h
45°
45°
Urban
Urban
Y/N
Y/N
4h
4h
2h
2h
h
h
2h
4h
Table 2(a)
Effect of embankment slope and breadth Dwelling
roof
pitch
45°
45°
45°
Embankment
slope
Embankment
breadth
H
W
Separation
distance
90°
90°
90°
h
h
h
2h
2h
2h
16h
16h
16h
2h
4h
8h
45°
45°
90°
90°
4h
4h
2h
2h
16h
16h
2h
4h
45°
45°
60°
60°
h
h
2h
2h
16h
16h
2h
4h
45°
45°
30°
30°
h
h
2h
2h
16h
16h
2h
4h
Table 2(b)
Effect of hill
Dwelling
roof
pitch
45°
45°
45°
H
W
B
Separation
distance
h
2h
3h
16h
16h
16h
4h
4h
4h
2h
2h
2h
16
3. EXPERIMENTAL METHODOLOGY
3.1 WIND TUNNEL MODELLING
3.1.1 Effect of adjacent buildings and topographical features
External wind pressure fields were investigated using 1:80 scale models of dwellings and
adjacent buildings or topographical features. The tests were carried out in BRE’s Environmental
Wind Tunnel, which is used primarily for studies into wind flow around buildings. It is an open­
jet wind tunnel with a working section of approximately 2m wide by 1.5m high.
The model scale was chosen to be small enough to allow a sufficient range of adjacent object
heights and separation distances to be represented, but large enough to allow sufficiently
detailed pressure measurements to be made. The models were mounted on a turntable in order to
test at any required wind direction. Appropriate mean velocity profiles and turbulence intensity
profiles for the surrounding suburban or urban terrain were modelled in the upwind fetch of the
wind tunnel at a scale to match the dwellings and adjacent objects. The aerodynamic roughness
height parameter, z0, which is a measure of the terrain roughness, was 0.35m (full scale) for the
suburban terrain simulation, and 0.70m (full scale) for the urban terrain simulation, which are
within the range of z0 given by ESDU (1993) for these terrain types.
Measurements of surface pressures on the roof and façades of the dwelling, and pressures at two
additional heights above the roof of the dwelling were made. These heights corresponded to flue
heights of 0.6m and 1.5m at full scale.
3.1.2 Ridge vent investigation
Pressures at locations coinciding with ridge vents were carried out at 1:60 scale, on existing
wind tunnel models that were used previously in Upton et al. (1999(b)). Measurements of
surface pressure were made only. Suburban terrain was modelled in the upwind fetch of the
wind tunnel using the same roughness simulation as in the previous investigation. Thus the
roughness height parameter, zo, for this investigation was 0.26m (full scale).
3.2 MODEL CONSTRUCTION
Dwelling models were constructed in a similar way to the models used in Upton et al. (1999(b)).
The models were constructed using 6mm perspex sheet and were instrumented in different
ways, according to whether surface pressures, or pressures above the roof, or pressures at ridge
vents were measured. These are described in the following sections. The adjacent buildings and
topographical features were constructed as block models from wood, and were painted matt
black. Non-instrumented block models of dwellings were also constructed from wood and
painted matt black.
3.2.1 Surface tapped models
To measure surface pressures, dwelling models were instrumented using conventional surface
pressure taps which consisted of 2mm diameter brass tubes inserted into the perspex model,
with the ends flush with the external surface of the model. The taps were connected to the
measurement system using small-bore pvc tubing. A restrictor was fitted in each line, in order to
remove any distortion introduced into pressure traces by the tubing (Cook (1990)).
17
Pressure-tapped models were constructed for dwellings with 45° and 30° roof pitches. An
additional section which represented the flat roof extension was also constructed. When
modelling terraces, non-instrumented block models were used alongside the instrumented model
to represent additional houses in the terrace.
Pressure taps were densely distributed over the roofs of the instrumented models. In addition,
pressure taps were introduced into the façades of the models, in order to measure pressures at
locations coinciding with ventilation openings. The arrangements of pressure taps are shown in
photographs in Figures 1 and 2.
3.2.2 Vertical tube models
To measure pressures at heights above the surface of the roof, dwelling models were
instrumented with 2mm diameter brass tubes. Tubes were inserted in the roofs of the models
and were fixed in a vertical position, independent of the roof angle. The height of the vertical
tubes was adjustable, allowing pressures to be measured at any height above the roof surface.
Surface taps were provided in the façades of the dwelling model, with an arrangement similar to
that for the models with surface taps on the roof. Again, the vertical tubes and surface taps were
connected to the pressure measurement system using small-bore pvc tubing and a restrictor was
fitted in each line.
Vertical tube models were constructed for dwellings with 30° and 45° pitch roofs, and for the
flat roof extension. The 5 by 4 array of vertical tubes on each slope of the 45° pitch roof is
shown in the photograph in Figure 12a. The 5 by 3 array of tubes on each slope of the 30° pitch
roof model is shown in 12b.
Figure 12a
Vertical tube model of dwelling with 45° pitch roof
18
Figure 12b
Vertical tube model of dwelling with 30° pitch roof
3.2.3 Models used for ridge vent tests
To measure ridge pressures, the models were fitted with surface taps located along the ridges of
the 3 house terrace models, and were connected to the pressure measurement system in the same
way as described previously. Terraces with roof pitches of 30° and 45° were modelled. Dummy
block models of 3 house terraces were used to extend the 3 house terrace to a 9 house terrace.
The instrumented terrace model and 2 dummy block models were arranged in different ways in
order to allow pressures to be measured along half the length of the 9 house terrace, taking
advantage of a plane of symmetry. The arrangement of pressure taps is shown in Figure 12c.
Figure 12c
Ridge taps on 30° pitch model (45° model similar)
19
3.3 MEASUREMENT OF PRESSURES
Pressures were measured using a Scanivalve pressure measurement system comprising ZOC
electronic pressure scanners and a Digital Service Module (DSM3200), which allows the
electronic pressure scanners to be interfaced to an Ethernet network. The system was run from a
separate computer using DSMLINK software, allowing experimental setups for the
measurement hardware to be configured. The following pressure scanners were used: a 64
channel scanner (ZOC33/64Px), 2 ´ 32 channel scanners (ZOC22B) and 1 ´ 32 channel scanner
comprising 4 individual banks of 8 channel sensors (ZOC23B). Thus pressures could be
measured at up to 160 points simultaneously, at a maximum sampling rate of 100kHz
(depending on the number of scanners connected to the DSM at any one time). Each scanner
module has its own unique factory set calibration coefficients, provided by the module supplier,
and is installed with an RTD, allowing temperature compensation to be carried out.
For a particular dwelling model, pressure data were acquired at all vertical tubes and surface
taps simultaneously, at a sampling rate which allowed high pressure 'events' that have been
found to tend to cause flue flow reversal to be resolved. For example, Upton et al. (2000) found
that typically a flue flow reversal may occur if a low to moderate wind speed (say, 2-4m/s)
occurred for a period of 5-10 seconds. The sampling rate has been set to resolve this event using
~5 sampling points. The timescales (T), length scales (L) and velocity scales (V) are related
between full scale and model scale in the following way:
Lm
Lf
=
Tm Vm Tf Vf
where the subscripts 'm' and 'f' denote model scale and full scale respectively.
Taking Lf = 80 (model scale 1:80), Vf = 3m/s and Tf = 10 seconds, if the wind tunnel speed Vm =
6m/s and Lm = 1, therefore the corresponding wind tunnel time period Tm = 0.06s. Thus a
sampling frequency of ~100Hz is required to obtain ~5 sampling points in order to resolve the
event. Pressures have been sampled at 100Hz over a period of approximately 10 seconds at
model scale (corresponding to a sampling period of about 15 minutes at full scale).
A pitot-static tube was placed at a height of 300mm above the wind tunnel floor in the
undisturbed incoming flow (slightly upwind and to one side of the model). The dynamic and
static pressures from the pitot-static tube were measured using each of the transducer modules
being used (1 to 3), and the average pressure difference, corrected to eaves height, was used to
normalise the measured pressures to coefficient form using the standard definition:
CP =
p x - ps
1 rV 2
ref
2
where:
Cp is the pressure coefficient
px is the pressure measured at the surface tap or vertical tube
ps is the static pressure measured by the pitot-static tube
Vref is the reference velocity (wind speed at eaves height in undisturbed flow)
r is the air density
1 rV 2
ref
2
is the total pressure at reference height
20
To minimise the effect of system drift, data acquisition was carried out in blocks, with zero
calibrations (to determine zero offsets) performed immediately before and after the wind tunnel
was run. If the drift was large, the data set was repeated; otherwise, the average offset was
removed from the data during analysis.
3.4 DATA ANALYSIS
3.4.1 Effect of adjacent buildings and topographical features
Pressure records containing instantaneous sampled pressures have been analysed in the
following ways:
· Time-averaged pressure coefficients have been evaluated over the roof (measured
using both surface taps and vertical tubes), and on façades (measured using surface taps
only);
· The minimum time-averaged façade pressure coefficients for each scenario have been
identified in order to determine the maximum difference between the pressure field over
the roof and the interior pressure (i.e. ‘worst-case’ internal/external pressure difference).
The maximum pressure difference will tend to occur when an open window is located
on the façade at a point coinciding with the minimum pressure. In this case, the interior
pressure will be close to the measured façade pressure, since the window acts as a
'dominant opening' (Cook (1990));
· Time-averaged roof and façade pressures have been used as boundary conditions to a
computational model (discussed further in Section 4) in order to predict the impact of
the external wind pressure field on the interior pressure, and to determine the sensitivity
of the interior pressure to factors such as the ventilation strategy, internal/external
temperature difference and orientation of the dwelling relative to the adjacent structure.
Results are presented in Sections 5.1 to 5.3.
3.4.2 Ridge vents
Pressure records containing instantaneous pressures sampled over a period of approximately 10
seconds at model scale have been analysed in the following ways:
· Time averages have been evaluated for pressures measured at the ridge;
· Intermittency characteristics of the pressure fields have been plotted by determining the
percentage of time that pressure coefficients are positive for time periods greater than
10 seconds (at full scale) at a nominal wind speed of 3m/s.
Results are presented in Section 5.5.
21
22
4. COMPUTATIONAL MODELLING
Computational modelling has been used to investigate further the effect of the wind pressure
field caused by an adjacent building or topographical feature on the interior pressure. From this,
the difference between the interior pressure and the pressure at a point over the roof coinciding
with the flue terminal may be assessed. An indication of the maximum possible pressure
difference resulting from an open window in a region of low pressure has been obtained from
the simplified analysis described in Section 5.3. The main purpose of the computational
modelling is to investigate a greater range of factors influencing the interior pressure, including
ventilation opening areas, orientation of the building relative to the adjacent structure, wind
speed and internal/external temperature difference.
Computational modelling has been carried out using the BRE's flow model, BREEZE. This is a
flow/pressure analysis program which applies methods of network flow computation to
calculate mass flows and pressure differences throughout a building. The building model used
in this analysis is of BRE test house B54.2, and is similar to that used in Plathner & Stephen
(2001). This model has been selected in order to provide consistency between previous
BREEZE modelling work carried out for HSE and the current program of BREEZE modelling.
Also, the model has previously been validated against test data, and its dimensions are similar to
the scale model of the dwellings tested in the wind tunnel.
In all cases, the internal temperature was set to 20°C and the 50th percentile house envelope
leakage was selected, corresponding to a leakage coefficient of 140.934m3.hr-1.Pa-1 (Plathner &
Stephen (2001)). The trickle vent sizes and locations were chosen in accordance with Approved
Document F1 (Department of the Environment and The Welsh Office (1995)). The kitchen
window dimensions from the BRE test house were used, and the effective area of the window
open 90° was calculated using CIBSE Guide A (1999).
The flue was modelled to represent closely the existing flue in building 54.2, and was situated
within the kitchen. The main lineaments of flue construction are outlined as follows:
· Flue cross-sectional area of 0.012m2;
· Duct length of 6.24m which includes 4.74m to match base of duct to roof height and
1.5m for the flue above the roof;
· A wire mesh inlet;
· Standard cowling terminal type.
BREEZE is able to accommodate pressure losses both due to inlet/outlet terminal losses and
friction losses along the length of the duct. To model the inlet/outlet pressure losses, the user
needs to enter the velocity pressure loss factors. Based on values presented in CIBSE Guide C
(1996), a loss factor of 0.5 was chosen for the inlet and 1.0 for the outlet. To model the friction
losses, the user needs to enter a coefficient of friction. Based on values presented in CIBSE
Guide C (1996), a coefficient of 0.03 was chosen. A sensitivity study was undertaken to
determine the impact of reasonable variations of both the pressure loss factors and the
coefficient of friction. This study showed that the results were fairly insensitive to variations in
these parameters. Indoor temperatures were used as the flue temperature for the part of the flue
up to the height of the roof, and external temperature were used for the part of the flue above the
23
height of the roof. This is a worst-case scenario, corresponding to the flue temperature likely to
be encountered on start-up from cold, when buoyancy forces which would normally enhance the
exhausting of flue gases are small.
The following scenarios, representing different pressure field regimes, have been modelled:
·
Base case - isolated dwelling (no adjacent structures).
·
Five additional scenarios:
(1) A dwelling within a positive pressure field caused by a downwind structure;
(2) A dwelling within a negative pressure field caused by an upwind structure;
(3) A dwelling with a large structure situated to one side of it;
(4) A dwelling with the adjacent structure offset relative to the dwelling;
(5) A dwelling with adjacent structure rotated relative to the dwelling.
For the base case and cases (1) and (2), sensitivity analyses have been carried out to determine
the effects of the following factors on the interior pressure:
·
Internal/external temperature difference;
·
Ventilation openings;
·
Open/closed doors;
·
Orientation of kitchen relative to adjacent structure.
Four internal/external temperature differences (0°C, 5°C, 10°C and 15°C have been considered,
corresponding to an internal temperature of 20°C and external temperatures of 20°C, 15°C,
10°C and 5°C, respectively. When the external temperature is below the internal temperature,
the interior pressure is negative, and therefore will tend to have an adverse impact on flue
performance. Ventilation openings have been varied by considering a scenario with background
ventilation (i.e. trickle vents only) and a scenario with additionally an open window. Different
kitchen orientations have been considered by placing the kitchen and open window on the
upwind and downwind sides of the dwelling (as shown in Figure 13).
For the non-symmetrical cases ((3), (4) and (5)), the impact of the ventilation openings only has
been investigated. The kitchen has been oriented so that the open window is located on the
façade which experiences the minimum negative pressure (a 'worst-case' scenario).
24
Figure 13
Kitchen orientations S1 and S2 (C4 is kitchen with W marking kitchen window; arrow
denotes wind direction)
A description of the cases modelled, along with external pressure coefficients at locations
coinciding with the kitchen window are given in Table 3. The external pressure coefficients
have been obtained directly from façade pressure measurements carried out in the wind tunnel.
For the base case and cases 1 and 2, the pressure coefficients are those measured at a location
coinciding with the kitchen window, which is situated on either the upwind (S2) or downwind
(S1) side of the dwelling. For cases 3, 4 and 5 the pressure coefficients correspond to the
minimum negative façade pressures measured in each case, on any of the four facades.
The output obtained from the BREEZE model includes the kitchen pressure relative to
atmospheric pressure (referred to as ‘interior pressure’). Results are presented in Section 5.4.
25
Table 3
Cases modelled using BREEZE
Case
Description
External pressure
coefficient at
window (S1)
External pressure
coefficient at
window (S2)
Base case
Isolated dwelling
-0.217
0.487
Case 1
8h x 2h adjacent structure
downwind at a separation 4h
0.050
0.586
Case 2
8h x 2h adjacent structure
upwind at a separation 4h
-0.135
0.060
Case 3
4h x 4h adjacent structure to
one side at a separation 2h
-0.625
-
-0.257
-
-0.232
-
Case 4
Case 5
2h x 4h adjacent structure
offset by 2h at a separation
2h, wind direction 15°
2h x 4h adjacent structure
rotated 60° at a separation
2h, wind direction 0°
26 5. RESULTS
5.1 IMPACT OF ADJACENT STRUCTURES AND TOPOGRAPHICAL FEATURES
ON THE EXTERNAL PRESSURE FIELD OVER DWELLING ROOFS
5.1.1 Introduction
Surface pressure tap measurements have been used in the first instance to determine the impact
of adjacent structures and topographical features on the external pressure field over dwelling
roofs. Vertical tube measurements are discussed later in Section 5.2. In the present section, the
variation in the pressure field with wind direction has been investigated first, in order to
determine wind directions with the most significant impact. The results of this investigation are
discussed in Section 5.1.2. In subsequent sections, only results for the wind directions with the
most significant impact are presented, except where asymmetries in the arrangements of
adjacent structures and dwellings require testing of additional wind directions (for example,
where the adjacent structure is rotated or offset relative to the dwelling). Section 5.1.3 presents
results on the magnitude and extent of the adjacent structure’s zone of influence as a function of
its dimensions. The results of sensitivity analyses are presented in Sections 5.1.4 - 5.1.8. Results
for topographical features are discussed in Section 5.1.9.
5.1.2 Effect of wind direction
In order to determine the wind direction for which the maximum mean positive pressure over
the roof occurs, the maximum mean Cp has been plotted as a function of wind direction for an
isolated dwelling, a dwelling with adjacent structure of dimensions 4h x 2h (height of 4h, width
of 2h) at a separation of 4h and a dwelling with adjacent structure of dimensions 2h x 16h at a
separation of 4h (Figures 14, 15 and 16, respectively). The maximum positive surface pressure
has been plotted along with pressures measured by the 12.5mm and 31.5mm tubes. For the
isolated dwelling, the variation in maximum pressure with wind direction is roughly
symmetrical about a 90° wind direction. For both the surface pressure and the vertical tube
measurements, the maximum positive pressure occurs at near 0° and 180°. The minimum
pressure occurs in the range 60° to 120°.
When an adjacent structure is present, the variation in maximum pressure with wind direction is
no longer symmetrical about the 90° wind direction. Maximum pressures are equal to or greater
than the isolated dwelling case, and occur for 180°, when the obstruction is downwind of the
dwelling. When the obstruction is upwind of the dwelling, the maximum pressure is lower than
the isolated case, and in the case of the 2h x 16h adjacent structure, is negative at wind
directions close to 0°. Comparing surface pressure measurements with and without the adjacent
structure present, these indicate that the Cp difference for a particular wind direction is greatest
when the adjacent structure is upwind of the dwelling (0°), and is smallest when it is downwind
(180°). Thus the wake downwind of an obstruction has a greater influence than the flow pattern
produced by an obstruction on its upwind side. For both adjacent structures, the maximum
surface pressure for the 90° wind direction is lower with the structure present than for the
isolated dwelling, due to funnelling of wind between the dwelling and structure.
The results illustrate that the maximum positive pressures occur for the 180° wind direction.
Since positive pressures will tend to contribute towards flue flow reversal, this wind direction
has been considered primarily for situations where the arrangement of adjacent structure and
dwelling has a plane of symmetry. The 0° wind direction has also briefly been considered.
27
Maximum mean Cp over the roof of isolated 45 degree
pitch dwelling for different wind directions
0.6
0.4
Cp
0.2
Surface
12.5 tubes
0
180
150
120
90
60
30
0
31.5 tubes
-0.2
-0.4
-0.6
Wind direction
Figure 14
Effect of wind direction on the maximum roof pressure
Isolated dwelling
Maximum mean roof Cp on dwelling
with 4h x 2h obstruction at 4h
0.6
0.4
0.2
Surface
Cp
12.5 tubes
31.5 tubes
0
180
150
120
90
60
30
0
Isolated (surface)
-0.2
-0.4
-0.6
Wind direction
Figure 15
Effect of wind direction on the maximum roof pressure
4h x 2h adjacent structure at 4h separation
28
M aximum mean roof Cp on dwelling
with 2h x 16h obstruction at 4h
0.6
0.4
0.2
Sur f ace
Cp
12.5 tubes
31.5 tubes
0
0
30
60
90
120
150
180
Isolated ( sur f ace)
-0.2
-0.4
-0.6
Wind direction
Figure 16
Effect of wind direction on the maximum roof pressure
2h x 16h adjacent structure at 4h separation
5.1.3 Effect of adjacent structure dimensions and separation distance
The effect of adjacent structure dimensions has been investigated by varying the heights and
widths of adjacent structures, and the separation distances between dwellings and adjacent
structures. When the wind direction is 0° (obstruction upwind) or 180° (obstruction downwind),
the maximum roof pressure occurs on the roof centre line. Thus the mean roof pressure
coefficient Cp has been plotted as a function of its location (1 to 12) along the roof centre line.
Locations 1 and 12 are adjacent to the eaves, while locations 6 and 7 are either side of the ridge.
For 180°, locations 1 to 6 are on the upwind side of the roof furthest away from the adjacent
structure, and locations 7 to 12 are on the downwind side. For 0°, locations 1 to 6 are on the
downwind side (again furthest away from the adjacent structure) and locations 7 to 12 are on the
upwind side.
Figures 17 and 18 illustrate the effect that an adjacent structure (in this case with dimensions 8h
x 2h) has on the mean external roof pressure field when wind directions are 180° and 0°,
respectively. When the adjacent structure is downwind of the dwelling (180°), the pressure field
over the whole roof is more positive relative to the isolated dwelling. When the adjacent
structure is upwind (0°), the pressure field is more negative on the upwind side of the roof, but
less negative on the downwind side, relative to the isolated dwelling. For the 180° case,
pressures become monotonically less positive with increasing separation distance, converging
towards the isolated case at large separation distances, when the dwelling is outside the
influence zone of the adjacent structure. On the upwind side of the roof of the 0° case, pressures
monotonically increase with increasing separation distance, again converging to the isolated
case at large separation distances.
29
Roof centre line Cp
8h x 2h obstruction at 4h, 8h and 12h downwind
0.8
0.6
0.4
Cp
0.2
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
12
4h
8h
12h
Isolated
-0.4
-0.6
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 17
Effect of separation distance on the roof centre line Cp
8h x 2h obstruction downwind (180° wind direction)
Note: on this graph and all similar ones, “upwind eaves to downwind eaves” indicates that
upwind is to the left of the graph and downwind to the right
8h x 2h obstruction at 4h, 8h and 12h upwind
roof centre line Cp
0.8
0.6
0.4
4h
0.2
Cp
8h
12h
0
1
2
3
4
5
6
7
8
9
10
11
12
-0.2
-0.4
-0.6
Tap position on roof centre line
(downwind eaves to upwind eaves)
Figure 18
Effect of separation distance on the roof centre line Cp
8h x 2h obstruction upwind (0° wind direction)
30
isolated
In both cases, the variation in Cp with separation distance is greatest on the side of the roof
which is closest to the adjacent structure. For example, for the 180° case, the variation in Cp
with separation distance is greater for locations 7 to 12 (closest to the adjacent structure) than
for locations 1 to 6. Similarly, for the 0° case, the variation in Cp with separation distance is
greater for locations 7 to 12 (again, closest to the adjacent structure) than for locations 1 to 6. In
both cases, the adjacent structure also tends to reduce the variation in pressure between the
upwind and downwind sides of the roof.
Figures A1 to A3 in Appendix A illustrate the variation in the impact of directly downwind
structures of width 2h and height 2h, 4h and 8h respectively. The separation distances plotted
have been chosen as a function of the adjacent structure height, (multiplied by 0.5, 1 and 1.5).
The figures illustrate the increase in the magnitude and extent of the influence zone with height
of structure; for a given separation distance, the magnitude of the positive pressure field
increases with increasing structure height.
In Figures A4 to A7 in Appendix A, graphs have been plotted to illustrate the impact of adjacent
structures of the same height 2h, but with varying widths 2h, 4h, 8h and 16h. The magnitude
and extent of the influence zone increases with increasing width. These results are summarised
in Figure A8 for a single separation distance 2h.
Figure A9 shows the effect of distance for an adjacent building of height 2h and width 4h
downwind. It is clear that the case with the building at 6h distance is closer to the isolated case
than the cases at distances of 2h and 4h. With a larger adjacent building of 4h high and 4h wide
downwind, figure A10 shows the greater increases in the roof centre line Cp with a monotone
increase in the effect at every point on the centre line as the distance of the building decreases.
Figures A11-A14 of Appendix A illustrates the variation in the impact of adjacent structures
with the adjacent structure upwind of the dwelling. The significant effect, common between the
cases, is that pressures on the upwind slope are reduced by comparison with the isolated case,
with the effect increasing with proximity.
Figure A11 shows that for adjacent buildings of height 2h at a distance of 2h upwind the
pressures on the upwind roof of the dwelling are reduced. The effect increases with the width of
the adjacent building, with all mean roof pressures being brought below zero for cases where it
is above 4h wide. Pressures on the downwind slope are increased compared to the isolated case,
but not to above zero, except in the case of a 16h wide building, where all centre line pressures
are reduced.
Figure A12 shows the effect of the distance of an adjacent upwind building of height 2h and
width 2h. The pressures on the upwind roof slope are reduced, with the effect increasing as the
adjacent building becomes closer. As in figure A11, pressures on the downwind roof slope are
increased, but not to above zero.
Figure A13 shows the effect of a building of height 4h and width 2h. Pressures are decreased
across the whole roof of the dwelling, particularly on the upwind slope, with a much greater
effect when the adjacent building is nearer.
Figure A14 shows the effect of a building of height 4h and width 4h at different distances
upwind. On the upwind roof slope, pressures are strongly reduced, with all pressures being
below zero if the adjacent building is at 6h or nearer. For buildings at 4h or further, downwind
roof pressures are increased, but not above zero, but when the neighbouring building is at 2h, all
roof pressures are reduced.
31
The influence of the downwind adjacent structure is illustrated more clearly by examining the
variation in maximum Cp difference with normalised separation distance (Figure 19). The
maximum Cp difference is defined as the difference between the maximum roof and minimum
façade pressure coefficients. Separation distance, x, has been normalised by the adjacent
structure height, H. The zone of influence of the adjacent structure is the value of x/H at which
the maximum Cp difference tends to the isolated case.
Variation of max Cp difference with separation distance
1.2
1
W/h:
=4
=4
=8
Max Cp difference
W/h=2
0.8
W/h = 2
2h x 2h
8h x 2h
isolated
4h x 4h
0.6
2h x 4h
2h x 8h
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
x/H
Figure 19
Variation of maximum Cp difference with separation distance
for adjacent buildings (labelled height x width)
When the adjacent structure is relatively narrow in comparison to the dwelling (W/h = 2, where
W is the width of the adjacent structure and h is the dwelling eaves height), the zone of
influence is approximately 1.5 times the height of the adjacent structure. When the adjacent
structure is wider than the dwelling (W/h ≥ 4), the zone of influence extends to at least 2 times
the height of the adjacent structure.
The impact of these results on the likelihood of flue flow reversal is discussed in Section 6.
5.1.4 Effect of dwelling roof pitch
Decreasing the pitch of a roof decreases the range of pressures on the roof. Figure 20a illustrates
that for an isolated dwelling, roof centre line Cp is less positive on the upwind slope and less
negative on the downwind slope in the 30° pitch case compared to the 45° pitch one.
32
The effect of roof pitch on roof centre line Cp
for an isolated dwelling
0.6
0.4
0.2
Cp
0
0
1
-0.2
30 pitch
45 pitch
-0.4
-0.6
-0.8
Position on roof centre line
(downwind eaves to upwind eaves)
Figure 20a
Effect of dwelling roof pitch on the roof centre line Cp
Figures 20b and 20c show the effect of a downwind obstruction on a dwelling with a 30° pitch
roof and a 45° pitch roof respectively. In both cases the pressure is increased at all points.
Figure 20d shows that on the upwind slope the effect on the pressures is greater in the case of
the 30° pitch roof, with the peak pressures for the 30° and the 45° roofs being significantly
closer in the obstructed case than when the dwellings are isolated.
The effect of a 2h x 4h obstruction at 4h downwind on roof centre
line Cp for a 30 degree pitched roof
0.4
0.3
0.2
Cp
0.1
0
0
1
30 pitch
30 pitch +obs
-0.1
-0.2
-0.3
-0.4
Position on roof centre line
(downwind eaves to upwind eaves)
Figure 20b
Effect of a 2h x 4h downwind obstruction on 30° pitch roof centre line Cp
33
The effect of a 2h x 4h obstruction at 4h downwind on roof centre
line Cp for a 45 degree pitched roof
0.6
0.4
0.2
Cp
0
0
1
-0.2
45 pitch
45 pitch +obs
-0.4
-0.6
-0.8
Position on roof centre line
(downwind eaves to upwind eaves)
Figure 20c
Effect of a 2h x 4h downwind obstruction on 45 pitch roof centre line Cp
The effect of roof pitch on roof centre line Cp with a 2h x 4h
obstruction at 4h downwind
0.6
0.4
0.2
Cp
0
0
1
-0.2
30 pitch
45 pitch
30 pitch +obs
45 pitch +obs
-0.4
-0.6
-0.8
Position on roof centre line
(downwind eaves to upwind eaves)
Figure 20d
Comparison of effects of a 2h x 4h obstruction at 4h upwind
on 30 and 45 pitch roof centre line Cp
5.1.5 Effect of location of dwelling in a terrace
When a dwelling is located in a terrace of similar dwellings, the effect of a nearby building on
the roof pressure field is dependent on both the location of the adjacent building and the
position of the dwelling in the terrace.
34
Figures 21a and 21b examine the case where a dwelling has an adjacent structure, which is in
the same position downwind of it in each case, but the dwelling is either in the middle or at the
end of a terrace of three similar houses. The long axis (i.e. ridge) of the terrace is perpendicular
to the wind direction. Figure 21c looks at the case where the ridge of the terrace is parallel to the
wind. Here again the dwelling may be either at the end of the terrace near the adjacent building
or in the middle of the terrace.
With the terrace perpendicular to the wind direction, and an obstruction downwind, mid-terrace
dwellings have much higher roof pressures than end-terrace cases. In both of the cases in
Figures 21a and 21b the end-terrace case has even lower roof pressures than in the case where
there is no downwind obstruction, whereas the mid-terrace case is comparable to the case where
the dwelling is detached.
In Figure 21c it is clear that the mid-terrace house again has higher roof pressures, despite being
further from the downwind obstruction, contrasting with results for a detached dwelling where
there was a monotone decrease in roof pressures as the dwelling was moved further upwind of a
moderate sized obstruction. It seems that the increase in the roof pressures due to being in the
middle of a terrace is greater than the decrease due to the greater distance from the nearby
building.
Roof centre line Cp for end-terrace, mid-terrace and
detatched dwellings with a 4hx2h building at 2h
downwind
0.6
0.4
Cp
0.2
0
-0.2
1
2
3
4
5
6
7
8
9
10
-0.4
11
12
mid-terrace
end-terrace
detached
isolated
-0.6
-0.8
Tap position on roof centre line
(downwind eaves to upwind eaves)
Figure 21a
Effect of location in terrace on roof centre line Cp for the terrace ridge perpendicular to
the wind direction; downwind building of height 4h and width 2h at 2h distance
35
Roof centre line Cp for end-terrace, mid-terrace and
detached dwellings with a 4h x 4h building at 4h
downwind
0.6
0.4
0.2
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
12
mid-terrace
end-terrace
detached
iso lated
-0.4
-0.6
Tap position on roof centre line
(downwind eaves to upwind eaves)
Figure 21b
Effect of location in terrace on roof centre line Cp for a terrace ridge perpendicular to the
wind direction; downwind building of height 4h and width 4h at 4h distance
Effect of a 4h x 2h building at 2h downwind along
ridge of a terrace
0.18
0.16
0.14
Cp
0.12
0.1
0.08
end-terrace
centre-terrace
0.06
0.04
0.02
0
1
2
3
4
5
6
7
8
9
10 11 12
Tap position on roof centre line
(downwind eaves to upwind eaves)
Figure 21c
Effect of location in terrace on roof centre line Cp for the terrace ridge parallel to the
wind direction; downwind building of height 4h and width 2h at 2h distance
36
5.1.6
Effect of adjacent structure roof pitch
Figure 22 examines the effect of the roof pitch of a neighbouring building of height 4h and
width 2h. The measured roof centre line Cp values indicate that the effect of the roof pitch of
the adjacent building in this case were quite small, with little difference in the range of roof
pressures measured. There is an indication that the highest pressure on the roof of the dwelling
occurs in a different place for the cases with a pitch roof building downwind, but this effect is a
small one. These results show that in this scenario the pitch of an adjacent structure has no
significant effect on the risk factors for flue flow reversal.
Effect of adjacent structure roof pitch
4h x 2h neighbour at 2h downwind
roof centre line Cp
0.6
0.4
Cp
0.2
Flat roof neighbour
Pitch 30 neighbour
0
Pitch 45 neighbour
1
2
3
4
5
6
7
8
9
10 11 12
Isolated
-0.2
-0.4
-0.6
Tap position on roof centre line
(downwind eaves to upwind eaves)
Figure 22
The effect on roof centre line Cp of the pitch of the roof of a downwind structure of
height 4h and width 2h
5.1.7
Effect of offset or rotation of adjacent building relative to dwelling
Figure 23 shows the effect on maximum roof Cp of a 2h x 4h obstruction at 2h downwind, offset
by 0h (symmetric), h or 2h, for each wind angle from 0 to 180. The maximum Cp occurs for an
adjacent structure offset by 2h, when the wind angle is 15°; however, the increase in the
maximum Cp relative to the isolated case is reasonably small.
37
2h x 4h obstruction at 2h, offset by 0, h or 2h
maximum roof Cp
0.6
0.5
Max roof Cp
0.4
0.3
Offset 2h
Offset h
0.2
Symmetric
Isolated
0.1
0
0
15
30
45
60
75
90 105 120 135 150 165 180
-0.1
-0.2
Wind angle
Figure 23
Maximum roof Cp for a range of wind angles for a dwelling with a 2h x 4h neighbour at
2h distance, offset by 0, h or 2h
Figure 24 shows the maximum roof pressure for a range of wind angles for dwellings with a 2h
x 4h neighbour at 2h, rotated by 0, 30° or 60°, For the range of wind angles modelled , there is
little evidence that there is greater risk in the angled cases than the symmetric case. The only
angle where the maximum roof pressure is greater in the angled cases is when the neighbour is
directly upwind of the dwelling, when the amount of shelter provided by the neighbour
decreases dramatically as it is rotated. However for almost all other wind angles, the rotated
neighbours cause lower maximum roof pressures than in the symmetric case, indicating
probable lower risks of flue flow reversals.
38
2h x 4h obstruction at 2h, angled at 0, 30 and 60
maximum roof Cp
0.6
0.5
0.4
Max roof Cp
0.3
Isolated
0.2
Angle 30
Angle 60
0.1
Angle 0
0
0
15
30
45
60
75
90
105 120 135 150 165 180
-0.1
-0.2
-0.3
Wind angle
Figure 24
Maximum roof Cp for a range of wind angles for a dwelling with a 2h x 4h neighbour at
2h, rotated by 0, 30° or 60°
5.1.8
Effect of neighbouring dwellings
Figures 25 and 26 compare the roof centre line Cp for a dwelling with a 2h x 4h building at 2h
downwind in the cases where it is on its own and where there are two identical dwellings
located at the width of the dwelling to each side. It is apparent that roof pressures are somewhat
higher in the case with the two neighbours, but that the peak roof Cp is very similar. The effects
on risk of flue flow reversal might be expected to be small, as sizeable increases are only visible
at points where the roof pressure is negative.
39
Effect of neighbours
2h x 4h obstruction at 2h downwind
roof centre line Cp
0.6
0.4
0.2
isolated
no neighbours
0
Cp
1
2
3
4
5
6
7
8
9
10
11
12
neighbours
-0.2
-0.4
-0.6
-0.8
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 25
Roof centre line Cp for a dwelling with a 2h x 4h building at 2h downwind, with
and without two nearby neighbours
Effect of neighbours
4h x 2h obstruction at 2h downwind
roof centre line Cp
0.6
0.4
0.2
Isolated
Cp
0
-0.2
no neighbours
1
2
3
4
5
6
7
8
9
10 11 12
neighbours
-0.4
-0.6
-0.8
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 26
Roof centre line Cp for a dwelling with a 4h x 2h building at 2h downwind,
with and without two nearby neighbours
40
5.1.9
Effect of adjacent topographical features
Topographical features such as embankments and hills can have a large effect on the pressure
field around a building. Relevant factors are the distance to the feature, its height, breadth and
the angles of its slopes.
Figure 27a illustrates the effect of the height of a small downwind hill with the base always at
2h separation. It is clear that the higher the hill, the larger the pressure on the roof of the
dwelling, with peak coefficients being higher than in the isolated case and positive over the
whole centre line, rather than just one half of it as in the isolated case. The effect is much
greater on the downwind side of the roof (closest to the hill), although risks are clearly higher
on the upwind slope.
Figure 27b shows the effect with a hill of different heights with base at 2h upwind of the
dwelling. The effect here is strikingly different to cases (for example figure A14) where there is
a large building upwind. In the case of a hill of height h, the roof pressures are decreased
slightly where they are highest, but when the hill is of height 2h, maximum roof pressures are
increased. In both cases roof pressures on the downwind side of the roof are substantially
increased, but not above zero.
Figure 28a illustrates the effect of the downwind dimension (breadth) of a downwind
embankment (both examples have height 2h and crosswind dimension 16h). The effects of two
vertical embankments of breadth h and 4h are shown. It is clear that the smaller embankment
increases the pressures on the upwind slope of the roof much more than the larger embankment.
Figure 28b compares the effects of downwind embankments with different slopes. In each case
the distance of the dwelling to the top of the embankment is the same, as are the height, width
and breadth of the top of flat part of the embankment. In this case, all of the embankments have
a similar effect on the upwind roof slope (a small increase in pressure coefficients compared to
the isolated case). On the downwind slope, pressure coefficients are increased, with the greatest
increase being caused by the 60° embankment and the least by the vertical embankment.
41
Roof centre line Cp with hills of different heights at 2h
downwind
0.8
0.6
0.4
Hill h
0.2
Cp
Hill 2h
Hill 3h
0
1
2
3
4
5
6
7
8
9
10
11
12
isolated
-0.2
-0.4
-0.6
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 27a
Roof centre line Cp with hills of different heights at 2h downwind
Hill upwind of dwelling
roof centre line Cp
0.6
0.4
Cp
0.2
0
1
2
3
4
5
6
7
8
9
10
11
12
-0.2
-0.4
-0.6
Tap position on roof centre line
(downwind eaves to upwind eaves)
Figure 27b
Roof centre line Cp with hills of different heights at 2h upwind
42
Hill h
Hill 2h
isolated
Roof centre line Cp with an vertical embankment of different
breadths at 2h downwind
0.8
0.6
0.4
Cp
0.2
Breadth h
Breadth 4h
Isolated
0
1
-0.2
2
3
4
5
6
7
8
9
10
11
12
-0.4
-0.6
-0.8
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 28a
Roof centre line Cp with embankments of height 2h
and different breadths at 2h downwind
Roof centre line Cp with embankments of different slopes at 4h
downwind
0.6
0.4
Cp
0.2
0
1
2
3
4
5
6
7
8
9
10
-0.2
-0.4
-0.6
11
12
Slope 90
Slope 60
Slope 30
Isolated
-0.8
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 28b
Roof centre line Cp with embankments of different slopes at 4h downwind
5.2 EFFECT OF FLUE HEIGHT ON THE EXTERNAL PRESSURE AT THE FLUE
TERMINAL
Pressures have been measured at 2 flue terminal heights above the dwelling roof, using vertical
tubes of length 12.5mm and 31.5mm (model scale). The results of the vertical tube
measurements are compared with surface pressure measurements with the adjacent structure
present, and additionally with surface pressure measurements for the isolated dwelling.
43
Figure 29 shows vertical tube measurements for an adjacent structure of 8h x 2h placed at a
separation distance of 4h downwind of the dwelling. This figure illustrates the decrease in
maximum positive pressure with an increase in height above the roof. Additional graphs have
been plotted for adjacent structures with the same width and different heights (2h x 2h, 4h x 2h)
in Appendix B, along with adjacent structures with the same height and varying widths (2h x
4h, 2h x 16h). The results show that the difference in maximum positive Cp between the surface
measurements and the 12.5mm tubes is generally about 0.1, and between the surface
measurements and the 31.5mm tubes is about 0.2. The variation in Cp with height above the
surface is generally smaller for the downwind side of the dwelling (where roof pressures tend to
be negative) than the upwind side (where roof pressures are positive).
8h x 2h obstruction at 4h downwind
roof centre line Cp
0.8
0.6
Cp
0.4
12.5 tubes
0.2
31.5 tubes
surface
0
isolated
0
-0.2
-0.4
-0.6
Position on roof centre line
(dow nw ind eaves to upw ind eaves)
Figure 29
Variation in Cp with height above the roof, 8h x 2h obstruction at 4h downwind
Figure 30 shows vertical tube measurements for an 8h x 2h adjacent structure at a separation
distance of 4h upwind of the dwelling. Again, additional cases have been presented in Appendix
B. The variation in Cp with height above the roof again is greatest on the upwind side of the
roof, and in the case of the 8h x 2h obstruction, the pressure becomes more negative with height
above the roof. However, for some of the cases plotted in Appendix B where the height of the
adjacent structure is closer to the dwelling height, the pressure becomes slightly less negative
with increasing height, although remains negative.
44
8h x 2h obstruction at 4h upwind
roof centre line Cp
0.6
0.4
Cp
0.2
31.5 tubes
12.5 tubes
0
surface
0
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure 30
Variation in Cp with height above the roof
8h x 2h obstruction at 4h upwind
5.3 IMPACT OF ADJACENT STRUCTURES AND TOPOGRAPHICAL FEATURES
ON MAXIMUM INTERNAL/EXTERNAL PRESSURE DIFFERENCES IN THE CASE
OF A DOMINANT OPENING
The roof centre line Cp difference has been calculated as the roof centre Cp values minus the
minimum façade Cp. This represents a 'worst-case' scenario, when the flue terminates in a
region where maximum positive pressure is encountered on the roof centre line, and the boiler is
situated in a room with an open window. In this case, the interior pressure will be close in value
to the external pressure at a location coinciding with the open window. Therefore, the Cp
difference profiles plotted are a measure of the maximum internal/external pressure difference
that may occur with an open window.
Results for three scenarios have been plotted in Figures 31 to 33, where the adjacent structure is
located downwind of the dwelling. The scenarios include a 8h x 2h downwind obstruction
(Figure 31), a 4h x 4h downwind obstruction (Figure 32) and an 2h x 16h downwind obstruction
(Figure 33), and represent a range of different adjacent structure geometries. The maximum Cp
difference is approximately equal to 1.0, and generally occurs for the isolated case or for a
situation where the adjacent structure is far enough from the dwelling that it does not have a
large influence on the dwelling pressure field. In these cases, the profiles are shifted upwards
since the minimum façade pressure is negative (Cp ~ -0.6). In cases where the dwelling is very
close to the downwind adjacent structure, the minimum façade pressure becomes positive (for
example, at downwind separation distances of 2h, for the 2h x 16h and 4h x 4h adjacent
structures, minimum façade pressures are equal to 0.07 and 0.09 respectively). Thus the profiles
are shifted downwards. For all other separation distances where the adjacent structure has a
significant effect, the minimum façade pressures are negative, although they are smaller in
magnitude than the isolated case.
45
8h x 2h obstruction downwind
roof centre line Cp difference
1.2
1
4h
8h
12h
isolated
0.6
0.4
0.2
0
1
2
3
4
5
6
7
8
9
10
11
12
-0.2
Tap position on roof centre line
(upwind eaves to downwind eaves)
Figure 31
Effect of separation distance on the roof centre line Cp difference
8h x 2h obstruction downwind (180° wind direction)
4h x 4h obstruction downwind
roof centre line Cp differe nce
1.2
1
0.8
isolated
Cp difference
Cp difference
0.8
0.6
8h
6h
0.4
4h
2h
0.2
0
1
-0.2
Position on roof centre line (downwind
eaves to upwind eaves)
Figure 32
Effect of separation distance on the roof centre line Cp difference
4h x 4h obstruction downwind (180° wind direction)
46
2h x 16h obstruction downwind
roof centre line Cp diffe rence
1.2
1
Cp difference
0.8
0.6
2h
0.4
4h
isolated
0.2
0
1
-0.2
-0.4
Position on roof centre line
(upwind eaves to downwind eaves)
Figure 33
Effect of separation distance on the roof centre line Cp difference
2h x 16h obstruction downwind (180° wind direction)
Results for one example of an upwind adjacent structure, 8h x 2h, are shown in Figure 34.
Again the maximum Cp difference occurs for the isolated case. With the adjacent structure
present, the minimum façade pressures are -0.18, -0.22 and -0.30 for separation distances of 4h,
8h and 12h respectively. Thus the minimum façade pressure becomes less negative when the
dwelling is close to the adjacent structure, and profiles are shifted upwards by an amount which
increases with increasing separation distance.
47
8h x 2h obstruction upwind
roof centre line Cp difference
1.2
1
Cp difference
0.8
0.6
4h
8h
0.4
12h
isolated
0.2
0
1
-0.2
-0.4
Position on roof centre line
(downwind eaves to upwind eaves)
Figure 34
Effect of separation distance on the roof centre line Cp difference
2h x 16h obstruction upwind (0° wind direction)
Results for non-symmetrical cases, where the adjacent structure is offset or rotated relative to
the dwelling, are shown in Figures 35 and 36. The variation in maximum Cp difference with
wind angle is plotted. The graphs show that the maximum Cp difference again occurs for the
isolated case.
These results show how an adjacent structure tends to moderate the pressure difference
encountered over the dwelling, both for the upwind and downwind cases. The results also
demonstrate that, for the situation where a dominant opening is located close to where the
minimum façade pressure occurs, the internal/external pressure difference will tend to be larger
for the isolated dwelling than dwellings with adjacent structures.
48
2h x 4h obstruction at 2h, angled by 0, 30 or 60
maximum Cp difference
1
0.9
Cp difference
0.8
0.7
Isolated
0.6
Angle 0
0.5
Angle 30
0.4
Angle 60
0.3
0.2
0.1
0
0
15
30
45
60
75
90
105 120 135 150 165 180
Wind angle
Figure 35
Effect of wind angle on the maximum Cp difference
2h x 4h obstruction at 2h, angled by 0°, 30° and 60°
Cp difference
2h x 4h obstruction at 2h, offset by 0,h and 2h
m axim um Cp difference
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Offset 2h
Offset h
Symmetr ic
Isolated
0
15
30
45
60
75
90
105 120 135 150 165 180
Wind angle
Figure 36
Effect of wind angle on the maximum Cp difference
2h x 4h obstruction at 2h, offset by 0, h and 2h
49
5.4 BREEZE MODELLING
5.4.1 Sensitivity of interior pressure to internal/external temperature difference
Results for the sensitivity analyses for the base case, case 1 and case 2 are presented first.
Figures 37 to 39 show the impact of internal/external temperature difference on the kitchen
interior pressure for a standard reference meteorological wind speed of 4m/s. The reference
wind speed has been corrected to the local wind speed at eaves height, taking into account the
effect of the surrounding suburban terrain. In all cases, the minimum interior pressure occurs
when the internal/external temperature difference is at a maximum of 15° C (i.e. the internal
temperature is 20° C and the external temperature is 5° C), which will occur during winter
months. At this temperature difference, the interior pressure is negative. When background
ventilation is provided, the minimum interior pressures for the base case, upwind case and
downwind case are -2.0Pa, -2.4Pa and -1.6Pa respectively (i.e. the wind pressure field has a
small influence on the interior pressure). With an open window, the sensitivity of the interior
pressure to the internal/external temperature difference decreases, and the interior pressure
becomes more dominated by the external wind pressure at the open window, although at this
wind speed, pressure differences associated with buoyancy forces still have a significant effect.
At larger wind speeds, it is likely that the internal/external temperature difference will be less
significant in the case of an open window. In subsequent models, the internal/external
temperature difference has been taken as 15° C, as this has the largest adverse effect on the
kitchen interior pressure.
Isolated dwelling
0.5
0
Interior pressure (Pa)
0
5
10
15
20
25
-0.5
-1
-1.5
-2
-2.5
-3
External temperature
S1; background; doors open, met wspeed 4m/s
S1; window; doors open, met wspeed 4m/s
Figure 37
Effect of internal/external temperature difference on kitchen interior pressure
Isolated case – orientation S1 (kitchen downwind side of house)
50
Adjacent structure downwind
0
5
10
15
20
25
0.5
Interior pressure (Pa)
0
-0.5
-1
-1.5
-2
-2.5
-3
External temperature
S1; background; doors open, met wspeed 4m/s
S1; window; doors open, met wspeed 4m/s
Figure 38
Effect of external/internal temperature difference on kitchen interior pressure
Adjacent structure downwind
Adjacent structure upwind
0.5
Interior pressure (Pa)
0
0
5
10
15
20
25
-0.5
-1
-1.5
-2
-2.5
-3
External temperature
S1; background; doors open, met wspeed 4m/s
S1; window; doors open, met wspeed 4m/s
Figure 39
Effect of external/internal temperature difference on kitchen interior pressure
Adjacent structure upwind
51
5.4.2 Sensitivity of interior pressure to wind speed, as a function of dwelling
ventilation strategy and orientation
Figures 40 to 45 illustrate the sensitivity of the interior pressure to the wind speed, as a function
of the ventilation strategy for the dwelling and the orientation of the dwelling relative to the
wind direction. Figures 40 and 41 show these results for the base case (isolated dwelling). For
both orientations, the interior pressure is much more sensitive to the wind speed for the open
window case than the background ventilation case, and as would be expected rapidly becomes
more positive with increasing wind speed when the kitchen is on the upwind side of the house
(orientation S2), and more negative with increasing wind speed when the kitchen is on the
downwind side of the house (orientation S1). The variation of interior pressure with wind speed
over the range 0m/s to 4m/s (at eaves height) is less than 0.5Pa for the background ventilation
case, and the pressure remains negative. For orientation S2 (open window), the variation in
pressure is ~2Pa (more than 4 times greater) over the same range of wind speeds. Note that the
wind pressure coefficient at a location coinciding with the open window is -0.217, and at higher
wind speeds, the interior pressure is close to the external wind pressure at the window. The
small variation of interior pressure with wind speed in the background ventilation case arises
due to the averaging out of positive and negative façade and roof pressures over the dwelling,
since the ventilation openings are relatively well distributed. This will tend to result in a net zero
interior pressure in the absence of the stack effect.
The sensitivity of the interior pressure to ventilation strategy and orientation is shown in Figures
42 and 43 for case 1 (downwind obstruction) and in Figures 44 and 45 for case 2 (upwind
obstruction). In both cases, the variation in interior pressure for the background ventilation case
is greater relative to the base case, and the pressure tends to increase with wind speed for case 1
and decrease with wind speed for case 2. This is because in case 1, the averaging of façade and
roof pressures over the dwelling results in net positive wind-induced interior pressures, and in
case 2 it results in net negative wind-induced interior pressures. The sensitivity of interior
pressure to wind speed for the open window case relative to the background ventilation depends
on the magnitude of the external pressure coefficient at a location coinciding with the window.
For example, for case 1, the external pressure coefficient at the window for orientation S1 is
close to zero, and therefore the variation is relatively small. Conversely, the external pressure
coefficient at the window is large and positive (0.586) for orientation S2 and therefore the
interior pressure rises rapidly with wind speed relative to the background ventilation case. For
case 2 (adjacent structure upwind of dwelling), the largest variation occurs when the open
window is on the downwind façade of the dwelling (orientation S1), when the pressure
coefficient is negative. The pressure coefficient on the upwind façade of the dwelling is close to
zero, and therefore when the window is on this façade (orientation S2) the variation is small.
52
Isolated dwelling
0
0
1
2
3
4
5
Interior pressure (Pa)
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
Figure 40
Effect of wind speed on kitchen interior pressure
Isolated case – orientation S1 (kitchen downwind side of house)
Isolated dwelling
5
Interior pressure (Pa)
4
3
2
1
0
-1
0
1
2
3
4
-2
-3
Wind speed (m/s)
S2; background; doors open, T diff 15
S2; 90 deg window; doors open, T diff 15
Figure 41
Effect of wind speed on kitchen interior pressure
Isolated case – orientation S2 (kitchen upwind side of house)
53
5
Adjacent structure downwind
6
5
Interior pressure (Pa)
4
3
2
1
0
-1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-2
-3
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
Figure 42
Effect of wind speed on interior pressure
Adjacent structure downwind – orientation S1 (kitchen downwind side of house)
Adjacent structure downwind
6
5
Interior pressure (Pa)
4
3
2
1
0
-1
0
0.5
1
2
1.5
2.5
3
3.5
4
4.5
-2
-3
Wind speed (m/s)
S2; background; doors open, T diff 15
S2; 90 deg window; doors open, T diff 15
Figure 43
Effect of wind speed on interior pressure
Adjacent structure downwind – orientation S2 (kitchen upwind side of house)
54
Adjacent structure upwind
0
0
1
2
3
4
5
Interior pressure (Pa)
-0.5
-1
-1.5
-2
-2.5
-3
Wind speed (m/s)
S2; background; doors open, T diff 15
S2; 90 deg window; doors open, T diff 15
Figure 44
Effect of wind speed on interior pressure
Adjacent structure upwind – orientation S2 (kitchen downwind side of house)
Adjacent structure upwind
0
0
1
2
3
4
5
Interior pressure (Pa)
-0.5
-1
-1.5
-2
-2.5
-3
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
Figure 45
Effect of wind speed on interior pressure
Adjacent structure upwind – orientation S1 (kitchen upwind side of house)
55
5.4.3 Sensitivity of interior pressure to wind speed, as a function of internal door
setting
Figures 46 to 48 illustrate the sensitivity of the interior pressure to the internal door setting
(open or closed) for the base case and cases 1 and 2. With the window open, the internal door
setting does not have a significant influence on the interior pressures, which are again
dominated by the external pressure at the window. In the case of background ventilation, the
influence of internal door setting is greatest for the isolated case. This is likely to be due to the
greater variation of pressure over different façades of the dwelling, which will tend to cause
variations in interior pressure between different rooms in the dwelling when doors are closed,
but will be equalised over the inside of the dwelling when doors are open. In the isolated case,
the kitchen is on the downwind side of the dwelling, and therefore when doors are closed the
negative external pressures acting on leakage and ventilation paths to the kitchen from outside
will tend to dominate the kitchen interior pressure.
Isolated case
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Interior pressure (Pa)
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
S1; background; doors closed, T diff 15
S1; 90 deg window; doors closed, T diff 15
Figure 46
Effect of door opening and ventilation strategy on interior pressure
Isolated case – orientation S1 (kitchen downwind side of house)
56
Adjacent structure downwind
0
Interior pressure (Pa)
-0.5
0
1
2
3
4
5
-1
-1.5
-2
-2.5
-3
-3.5
Wind speed (m /s)
S1 ; background; doors open, T diff 15
S1 ; 90 deg w indow; doors open, T diff 15
S1 ; background; doors cl osed, T di ff 15
S1 ; 90 deg w indow; doors cl osed, T diff 15
Figure 47
Effect of door opening and ventilation strategy on interior pressure
Case 1 – orientation S1 (kitchen downwind side of house)
Adjacent structure upwind
0
Interior pressure (Pa )
-0.5
0
1
2
3
4
5
-1
-1.5
-2
-2.5
-3
-3.5
Wind speed (m/s)
S1 ; background; doors open, T diff 15
S1 ; 90 deg wi ndow; doors open, T diff 15
S1 ; background; doors cl osed, T diff 15
S1 ; 90 deg wi ndow; doors cl osed, T di ff 15
Figure 48
Effect of door opening and ventilation strategy on interior pressure
Case 2 – orientation S1 (kitchen downwind side of house)
57
5.4.4 Results for Cases 3, 4 and 5
Results for cases 3, 4 and 5 are shown in Figures 49 to 51. For cases 3, 4 and 5, the dwelling has
been orientated so that the open window is located where the minimum negative external
pressure occurs. For case 3, the proximity of a large adjacent structure to one side of the
dwelling causes extremely large external pressures on the façade closest to the adjacent
structure. Therefore, the sensitivity of the interior pressure to wind speed is very large, and the
interior pressure becomes large and negative at high wind speeds. For the other cases, the
minimum interior pressures are similar to those found in the isolated case.
5.4.5 Further comments on interior pressures
Interior pressures expressed in coefficient form for the base case and 5 additional cases are
shown in Table 4 for scenarios with no internal/external temperature difference (i.e. interior
pressure is generated solely by wind). For open window cases, the interior pressure is
approximately the same as the external pressure at a location coinciding with the window (see
Table 3 in Section 4), since an open window acts as a ‘dominant opening’. For background
ventilation cases, the interior pressure is closer to the external pressure field averaged over the
whole dwelling, since openings are small and reasonably well distributed. For this reason, the
interior pressure does not vary greatly with orientation, and is considerably less negative than
the most negative pressure on the façade. The latter point is most clearly illustrated for cases 3,
4 and 5 where the open window has been chosen to coincide with the most negative pressure. In
these cases, interior pressures are considerably more negative when there is a dominant opening
than when the ventilation is well distributed.
The table also illustrates the magnitude of the positive and negative interior pressures
experienced for the background ventilation cases, relative to the external pressure fields
presented in Section 5.1. Considering the upwind case, Figure 18 shows that the maximum roof
pressure for an 8h x 2h obstruction at a distance 4h upwind of a dwelling is -0.17, which is more
negative than the interior pressure (-0.09). Thus, when background ventilation is provided, the
net pressure (roof pressure minus interior pressure) remains negative, which will tend to
enhance flue performance. Considering the downwind case, Figure 17 shows that the maximum
roof pressure for an 8h x 2h obstruction at a distance 4h downwind of a dwelling is 0.55, which
is more positive than the interior pressure (0.16). Therefore, the net pressure remains positive,
although the positive interior pressure helps to reduce the magnitude of the net pressure relative
to the roof pressure. Note that the net pressure for this case is similar to the isolated case (~0.4).
The greatest net pressure occurs for Case 3, where there is a large structure to one side of the
dwelling. In this case, the maximum roof pressure is 0.35, and the interior pressure is -0.22,
giving a net pressure of 0.57.
Table 4
Interior pressures expressed in coefficient form (Cp)
Case
Orientation S1
background
Base case (isolated dwelling)
Case 1 (downwind adjacent structure)
Case 2 (upwind adjacent structure)
Case 3 (side adjacent structure)
Case 4 (offset adjacent structure)
Case 5 (rotated adjacent structure)
-0.01
0.16
-0.07
-0.22
-0.01
-0.11
58
open
window
-0.21
0.05
-0.13
-0.62
-0.25
-0.23
Orientation S2
background
0.00
0.18
-0.09
-
open window
0.48
0.58
0.06
-
Side adjacent structure
0
Interior pressure (Pa)
-1
0.5
0
1
1.5
2
2.5
3
3.5
4
4.5
-2
-3
-4
-5
-6
-7
-8
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
Figure 49
Effect of wind speed on interior pressure
4h x 4h adjacent structure to one side – orientation S1 (kitchen downwind side of house)
Offset adjacent structure
0
Interior pressure (Pa)
-0.5
0
1
2
3
4
5
-1
-1.5
-2
-2.5
-3
-3.5
-4
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
Figure 50
Effect of wind speed on interior pressure
Adjacent structure at 2h upwind – orientation S1 (kitchen downwind side of house)
59
Rotated adjacent structure
0
Interior pressure (Pa)
-0.5
0
1
2
3
4
5
-1
-1.5
-2
-2.5
-3
-3.5
-4
Wind speed (m/s)
S1; background; doors open, T diff 15
S1; 90 deg window; doors open, T diff 15
Figure 51
Effect of wind speed on interior pressure
Adjacent structure upwind – orientation S1 (kitchen downwind side of house)
5.5 RIDGE VENT INVESTIGATION
5.5.1 Influence of wind direction on ridge vent pressures
Ridge vent pressure coefficients are affected by five principal factors – wind direction, the
location of the ridge vent on the roof, the pitch of the roof, the length of the building and the
nearby buildings and terrain. In this study, the first four factors have been studied.
Figures 52a and 52b show the effect of changing wind directions on the ridge vent pressure
coefficients, as measured on wind tunnel models, for a 45° pitch roof and a 30° pitch roof
respectively. Note that the ridge vent locations are at a distance of 3mm (model scale) from the
ridge, corresponding to a full scale distance of 0.24m. Thus for the 0° wind direction, the ridge
vents are on the upwind side of the ridge, whereas for the 180° wind direction, the ridge vents
are on the downwind side of the ridge.
There is a dramatic contrast in the behaviour between the 45° and 30° cases. In the 45° case
positive pressures are only seen when the wind is from near to 0° (0° or 30° in the cases
examined). By contrast, with a 30° pitch roof, positive pressures only occur in the case where
the ridge of the dwelling is parallel to the wind direction.
A direct comparison of the worst cases for each are given in Figures 52c and 52d. Figure 52c
illustrates the difference when the ridge is perpendicular to the wind: the 45° pitch roof dwelling
has positive pressures over a large fraction of the ridge, whereas the 30° pitch roof has negative
pressures along the whole ridge. In the case where the ridge is parallel to the wind direction
(Figure 52d), the pattern is similar for each of the two buildings (lower pressure at the windward
60
end and higher pressure at the leeward end) but the 30° pitch roof dwelling has slightly higher
pressure along the whole of the ridge. For the 45° case the pressure remains negative along the
entire ridge (although is close to zero at the leeward end), whereas for the 30° case, the pressure
becomes positive (although again is close to zero) at the leeward end of the ridge.
Ridge vent Cp for different wind directions on an isolated 45
degree pitched roof dwelling
0.2
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Cp
-0.2
-0.4
-0.6
0
30
60
90
120
150
180
-0.8
-1
Ridge vent position
(upwind to right in angled cases)
Figure 52a
Effect of changing wind direction on ridge vent pressure coefficients for a 45° pitch roof
dwelling
61
Ridge vent mean pressure coefficients for different wind
directions for a 3 house terrace with a 30 degree pitch roof
0.1
0
-0.1 1
3
5
7
9
11
13
15
17
0
30
60
90
120
150
180
-0.2
Cp
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
Ridge vent location
(upwind to right for angled cases)
Figure 52b
Effect of changing wind direction on ridge vent pressure coefficients for a 30° pitch roof
dwelling
Comparison of roof vent Cp for 30 and 45 pitch roof 3 house terrace
perpendicular to wind direction, ridge vents facing wind
0.15
0.1
0.05
Cp
0
-0.05
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18
45 pitch
30 pitch
-0.1
-0.15
-0.2
-0.25
Ridge vent position (left ridge end to right)
Figure 52c
Ridge vent Cp for 30° and 45° pitch roof dwellings with ridge perpendicular to the wind
direction
62
Comparison of roof vent Cp for 30 and 45 pitch roof 3 house
terrace, with wind from 90 degrees
0.1
0
-0.1 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18
-0.2
Cp
-0.3
45 pitch
30 pitch
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
Roof vent position
(from downwind end to upwind end)
Figure 52d
Ridge vent Cp for 30° and 45° pitch roof dwellings with ridge parallel to the wind
direction
5.5.2 Intermittency in ridge pressures
The intermittency in ridge pressures has been analysed in order to determine the proportion of
time that the pressure occurs for a continuous period greater than 10 seconds at a nominal wind
speed of 3m/s. As an illustration of the intermittency characteristics of the pressure, the
variation in pressure with sample number has been plotted in Figure 53. Individual
measurement points have been chosen on the 30° and 45° pitch roofs. The graph shows that
large fluctuations in pressure about the mean can occur. These may be sustained over a range of
timescales. The variation in pressure over time is associated with fluctuations in the flow
patterns caused both by atmospheric turbulence and the unsteady flows generated by the
dwelling itself.
Figures 54a and 54b show the variation with location on the ridge and wind direction of the
percentage of time that sustained positive pressure ‘events’ occur for the two roof pitches.
Generally, when the mean pressure is large and negative (e.g. for the downwind end of the
terrace), sustained positive pressure events never occur (the percentage of time is zero). When
the mean pressure is small and negative (e.g. for the 90° wind direction for both roof pitches),
sustained positive pressure events can occur for a small proportion of the time (~10%).
63
Isolated dwelling ridge vent pressure
50
40
45 pitch max tap
Pressure (Pa)
30
30 pitch max tap
20
10
0
-10
1 11 21 31 41 51 61 71 81 91 101 111 121 131141 151 161 171181 191
-20
-30
-40
Sam ple num ber
Figure 53
Variation in pressure with sample number
F r e q u e n c y o f s u s ta in e d p o s itiv e p r e s s u r e o n 4 5 d e g r e e r o o f
fo r d iffe r e n t w in d d ir e c tio n s
more than 9 seconds real time
Percentage of time Cp positive for
60
50
0
30
60
90
120
150
180
40
30
20
10
0
1
2
3
4
5
6
7
8
9
-1 0
R id g e n u m b e r (e n d o f r id g e to m id d le )
Figure 54a
Percentage of time that sustained positive pressure events occur: 45° pitch roof
64
F re q u e n c y o f s u s ta in e d p o s itiv e p re s s u re e v e n ts w ith 3 0
d e g re e p itc h ro o f
12
10
9 seconds real time
Percentage of time Cp positive for more than
14
0
8
30
60
6
90
120
4
180
2
0
1
2
3
4
5
6
7
8
9
-2
R id g e v e n t p o s itio n (e n d o f rid g e to m id d le )
Figure 54b
Percentage of time that sustained positive pressure events occur: 30° pitch roof
5.5.3 Effect of terrace length on ridge pressure
The variation in pressure has been investigated for ridge vents on 45° pitch roof dwellings in a
terrace of nine. The results are shown in Figure 55 for the 'worst case' 0° wind direction (wind
perpendicular to the ridge), and compared with the three-house terrace case. For these cases, the
profiles along one half of the complete terrace are shown. Comparing the two profiles, for both
terrace lengths, the maximum positive pressure occurs towards the midpoint of the terrace.
However, in the case of the 9-house terrace, the pressure becomes positive between about half
and two thirds of the way from the centre to the end.
Figure 56 shows the pressure coefficients at ridge vents on a terrace of 9 houses with a 45°
pitch roof, for the cases where the ridge is parallel to the wind direction. Ridge vent pressures
become positive for all houses after the upwind two houses on the terrace, and are greater in
magnitude than in the case of the 3 house terrace by Cp ~ 0.1.
65
Variation in ridge vent mean Cp
3-house and 9-house terraces
0.15
0.1
0.05
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
Cp
-0.05
-0.1
-0.15
3-house terrace
-0.2
9-house terrace
-0.25
-0.3
Tap position (from centre to end of terrace)
Figure 55
Variation in pressure along the ridge for winds perpendicular to the ridge
Three- and nine-dwelling terraces
(Upwind half of the terraces only)
Mean ridge vent Cp for a 45 pitch roof terrace of 9 houses
parallel to the wind direction
0.2
0.1
0
-0.1 1
4
7
10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Cp
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
Location of ridge vent
(location 1: upwind end of terrace)
Figure 56
Variation in pressure along the ridge for a terrace of 9 houses with a 45° pitch roof
parallel to the wind direction
66
6. DISCUSSION
6.1 IMPACT OF ADJACENT STRUCTURES AND TOPOGRAPHICAL FEATURES
The wind tunnel results have shown how adjacent structures influence the pressure over the roof
of a dwelling, and in particular how an adjacent structure downwind of the dwelling produces
larger positive pressures over the roof of the dwelling than for the isolated case. Any increased
likelihood of flue flow reversal due to the presence of the adjacent structure, relative to the
isolated case, depends on a number of factors including:
· The increased magnitude of positive pressures on the roof at locations coinciding with
the flue terminal, compared to the isolated case;
· The impact of the wind pressure field on the interior pressure, which is a function of the
ventilation strategy (i.e. the size and distribution of ventilation openings);
· The relative timescales over which flue pressures and interior pressures respond
following any changes in wind speed or direction.
The likelihood of flue flow reversal also depends on the location of the flue on the roof,
although the following discussion assumes that the flue could be located at any point on the roof
including the point at which the maximum positive pressure occurs.
Considering all the experimental results obtained for the 45° pitch roof dwelling, the largest
maximum positive pressure coefficient has been found to occur when the dwelling is located
next to a 2h x 16h obstruction. In this case, the maximum surface pressure coefficient is 0.64,
which is approximately 50% greater than the isolated dwelling value of 0.43. Comparing
vertical tube results, the maximum pressure coefficient measured by the 31.5mm tubes with the
2h x 16h obstruction present is 0.38, which is approximately 60% greater than the isolated case
value of 0.24. Thus in the case of the 45° pitch roof, the pressure at the flue terminal could be up
to 60% greater when a large obstruction is present than when the building is isolated.
Note that the difference in roof pressures between the isolated and obstructed cases is larger for
the dwelling with the 30° pitch roof than the 45° pitch roof. For example, in the case of an
obstruction with dimensions 2h x 4h placed at a distance of 2h downwind of the dwelling, the
maximum positive surface pressure coefficient for the 30° pitch roof is 0.28. This is 75% greater
than the maximum pressure coefficient for the isolated case (Cp=0.16). In contrast, for the 45°
pitch roof adjacent to the same obstruction, the obstructed and isolated case values are 0.46 and
0.43, respectively, which is a difference of 7%. In addition, for the isolated building with a 30°
pitch roof, the pressure along the eaves is negative, whereas with the adjacent structure present,
the pressure becomes positive. In contrast, the pressure along the eaves for the 45° pitch roof
dwelling is positive both with and without the adjacent structure present.
The highest positive pressures will tend to occur when the adjacent structure is very close to the
dwelling (in the cases discussed above, the obstruction was at a distance of 2h from the
dwelling). However, an increase in positive roof pressures relative to the isolated dwelling will
tend to occur over a separation distance which is related both to the width of the adjacent
structure and its height. For example, for adjacent structures which are relatively narrow in
comparison to the dwelling dimensions (W = 2h), an increase in positive roof pressure will
occur up to a distance of ~1.5h, where h is the height of the adjacent structure. However, for
adjacent structures which are relatively wide in comparison to the dwelling dimensions (W ≥
4h), an increase in positive roof pressure will tend to occur for a distance of at least 2h.
67
Sensitivity analyses carried out for the 45° pitch roof dwelling indicate that for the cases
investigated, the maximum positive pressure on the roof of the dwelling is not significantly
increased by the adjacent structure geometry, the offset or rotation of the adjacent structure
relative to the dwelling, the location of the dwelling in a terrace or adjacent to neighbours, the
type of terrain or the topography. However, given the results for the 30° pitch roof, it is possible
that roof pressures for this dwelling are more sensitive to these factors.
The change in roof pressures due to the presence of an adjacent structure needs to be considered
alongside the impact that the corresponding change in dwelling façade pressures has on the
interior pressure. Investigation of the maximum Cp difference (i.e. the maximum roof pressure
minus the minimum façade pressure) provides an indication of a possible ‘worst-case’ scenario.
In this scenario, a dominant opening such as a window is located in the room in which the boiler
is housed, at a position which coincides with the minimum external façade pressure. This
analysis has shown that, although there is still a net positive pressure between the roof and
façade which may be larger or smaller than the roof pressure, this net pressure is smaller than
for the isolated dwelling case. This is because the adjacent structure tends to smooth out the
variation in pressure over the dwelling, resulting in a smaller difference between roof and
façade pressures. Thus the risk of flue flow reversal is greater for the isolated dwelling than the
dwelling with a downwind obstruction, when a dominant opening is located where the
minimum façade pressure occurs. This is also true for an obstruction located upwind of the
dwelling, although net pressures are considerably less positive than in the case of a downwind
obstruction.
Considering scenarios where background ventilation only is provided, and there is no dominant
opening, BREEZE modelling has shown that for an isolated dwelling, the interior pressure
coefficient is close to zero, and therefore the net pressure is approximately equal to the roof
surface pressure (net Cp is 0.44). When a downwind obstruction is present, the interior pressure
is positive, and therefore the net pressure is smaller in magnitude than the roof pressure. For
example, for an 8h x 2h downwind obstruction at 4h separation, the maximum roof surface
pressure coefficient is 0.55, and the interior pressure coefficient is 0.16 (c.f. Figure 17). Thus
the net Cp is 0.39, which is 11% lower than the isolated case. For an 8h x 2h upwind
obstruction, the maximum roof surface pressure coefficient is -0.17, and the interior pressure
coefficient is -0.09. Thus the net Cp is -0.08, which is 118% lower than the isolated case (c.f.
Figure 18). The highest positive net pressure has been found to occur when an extremely large
obstruction (4h x 4h) is placed to one side of the dwelling, in which case the net pressure is 0.57
(30% higher than the isolated case). This is the result of a relatively high negative interior
pressure (Cp = -0.22) induced by the acceleration of flow around the side of the adjacent
structure.
For cases where the adjacent structure is offset or rotated relative to the dwelling, net pressures
are 0.56 and 0.54 respectively (maximum roof pressure coefficients are 0.55 and 0.43
respectively, and interior pressures are negative). These values are 27% and 23% greater than
the isolated case, respectively. Net pressure coefficients are therefore also greater than the
isolated case when the adjacent structure is placed asymmetrically relative to the dwelling. In
this case, the dwelling is only partially enclosed by the positive pressure field created by the
adjacent structure, and large positive roof pressures can occur concurrently with relatively large
negative façade pressures.
For cases where the obstruction is downwind of the dwelling and arranged symmetrically, the
dwelling is more fully enclosed by the obstruction’s positive pressure field. In a steady-state
situation the apparent increase in the risk of flue flow reversal due to increased positive
pressures on the roof is moderated by the corresponding increase in interior pressures, as a result
68
of increased positive pressures acting on the façades. In fact, for the 8h x 2h adjacent structure,
the net positive pressure has been found to be smaller than for the isolated case. However, for a
reasonably airtight building with ventilation opening areas which are small in comparison to the
building dimensions, the timescale over which the interior pressure changes as a result of a
change in wind speed and/or direction is likely to be longer than the timescale for changes to
flue pressure. Thus it is possible that a change in wind speed and/or direction could cause an
immediate increase in positive pressure at the flue terminal, but that the influence of positive
pressure on the façades might only be experienced by the dwelling interior after a certain period
of time. The implications for flue flow reversal risk depend on the relative lengths of following
timescales:
· The timescale for the interior pressure to respond to changes in the wind pressure field
caused by a change in wind speed and/or direction;
· The timescale required for adverse wind pressures acting at the flue terminal to cause
sustained flue flow reversal.
Lawson (1980) gives an order of magnitude estimate of the time taken for a volume within a
building reach pressure equilibrium as:
-4 Vo
Dp
t i = 1.21 x 10
AD
where ti is the time taken for pressure equilibrium to be reached, Vo is the volume of the space,
AD is the volume of the openings and Dp is the average pressure across the openings. Taking a
mean wind speed of 5ms-1 and an average pressure coefficient difference of 0.6 gives Dp =
15Pa. If the volume of a room Vo = 100m3 and the trickle vent area AD = 0.004m3, the timescale
to reach equilibrium is approximately 12 seconds. Thus the timescale for the interior pressure to
respond to changes in the wind pressure field is approximately the same as that required for
adverse wind pressures acting at the flue terminal to cause sustained flue flow reversal (10-15
seconds). Therefore, the response of the interior pressure following changes in wind speed
and/or direction is likely to be sufficiently fast so that dynamic effects are not an important
consideration in the present analysis.
6.2 RIDGE VENTS
Pressures have been measured on either side of the ridge, at a small distance from the ridge
(0.24m at full scale). These measurements are representative of the pressures which would be
experienced on either side of a ridge vent terminal. The results indicate that for both the 45° and
30° roof pitches, the pressures are negative for most wind directions. Furthermore, for the 45°
case, when positive pressures occur on the upwind side of the ridge (0° and 30° wind
directions), much greater negative pressures occur on the downwind side, and thus the net wind
effect will tend to enhance ventilation from the terminal. For both roof pitches, the most
problematic wind direction occurs when the wind is along the ridge. In this case, although the
magnitude of the pressure is found to be small, sustained positive pressure 'events' can occur for
a significant proportion of the time. For example, in the case of the 45° roof, although time­
averaged pressures were found to be no greater than zero, positive pressure events greater than 9
seconds in length occurred for around 10% of the time, when the wind speed was moderate
(~3m/s). For this wind direction, slightly higher positive mean pressures are found to occur for
the 9 house terrace than the 3 house terrace, and thus the susceptibility of the terminal to adverse
wind effects is found to increase with increasing terrace length.
69
70
7. CONCLUSIONS
7.1 IMPACT OF ADJACENT BUILDINGS AND TOPOGRAPHICAL FEATURES ·
In the case of a ‘standard’ domestic dwelling with a 45° pitch roof, the maximum positive
pressure on the dwelling roof occurs when an adjacent structure is directly downwind of the
dwelling, and the dwelling is immersed in the positive pressure field produced by the
downwind structure. The magnitude of the pressure depends on the height and width of the
adjacent structure, and its distance from the dwelling. For the cases investigated, the
maximum increase in positive pressure is found to occur when the dwelling (eaves height h)
is located close to an embankment (height 2h). In this case, the maximum pressure
experienced by a flue terminal located above the roof may exceed that for the isolated
dwelling by up to 60%.
·
The reduction in maximum positive pressure with increasing height above the roof, as
measured by the vertical tubes, is similar for the isolated and obstructed cases. Typically,
the maximum pressure coefficient reduces by 0.1 at a distance of 12.5mm above the roof
and by 0.2 at a distance of 31.5mm above the roof, relative to the maximum surface
pressure coefficient. Since maximum positive surface pressures have been measured over
the range ~0.40 to ~0.65, pressure at height of up to 2.5m above the roof may remain
positive, and thus increases in flue height will not significantly reduce the risk of flue flow
reversal.
·
The zone of influence of an adjacent structure can be determined approximately from results
obtained here for the 45° pitch roof, and depends on the height and width of the adjacent
structure. For adjacent structures which are relatively narrow in comparison to the dwelling
dimensions (W = 2h), a significant increase in positive roof pressure will occur up to a
distance of ~1.5h, where h is the height of the adjacent structure. However, for adjacent
structures which are relatively wide in comparison to the dwelling dimensions (W ≥ 4h), an
increase in positive roof pressure will tend to occur for a distance of at least 2h. In all cases,
the maximum pressure tended to decrease with increasing separation distance.
·
The impact of an adjacent structure on pressures over the roof of a 30° pitch roof dwelling
is greater than for a 45° pitch roof. For the scenario investigated, the same adjacent structure
increased maximum roof surface pressures relative to the isolated dwelling cases by 75%
and 7% for the 30° pitch and 45° pitch respectively. In addition, pressures along the eaves
which were negative for an isolated 30° pitch roof dwelling became positive with the
adjacent structure present. However, maximum roof pressures were still found to be
considerably lower for the 30° pitch roof than the 45° pitch roof.
·
For the range of additional factors investigated, the maximum roof pressure has not been
found to increase significantly with the introduction of these factors. Sensitivity to the
following factors has been investigated: adjacent structure geometry; offset or rotation of
the adjacent structure relative to the dwelling; location of the dwelling within a terrace;
location of a dwelling adjacent to similar neighbours; geometry of topographical feature
(hill or embankment); terrain type.
·
For a worst case scenario where the interior pressure is dominated by the minimum external
façade pressure due to the existence of an open window or ‘dominant opening’ where the
minimum pressure occurs, the maximum difference between the roof pressure and the
interior pressure is greatest for the isolated dwelling. This is because an adjacent structure
71
tends to smooth out the variation in pressure over the dwelling, resulting in a smaller
difference between maximum roof and minimum façade pressures.
· When the dwelling is reasonably airtight and background ventilation only is provided, the
interior pressure tends to be positive when the adjacent structure is downwind of the
dwelling and the dwelling is fully enclosed by the positive pressure field. In the steady-state
case, the net positive pressure (difference between the maximum roof pressure and the
interior pressure) is smaller with the obstruction present than in the isolated case, for which
the interior pressure is close to zero.
· With background ventilation, and when the adjacent structure is to one side of the dwelling,
or is rotated or offset relative to the dwelling, the dwelling is only partially enclosed by the
positive pressure field created by the adjacent structure. In these cases, large positive roof
pressures can occur concurrently with relatively large negative façade pressures and net
positive pressures tend to be higher than the isolated case.
· For the moderate wind speeds at which Upton et al. (1999a) identified high flue flow
reversal risk, the influence of stack pressure on the interior pressure has been found to be
significant when internal/external temperature differences are large. In this case, negative
interior pressures occur when the external temperature is lower than the internal
temperature.
· The greatest increase in the risk of flue flow reversal with an adjacent structure present
relative to the isolated case has been found to occur when the ventilation strategy is
background ventilation, with no dominant opening provided by a window. The former
ventilation strategy is more likely to be used when internal/external temperature differences
are large (i.e. in winter), when the risk is further increased by the influence of the stack
effect on interior pressure.
7.2 RIDGE VENTS
· For both the 45° and 30° roof pitches, ridge vent pressures are found to be negative for most
wind directions. Furthermore, for the 45° case, when positive pressures occur on the upwind
side of the ridge (0° and 30° wind directions only), much greater negative pressures occur
on the downwind side, and thus the net wind effect will tend to enhance ventilation from the
terminal.
· For both roof pitches, the most problematic wind direction occurs when the wind is along
the ridge. In this case, although the magnitude of the pressure is found to be small, sustained
positive pressure 'events' can occur for a significant proportion of the time. For example, in
the case of the 45° roof, although time-averaged pressures were found to be no greater than
zero, positive pressure events greater than 9 seconds in length occurred for around 10% of
the time, when the wind speed was moderate (~3m/s).
· When the wind is parallel to the ridge, slightly higher positive mean pressures are found to
occur for the 9 house terrace than the 3 house terrace, and thus the susceptibility of the
terminal to adverse wind effects is found to increase with increasing terrace length.
72
8. RECOMMENDATIONS
8.1 IMPACT OF ADJACENT BUILDINGS AND TOPOGRAPHICAL FEATURES
· The risk of flue flow reversal has previously been found to be of concern for isolated
dwellings with roof pitches greater than 45° (Upton et al. 1999(b)). The present work has
found that the risk could increase further with an adjacent structure present, due to the
combination of higher wind pressures at the flue combined with larger negative pressures
experienced on building façades affecting interior pressures. Again, the risk has not been
found to reduce significantly by increasing the height of flues above the roof. Therefore,
similar recommendations apply as previously, namely that fanned draught systems should
be considered for dwellings with roof pitches greater than 45°, both with and without an
adjacent structure present.
· For dwellings with less steep roof pitches (30° or lower), the risk of flue flow reversal for
the isolated dwelling was previously found to be less of a concern. However, since the
increase in the risk of flue flow reversal in the presence of an adjacent structure has been
found to be more significant for the 30° pitch roof dwelling than the 45° pitch roof
dwelling, it is recommended that alternatives to natural draught systems should be
considered for the 30° roof pitch when an adjacent structure is present and at a sufficiently
close distance to the dwelling.
· In the present work, mainly the 45° pitch roof case was addressed. While results from the
45° pitch roof are useful for guidance on 30° pitch roofs and flat roofs (for example, the
zone of influence of an adjacent structure), additional wind tunnel experiments for the 30°
pitch roof and flat roof cases could be considered, in order to provide more information on
the impact of an adjacent structure on wind pressures over these roofs. In particular, the
effect of size and separation distance of the adjacent structure on negative pressure in the
eaves regions of the roofs could be investigated further.
8.2 RIDGE VENTS
· Comparing the results of the present investigation with results obtained previously for
isolated buildings with similar roof pitches, the risk of positive pressure events occurring at
ridge vents on 45° pitch roofs is significantly smaller than the risk of these events occurring
at flue terminals located near the eaves. In the case of 30° pitch roofs, the risk is slightly
reduced for ridge vents relative to flue terminals located near the eaves. Therefore, it is
suggested that the use of ridge vents is preferable to flue terminals located close to the eaves
for 45° pitch roofs.
73
74
9. ACKNOWLEDGEMENTS This work was funded by the Health and Safety Executive under Contract RSU Ref:
4355/R41.124.
Thanks are due to Stuart Upton and Paul Blackmore for their help with this work.
75
76
10. REFERENCES
1. BS 5440-1 (2001). Installation and maintenance of flues and ventilation for gas
appliances of rated input not exceeding 70kW (1st , 2nd and 3rd family gases) - Part 1 :
Specification for installation and maintenance of flues. BSI, 389 Chiswick High Rd,
London, W4 4AL. ISBN 0 580 33229 2.
2. CIBSE Guide A (1999) Environmental design, Chartered Institution of Building
Services Engineers, 1999.
3. CIBSE Guide C (1986) Reference data, Chartered Institution of Building Services
Engineers, 1986.
4. Cook, N.J. (1990) The designer’s guide to wind loading of building structures. Part 2
Static Structures. Building Research Establishment Report, Butterworths.
5. De Gids, W.F. & den Ouden, H.P.L. (1974) Three investigations of the behaviour of
ducts for natural ventilation, in which an examination is made of the influence of
location and ehight of the outlet, of the built-up nature of the surroundings and of the
form of the outlet. IG-TNO, Delft, Report number 529, November 1974. TNO Bouw,
POB 49, 2600 AA Delft, The Netherlands.
6. Department of the Environment and the Welsh Office, The Building Regulations 1991,
Approved Document F, 1995 Edition.
7. ESDU Data Sheet 82026 Amendments A-D (1993) Strong winds in the atmospheric
boundary layer. Part 1: Mean hourly wind speeds, April 1993.
8. Iles, P.J. (1998) Standard dwellings for energy modelling, BRE Client Report CR44/98,
BRE, Garston, January 1998.
9. Lilly, J.P. & Williams, T.P. (1996) The Design, Installation and Performance of Open
Flue Terminals. Paper No: T317. Proceedings of the Institute of Gas Engineers
Conference, Harrogate, May 1996.
10. Plathner, P.S. & Stephen, R.K. (2001) The effect of ventilation strategies on flue
performance. Part 2: BREEZE modelling of internal house pressures. BRE, Garston,
Watford, Herts, UK, WD25 9XX. Report number 205016.
11. Upton, S.L. Blackmore, P.A. & Tily, P.J. (1999(a)). Flue Performance of Domestic Gas
Burning Appliances. Aim 1. Full-scale Test House Measurements of the Flue
Performance of Open-Flued Gas Fired Appliances. BRE, Garston, Watford, Herts, UK,
WD25 9XX. Report Number CR 271/99.
12. Upton, S.L., Blackmore, P.A. & Tily, P.J. (1999(b)) Flue Performance of Domestic Gas
Burning Appliances. Aim 2. Wind tunnel tests on various model building configurations
for safe flue installation. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report
number CR109/99.
77
13. Upton, S.L., Blackmore, P.A. and Tily, P.J. (2000). Additional full-scale test house
measurements of the flue performance of open-flued gas fired appliances using internal
flues. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report number 200175.
14. Upton, S.L. & Tily, P.J. (2001) The effect of ventilation strategies on flue performance.
Part 1. Full-scale test house measurements. BRE, Garston, Watford, Herts, UK, WD25
9XX. Report number 205015.
15. Lawson, T.V., Wind Effects on Buildings, Volume 1: Design Applications, Applied
Science Publishers, 1980.
78
11. Appendix A
2h x 2h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
h
0.2
Cp
2h
3h
0
1
2
3
4
5
6
7
8
9
10
11
12
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A1
Effect of the distance of a 2h x 2h downwind
obstruction on the roof centre line Cp
4h x 2h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
2h
Cp
0.2
4h
6h
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
12
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A2
Effect of the distance of a 4h x 2h downwind
obstruction on the roof centre line Cp
79
isolated
Roof centre line Cp
8h x 2h obstruction at different distances downwind
0.8
Cp
0.6
0.4
4h
0.2
8h
12h
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
12
Isolated
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A3
Effect of the distance of an 8h x 2h downwind
obstruction on the roof centre line Cp
2h x 2h obstruction downwind
roof ce ntre line Cp
0.8
0.6
0.4
h
Cp
0.2
2h
3h
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
12
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A4
Effect of the distance of a 2h x 2h downwind
obstruction on the roof centre line Cp
80
isolated
2h x 4h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
2h
0.2
Cp
4h
0
isolated
1
2
3
4
5
6
7
8
10
9
11
12
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A5
Effect of the distance of a 2h x 4h downwind
obstruction on the roof centre line Cp
2h x 8h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
2h
Cp
0.2
4h
0
isolated
1
2
3
4
5
6
7
8
9
10
11 12
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A6
Effect of the distance of a 2h x 8h downwind
obstruction on the roof centre line Cp
81
2h x 16h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
2h
0.2
Cp
4h
isolated
0
1
2
3
4
5
6
7
8
9
10 11 12
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A7
Effect of the distance of a 2h x 16h downwind
obstruction on the roof centre line Cp
2h high obstructions of different widths, 2h downwind
roof centre line Cp
0.8
0.6
0.4
isolated
2h
Cp
0.2
4h
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
-0.4
-0.6
-0.8
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A8
Effect of the width of an obstruction of
height 2h on the roof centre line Cp
82
12
8h
16h
4h x 2h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
2h
Cp
0.2
4h
6h
0
1
2
3
4
5
6
7
8
9
10
11
12
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure A9
Effect of the distance of a 4h x 2h obstruction
on the roof centre line Cp
4h x 4h obstruction downwind
roof centre line Cp
0.8
0.6
0.4
isolated
8h
0.2
Cp
6h
0
4h
1
2
3
4
5
6
7
8
9
10
11
12
-0.2
-0.4
-0.6
Position on roof centre line
(downwind eaves to upwind eaves)
Figure A10
Effect of the distance of a 4h x 4h downwind
obstruction on the roof centre line Cp
83
2h
2h high obstructions of different widths at 2h upwind
roof centre line Cp
0.8
0.6
0.4
isolated
Cp
0.2
2h
4h
0
-0.2
1
2
3
4
5
6
7
8
9
10
11
12
8h
16h
-0.4
-0.6
-0.8
Position on roof centre line (downwind eaves to
upwind eaves)
Figure A11
Effect of the width of a 2h high upwind
obstruction on the roof centre line Cp
2h x 2h obstruction upwind
roof centre line mean Cp
0.6
0.4
Cp
0.2
h
2h
0
1
2
3
4
5
6
7
8
9
10
11
12
-0.2
-0.4
-0.6
Position on roof centre line (downwind eaves
to upwind eaves)
Figure A12 Effect of the separation distance of a 2h x 2h upwind obstruction on the roof centre line Cp 84
3h
isolated
4h x 2h obstruction upwind
roof centre line Cp
0.6
0.4
0.2
2h
Cp
0
-0.2
1
2
3
4
5
6
7
8
9
10 11 12
6h
isolated
-0.4
-0.6
-0.8
Position on roof centre line
(downwind eaves to upwind eaves)
Figure A13
Effect of the distance of a 4h x 2h upwind
obstruction on the roof centre line Cp
4h x 4h obstruction upwind
roof ce ntre line Cp
0.6
0.4
0.2
2h
0
Cp
1
2
3
4
5
6
7
8
9
10
11
12
-0.2
4h
6h
8h
-0.4
isolated
-0.6
-0.8
-1
Position on roof centre line (downwind
eaves to upwind eaves)
Figure A14
Effect of the distance of a 4h x 4h obstruction
upwind on the roof centre line Cp
85
86
12. Appendix B
2h x 2h obstruction at h downwind
roof centre line Cp
0.6
0.4
31.5 tubes
0.2
Cp
12.5 tubes
0
0
Surface taps
150
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(downwind eaves to upwind eaves)
Figure B1
Roof centre line Cp for a 45 pitch roof dwelling
with a 2h x 2h building at h downwind
4h x 2h obstruction at 2h downwind
roof centre line Cp
0.6
0.4
31.5 tubes
0.2
12.5 tubes
Surface taps
Cp
0
0
150
-0.2
-0.4
-0.6
Position on roof centre line (downwind eaves to upwind eaves)
Figure B2
Roof centre line Cp for a 45 pitch roof dwelling
with a 4h x 2h building at 2h downwind
87
isolated
8h x 2h obstruction at 4h downwind
roof centre line Cp
0.8
0.6
0.4
31.5 tubes
0.2
Cp
12.5 tubes
surface
0
0
1
5
0
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(downwind eaves to upwind eaves)
Figure B3
Roof centre line Cp for a 45 pitch roof dwelling
with a 8h x 2h building at 4h downwind
2h x 4h obstruction at 2h downwind
roof centre line Cp
0.6
0.4
0.2
31.5 tubes
0
0
1
Cp
5
0
12.5 tubes
surface
-0.2
isolated
-0.4
-0.6
-0.8
Position on roof centre line
(downwind eaves to upwind eaves)
Figure B4
Roof centre line Cp for a 45 pitch roof dwelling
with a 2h x 4h building at 2h downwind
88
2h x 16h at 2h dow nw ind
roof centre line Cp
0.8
0.6
0.4
31.5 tubes
0.2
Cp
12.5 tubes
0
0
1
5
surface
0
-0.2
isolated
-0.4
-0.6
-0.8
Position on roof centre line
(dow nw ind eaves to upw ind eaves)
Figure B5
Roof centre line Cp for a 45 pitch roof dwelling with a 2h x 16h
building at 2h downwind
2h x 2h obstruction at h upwind
roof centre line Cp
0.6
0.4
31.5 tubes
0.2
Cp
12.5 tubes
0
surface
0
150
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(downwind eaves to upwind eaves)
Figure B6
Roof centre line Cp for a 45 pitch roof dwelling
with a 2h x 2h building at 2h upwind
89
4h x 2h obstruction at 2h upwind
roof centre line Cp
0.6
0.4
0.2
31.5 tubes
Cp
0
0
1
5
0
12.5 tubes
Sur f ace taps
-0.2
isolated
-0.4
-0.6
-0.8
Position on roof centre line
(upwind eaves to downwind eaves)
Figure B7
Roof centre line Cp for a 45 pitch roof dwelling with a
4h x 2h building at 2h upwind
8h x 2h obstruction at 4h upwind
roof centre line Cp
0.6
0.4
Cp
0.2
31.5 tubes
12.5 tubes
0
0
150
surface
isolated
-0.2
-0.4
-0.6
Position on roof centre line
(upwind eaves to downwind eaves)
Figure B8
Roof centre line Cp for a 45 pitch roof dwelling
with a 8h x 2h building at 4h upwind
90
2h x 4h obstruction at 2h upwind
roof centre line Cp
0.6
0.4
0.2
31.5 tubes
12.5 tubes
0
0
150
Cp
surface
isolated
-0.2
-0.4
-0.6
-0.8
Position on roof centre line
(upwind eaves to downwind eaves)
Figure B9
Roof centre line Cp for a 45 pitch roof dwelling
with a 2h x 4h building at 2h upwind
2h x 16h at 2h upw ind
roof centre line Cp
0.6
0.4
31.5 tubes
0.2
12.5 tubes
0
0
1
5
0
Cp
surface
isolated
-0.2
-0.4
-0.6
-0.8
Position on roof centre line
(upwind eaves to downwind eaves)
Figure B10
Roof centre line Cp for a 45 pitch roof dwelling
with a 2h x 16h building at 2h upwind
91
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