- Email: [email protected]
Effects of roof shapes on wind-induced air motion inside buildings
Building and Enrironmenf. Vol. 32, No I, pp. I-I I, 1997 Copyright V> 1996 Elsevm Scmm Ltd. All nghts reserved Printed m Great Britain 036&1323/97 $17.00+0.00
Pergamon PII: SO360-1323(96)00021-2
Effects of Roof Shapes on Wind-Induced Air Motion Inside Buildings J. KINDANGEN* G. KRAUSS* P. DEPECKER*
f
(Accepted
7 February
1996)
The eflects of roof shape on wind-induced air motion inside buildings were analysed by using a numerical simulation: computationalfluid dynamics (CFD). The average indoor velocity coejjkient, a non-dimensional indoor air motion parameter, was used to calculate the relative strength of the interior air movement in the horizontal plane representative of the occupied space of the room. The distribution of average velocity between inlet and outlet, which enabled observation of the interior air motion behaviour, was analysed. The coefficient of spatial variation was used to investigate the uniformity of airflow inside the building. It was observed that the shape qf the roof directlyafjkcted the airjow pattern, especially the velocity magnitude. The importance of wind direction, building overhang and roof height was also investigated. Copyright 0 1996 Elsevier Science Ltd.
APPROACH
INTRODUCTION IN a number of regions with a warm climate, and particularly where it is hot and humid, natural ventilation proves to be a realistic alternative as an energy-conserving design strategy aimed at reducing the cooling loads of buildings and improving indoor thermal comfort. In regions with only two seasons (dry and rainy) the shapes of roofs and eaves should be considered as one of the main factors protecting the living areas from rain and glare from the sky. Roof shapes everywhere, in various climates and cultures, also have aesthetic and semiotic values. A certain type of roof is often a typical characteristic of a nature, a climate, a culture, local materials, construction techniques, etc. A number of researchers have studied the effect of some design parameters on interior airflow to improve the level of thermal comfort and increase energy savings, i.e. position and orientation of building, window sizes, interior partitions and effects of eaves; the methods they used were wind tunnel or in situ experimentation [l-5]. However, we have attempted to formally investigate the effects of roof shape on indoor airflow pattern by making use of computational fluid dynamics (CFD) codes. This paper describes the experiments designed to study the effect of roof shape on the average velocity coefficient, C,, generally considered as a non-dimensional indoor air motion parameter. In addition, the maximum C, and the coefficient of spatial variation, C,,, were computed to investigate the homogeneity of indoor airflow.
Some of the tests on houses were carried out by making use of CFD. Indoor air speed distributions were modelled for a basic model with fixed window openings, for five wind directions (0,30,45,60 and 90”) and 10 roof shapes. The roof shapes tested were selected to cover a wide range of possible roof shapes in tropical architecture. The model with a flat roof and without a roof overhang was used as a basic model for measurements of average velocity coefficients and enabled comparison with the other models. To observe the flow pattern inside buildings, we used the local average velocity coefficient as a function of the position in a horizontal section of 1.35m height. The average velocity coefficient was calculated according to number of grid in the building core. For buildings with small openings, such as cracks, it is reasonable to assume that, as the wind reaches the building, its kinetic energy is entirely converted into static pressure. On the contrary, buildings that are completely open, with wall porosity approaching loo%, only produce minor effects on the incoming winds, and do not create significant static pressure zones. In this case, indoor velocities are essentially a result of momentum (kinetic energy of wind). For window sizes in between these two extremes, the building slows down the incoming wind and creates an upwind positive pressure zone without, however, stopping it completely. As a result, the interior airflows are produced by a combination of mean static pressure and momentum. A momentum force is more effective at inducing indoor airflows than a static pressure of equal value as openings convert static pressure back into velocities. It is reasonable to consider that all design variables are interdependent and have an influence on interior airflow, but it is also useful to observe the influence of a design variable on its own to take its effects into consideration.
*Centre de Thermique de I’INSA de Lyon/Equipe Equipement de I’Habitat, Bltiment 307, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex ,France. tFaculty of Engineering, Architectural Department, University of Sam Ratulangi, Jl. Kampus Unsrat Bahu, Manado 95115, Indonesia. 1
2
J. Kindanyen
Fig.
et al.
I. Schematic view of simulation
with CFD
Thus, in this study, in order to analyse the effects of roof shape for different wind incidences, it was necessary to assume that all other parameters were constant.
EXPERIMENTAL
METHODS
Simulation with CFD In this study, we use FLUENT [9] software, and the standard k-c turbulence model is implemented to account for turbulent flow. The how and boundary conditions are isothermal. The reference wind velocity is 5.843 m/s at a reference height of 4.25 m with a reference turbulent intensity of 22%. The simulations were carried out with very rough mesh cells. According to Borth and Suter’s study [6], and as a result of the conclusions of our previous study [7], rough mesh cells are suitable for simulation with CFD. A coarse grid is sufficient to provide a good qualitative prediction of the fluid flow. Figure 1 shows a schematic view of the simulation and the cell dimension set; the building model was placed at the centre of the floor of the cube (3 1.20 x 3 1.20 x 12.00 m). This cube was subdivided into x-, y- and =-directions with grid sizes as shown in Table 1. The cell dimension distribution in the building core and its environment stayed constant for each test except for roof areas. The CFD results of average velocity coefficients (C,), local average velocity coefficients and coefficients of spatial variation (C,,,), as well as maximum C, (C,“,,J for
Table Length Min. 0.20
(x) Max.
I .20
1.Variation
of grid dimension
(m)
Width O_) Min. Max.
Height Min.
0.20
0.50
1.20
(z) Max. 1.20
Fig. 2. Dimensions of basic model (m).
each model were calculated when convergence had been reached, or when the global absolute residual iteration was small enough (less than 1E - 03 approximately). For our calculations we used the results of velocity magnitude at slices of 1.35m height. Building models For this study, simulations were carried out on 10 tested models. Window sizes were identical at inlet and outlet, 1.50m high and 3.78 m wide, corresponding to a wall porosity of 30%. This window size remained constant for all models. For this purpose the tested models had the same dimensions: 7.20 x 7.20 x 2.70 m (Fig. 2). The roof shapes were selected to provide a representative sample of room shapes and architecture; Model 3a has the same shape as Model 3, but the windward and leeward sides are inverted. The total height (ht) of each model is shown in Fig. 3.
Effects
of Roof Shapes on Wind-Induced
3
Air Motion
ht = 2.90 m
I
Model 4
Model 3 & 3a
Model 1 (basic model)
Model 5
Model 6
Model 9
Model IO
I
Fig. 3. Roof configurations.
Processing of results For each model tested, the following three non-dimensional indoor air motion parameters were computed, based on the 36 points measured according to grid distribution:
I
-cp cl1 i2
41516,
a b
(1)
3
- - m+u_-_’ c --e d
e
i
1
I / _ / ! I
6 Va = l/6 J, Vi
... 6 Vf = l/6 J, Vi
Fig. 4. Plan of indoor air velocity distribution at a height of 1.35m. where C, = average velocity coefficient C, = coefficient of spatial variation C “mrr= maximum local average velocity coefficient V, = mean velocity at interior location i (m/s) L’, = mean outdoor reference free-stream velocity at the height of4.25 m (5.843 m/s) as (VJ VJ = standard deviation of C, n = number of points measured in the model (36). C, is the measurement of the relative force of the interior air movement in the horizontal plane, which is representative of the occupied space of the room, in this case 1.35 m above floor level. We used the coefficient of spatial variation at the 36 measurement points to analyse the uniformity of indoor airflow, which is an indicator of airflow homogeneity, a low value of C,, indicating a uniform flow, and a high value being indicative of a greater spatial unevenness for the interior velocity distribution.
points estabFigure 4 shows the grid of measurement lished for the distribution of indoor inflow by using the average velocity per line (position). RESULTS AND DISCUSSION Importance of wind direction For each tested model, we observed that wind direction had a great influence on the interior airflow pattern: the highest C, was generally found for the wind direction of 0” (4 = 0). Except for Models 1, 3 and 3a, the highest velocity coefficient of these models was not found for a wind angle of 0” but for a wind incidence of 30”. The C, of each model for 4 = 90”, of course, is the lowest of all wind directions. Comparison of C, for all the models shows that Model 6 has the highest average indoor velocity coefficient for 4 = 0” (C, = 0.6214) and at the same time the lowest C,
J. Kindangen et al.
4 (a)
07
$
0.6
m Model i
Q
0.5
0
0.4
. Model 2 !-J Mode1 3 Model 3a Model 4
q Model 5
imModel 30”
45
60”
90”
I~Model10
wind dlrectlons
(b)
0.9
E
0.8
8
jmModel9
. Model 1 . Model 2 D Model 3 m Model 3i
n Model 4 l-~ Model 5
n Model 6 pi Model 7 . Model 8 30
45” wlnd dimctlons
60”
90”
w Model 9
q Model 1(
Fig. 5. (a) Effects of roof shape on C, for different wind directions. (b) Coefficients of spatial variation for all the models. (c) Maximum C, for all the models.
for 4 = 90” (C, = 0.0411) (Fig. 5a). The mean spatial variation coefficient of Model 6 for all wind angles is 0.5724; this corresponds to an average uniformity of indoor airflow. As shown in Fig. 5b, the difference of spatial variation coefficient for all models and for wind angles of 0, 30, 45 and 60” is not very significant. However, for a wind incidence of 90’ the difference of C,, for each model is remarkable. For this wind angle, the lowest C,, is found for Model 10 and the highest C,, for Model 7. The coefficient of spatial variation for Models 4 and 10 is lower than the mean C,, for all models and wind directions. Figure 5c shows the distribution of maximum C, for all the models and for different wind directions. The maximum C,, of Model 6 is the highest of all models and of all wind directions, corresponding to 0.948. However. when wind incidence is 60” Model 10 has the highest maximum C, of all models, likewise for a wind angle of 90 One rather surprising finding was that the C, of Model
6 is higher for winds between 0 and 30” from the normal, but not for other wind incidences (4 = 45, 60 and 90’). For a wind angle of 45 , C, of Model 6 is lower than that of Models 5, 9 and IO. Likewise for 4 = 60 ‘, the C, of Model 6 is lower than that of Models 3a, 4, 5, 9 and 10, and for d, = 90’ the C, of Model 6 is the lowest of all models. As noticed, in particular for wind angles of 0 and 30’) Model 6 is the most favourable model of all tested models to improve interior airflow. A comparison of kinetic energy of turbulence and of static pressure for the basic model and Model 6 as shown in Appendix A can explain why Model 6 has the highest C, only for wind incidences of 0 and 30’ A “momentum flow” component is present for small wind angles ($<30’) at the inlet. Model 6 has a more influential shape which increases the suction pressure. As was found by Ernest et al. [3], the suction pressure is more effective at inducing airflow through the model than the corresponding force of the positive pressure front on the windward side. We also noticed that the indoor airflow pattern for different wind directions, especially the pattern of indoor airflow direction, is identical for all models (Appendix 9). In other words, roof shapes induce interior air velocity, but have little intluence on pattern of interior airflow direction. In this case, the configurations of windows or openings were entirely responsible for the different interior airflow direction patterns. Casrs of’ohliyue wind direction Figure 5c shows the distribution of maximum C, for all the models and for different wind directions. One can see that there are special cases for Models 1, 2, 3, 3a and 8, whose maximum C, values for wind directions of 30, 45 and 60’ are higher than those found when the wind is perpendicular to the model (4 = 0”). However, a higher maximum C, value does not always produce a higher C,; this is the case of Models 2 and 8. Consequently, the highest C, for both these models is not found for wind directions when they have higher maximum C,. (4 = 30, 45 and 60’ ). but when wind direction is 0’ We noticed that most models (except for Models 1. 3 and 3a) have higher C,. when the wind is perpendicular to the model (4 = 0 ‘). As described, the coefficient of velocity for Models 1, 3 and 3a for oblique wind directions (I$ = 30” and/or 4 = 45 ) is higher than that for 4 = 0’. Therefore, it is difficult to draw the conclusion that when the windows are located in opposite walls, higher indoor velocities are obtained when the wind is at an angle with the windward windows, as was found by Givoni [l, 21. The present study, which in aggregate proved inconclusive about the effects of wind direction, suggested that wind direction cannot be studied independently of other design variables. Between
inlet
and outlet
Figure 6a-e show the distribution of average indoor air velocity between inlet and outlet in the centre section (Appendix C). When the inflow goes through the inlet its velocity is slowed down, while it accelerates near the outlet. This proves that airflow, for all models and for all wind angles, suffers a little kinetic energy loss. As was found by Murakami et ul. [8], airflow. when going into a
5
Effects of Roof Shapes on Wind-Induced Air Motion ,
,
position
position
zmnou,o.r_ (e) 6
of indoor
a
position
position
Fig. 6. Distribution
I
I
-D_ ------I Model 1 +
Model 2
-n_
Model 3a
Model 3 +
+
Model 4 +
Model 5
-_t
Model 6 -_t
Model 7
+
Model 6 +
Model 9
+
Model 10
air velocity at a height of 1.35 m in centre section for (a) 0, (b) 30, (c) 45, (d) 60 and (e) 90”.
room through an open window, retains a large part of its mean kinetic energy. A major part of the preserved energy is directly convected outside the room through the leeward window without interior dissipation, On the average, for the whole model and all wind directions, air velocity in d and e positions decreases to about 63% of inlet air velocity; after it has passed these positions it increases about 43%. A great dissipation of
air velocity between inlet and outlet is suffered by a wind incidence of 60”, on average about 45%, then followed by wind angles of 45 and 30” with 24 and 22% respectively. The air velocity at the inlet and the outlet for 4 = 0” was near identical, likewise for C$= 90”, while for Models 5, 6, 7 and 10 for a wind angle of 0” the outlet air velocity was higher than that of the inlet, and for Models 1, 3, 3a, 5, 6, 7, and 10 for a wind angle of 90”.
J. Kindangen et al
6
0”
30”
45”
60”
0”
90
-D_ Model I+
45”
60”
90
wind directions
wind directions
I
30”
+
Model 2
Model I+
Model 3 -_c
Model 4
Fig. 8. Effects of windward and leeward overhangs on a singlesloped roof building.
by about 37%. For wind angles of 30,45, 60 and 90” the C, of these models remains nearly identical. Therefore, for a single-sloped roof, the influence of an overhang on the windward and leeward sides is such that the indoor airflow only improves for a wind direction of 0”. 0”
30”
45”
60’
90” Importance
wind directions + r
Model 1 +
Model 5 -_t
Model 6
Fig. 7. Etfects of eaves on (a) a fat roof building and (b) a prism roof building.
For 4 = 90“, Model 10 has a greater outlet air velocity than that of the inlet, about 21% difference. This model also has at the same time the highest maximum C, and the lowest C,,; in this case, it certainly has the highest average velocity coefficient. Influence
qfoverhang
By comparing Model 1 with Model 2, and then Model 5 with Model 6, as shown in Fig. 7, the following can be seen. The effect of eaves on the flat roof building is really only significant for a wind incidence of O’, about 39% of amplification. In addition it becomes less significant for some other incidences of wind, 10% and 7% of amplification (30 and 45’) and for the last two incidences of wind (60 and 90”) values are nearly equal. The overhang of prism roof buildings such as Models 5 and 6 also has a significant influence on C, only for wind within 0” from normal (about 22%). Overhangs are adequate for protection against rain and glare, one of the problems facing buildings in hot humid regions. Figure 8 shows the average velocity coefficients for the single-sloped roof model with an overhang on windward and leeward sides, as compared to the model without eaves on either side. For a normal orientation (4 = 0”) the C, of the model having eaves on both sides increases
of roof height Among the models tested, those with a higher roof height have a higher C,; this means that roof height is one of factors that can improve indoor airflow. However, it is not the case for (p = 90’; by comparing Models 6 and 7, which have the same form but different total height, it is shown that there is an anomaly where the effect of roof height is the biggest for $J = 0” but the opposite for 4 = 90”. If the buildings have identical height (Models 6, 9 and lo), Model 6, with a double-sloped roof and overhangs on four sides, is more likely to induce a higher air motion inside the building. This is only true for a wind direction of O”, and for a wind direction of 30” air motion will decrease and Model 10 will gradually become better. Indeed, indoor airflows through large openings could be induced by mean pressures, momentum flow, or a combination of both. As expected, the pressure on the windward side when this side has more exposed projected area is greater than that when it has less exposed projected area. Consequently, we can presume that the building orientation of 6&90’ for Model 10, which has a larger exposed projected area of the windward side than that of Models 6 and 9, has a higher average positive pressure zone and a higher C,. This is shown in Fig. 9. In adddition, comparison of Models 3 and 3a shows that when the higher end of the roof is facing the wind the exposed projected area of the windward side has more influence on C, than when the lower end of the roof is facing the wind. On the contrary. if we compare Models 9 and 10, with the same roof profile, semi-cylindrical in shape but with different orientation or position, it can be seen that the C, of Model 9 for wind incidences of 0 and 30’ is lower than the C, of Model 10, while for wind incidences of 60 and 90- the reverse is true. They both have the same C, value for oblique wind angle (d = 45“). Unlike the
Effects of Roof Shapes on Wind-Induced Air Motion
0”
30”
45”
60”
90”
wind directions
Fig. 9. CLmparison
of Models 6, 9 and 10.
previous case, this comparison shows that the influence of streamlined form, allowing augmentation of the momentum-flow, has greater influence on C, than the influence of average positive pressure increasing on the windward side.
CONCLUSIONS Simulation using CFD to calculate average indoor velocity coefficients, uniformity of indoor airflow and maximum C,, has been carried out on a large set of roof configurations. The main results of this study are as follows. Increasing the average velocity coefficient induced by the shape form and its orientation together helps to improve airflow inside buildings. Influence of eaves on increasing of C, for flat roofs is quite significant. Overall,
the overhang of each model has a more significant influence for 4 I 30”. The height of the roof is also one of the important factors for interior airflow which, for large wind angles (60 5 4 I 90”) at the inlet, acts to increase the exposed projected area on the windward side, followed by the positive pressure of the windward side and then the C,. In particular for pyramidal, prism-shaped and semicylindrical roofs, for small wind angles (4530”) at the inlet, they induce an increase in the momentum flow contribution when they form a streamlined section to freestream flow. It appears that in order to promote indoor air motion, it is better to augment the strength of the suction zone rather than that of the positive pressure zone. This study has also investigated distribution of airflow into a room having cross-ventilation. It has been shown that the airtlow through an open window into the room still preserves a large part of its mean kinetic energy when it remains inside the room. A major part of its preserved energy is directly convected outside the room through the leeward window without interior dissipation. This study aims to find the best way to improve interior airflow in the design stage of naturally ventilated buildings. However, the interior airflow also depends on other design parameters to be taken into consideration, such as type of windows or openings, building layout, site planning, etc. These results also help to establish a data base for the C, as a function of the roof shape configurations; future work in this domain is needed to address the following issues. l
l l
Other architectural parameters such as natural ventilation in multi-room buildings, type of windows or openings, etc. Site plan, ground type or topography effects. Integration in a tool of building thermal comfort assessment in a humid tropical climate.
REFERENCES
1, B. Givoni, L’Homme, I’Architecture et le Climal, Edition du Moniteur, 2. 3. 4.
5. 6.
7.
8. 9.
7
Paris (1978). B. Givoni, Laboratory study of the effect of window size and location on indoor air motion. Architectural Science Reaiew 8, 4245 (1965). D. R. Ernest, F. S. Bauman and E. A. Arens, The prediction of indoor air motion for occupant cooling in naturally ventilated building. ASHRAE Transactions 97(l), 539-552 (1991). D. R. Ernest, F. S. Bauman and E. A. Arens, The effects of external wind pressure distribution on wind-induced air motion inside buildings. Journal of Wind Engineering and Industrial Aerodynamics 414,2539-2550 (1992). G. Gouin, Contribution Aerodynamique a 1’Etude de la Ventilation Naturelle de 1’Habitat en Climat Tropical Humide. These de Docteur, Universitt de Nantes (1984). J. Borth and P. Suter, Influence of mesh refinement on the numerical prediction of turbulent air flow in rooms. In Proceedings of Roomvent 1994, Air Distribution in Rooms, Fourth International Conference, Volume 1, pp. 1388148, Krakow, Poland (1994). J. 1. Kindangen, Modelisation Aeraulique en Climat Tropical Humide, d’un BPtiment Largement Ouvert sur 1’Exttrieur. Memoire de DEA, CETHIL/EEH, Institut National des Sciences Appliquees de Lyon (1994). S. Murakami, S. Kato, K. Mizutani and Y. D. Kim, Wind tunnel test on velocityypressure field of cross-ventilation with open windows. ASHRAE Transactions 97(l), 525-538 (1991).
FLUENT version 4.31, Fluent, Inc., New Hampshire (1993).
J. Kindangen et al.
8
APPENDIX A
Fig. Al. Distribution
of static pressure
and kinetic energy of turbulence model) and Model 6.
Apppendix B starts on.fhcing payr
in centre section for Model
I (basic
EfJkcts of Roof Shapes on Wind-Induced Air
JbfOhm
_. _ ._ ._ -_ -______.. ____-_.-1 _ I_ APPENDIX B
._- -
A_-.
-
.
-
~*
.-
_
-----
_
._
__ .-
-
-
.-
._
-_.
._ . _ ..I .
.
.
.
.
.
.
.
.._.-
_
_
_
-
60”
Fig, A2. Indoor
airflow pattern for different wind angles at a height of 1.35m for the basic model. There is the same tendency of indoor airflow pattern for all the models.
J. Kindangen et al.
10
APPENDIX C
Basic model
Model 3
Model 3a
Model 4
Model 5
Effects of Roof Shapes on Wind-Induced
Model 7
Model 6
Model 8
Model 9
Model 10 Fig. A3. Flow pattern
11
Air Motion
for all the models in centre section
Pergamon PII: SO360-1323(96)00021-2
Effects of Roof Shapes on Wind-Induced Air Motion Inside Buildings J. KINDANGEN* G. KRAUSS* P. DEPECKER*
f
(Accepted
7 February
1996)
The eflects of roof shape on wind-induced air motion inside buildings were analysed by using a numerical simulation: computationalfluid dynamics (CFD). The average indoor velocity coejjkient, a non-dimensional indoor air motion parameter, was used to calculate the relative strength of the interior air movement in the horizontal plane representative of the occupied space of the room. The distribution of average velocity between inlet and outlet, which enabled observation of the interior air motion behaviour, was analysed. The coefficient of spatial variation was used to investigate the uniformity of airflow inside the building. It was observed that the shape qf the roof directlyafjkcted the airjow pattern, especially the velocity magnitude. The importance of wind direction, building overhang and roof height was also investigated. Copyright 0 1996 Elsevier Science Ltd.
APPROACH
INTRODUCTION IN a number of regions with a warm climate, and particularly where it is hot and humid, natural ventilation proves to be a realistic alternative as an energy-conserving design strategy aimed at reducing the cooling loads of buildings and improving indoor thermal comfort. In regions with only two seasons (dry and rainy) the shapes of roofs and eaves should be considered as one of the main factors protecting the living areas from rain and glare from the sky. Roof shapes everywhere, in various climates and cultures, also have aesthetic and semiotic values. A certain type of roof is often a typical characteristic of a nature, a climate, a culture, local materials, construction techniques, etc. A number of researchers have studied the effect of some design parameters on interior airflow to improve the level of thermal comfort and increase energy savings, i.e. position and orientation of building, window sizes, interior partitions and effects of eaves; the methods they used were wind tunnel or in situ experimentation [l-5]. However, we have attempted to formally investigate the effects of roof shape on indoor airflow pattern by making use of computational fluid dynamics (CFD) codes. This paper describes the experiments designed to study the effect of roof shape on the average velocity coefficient, C,, generally considered as a non-dimensional indoor air motion parameter. In addition, the maximum C, and the coefficient of spatial variation, C,,, were computed to investigate the homogeneity of indoor airflow.
Some of the tests on houses were carried out by making use of CFD. Indoor air speed distributions were modelled for a basic model with fixed window openings, for five wind directions (0,30,45,60 and 90”) and 10 roof shapes. The roof shapes tested were selected to cover a wide range of possible roof shapes in tropical architecture. The model with a flat roof and without a roof overhang was used as a basic model for measurements of average velocity coefficients and enabled comparison with the other models. To observe the flow pattern inside buildings, we used the local average velocity coefficient as a function of the position in a horizontal section of 1.35m height. The average velocity coefficient was calculated according to number of grid in the building core. For buildings with small openings, such as cracks, it is reasonable to assume that, as the wind reaches the building, its kinetic energy is entirely converted into static pressure. On the contrary, buildings that are completely open, with wall porosity approaching loo%, only produce minor effects on the incoming winds, and do not create significant static pressure zones. In this case, indoor velocities are essentially a result of momentum (kinetic energy of wind). For window sizes in between these two extremes, the building slows down the incoming wind and creates an upwind positive pressure zone without, however, stopping it completely. As a result, the interior airflows are produced by a combination of mean static pressure and momentum. A momentum force is more effective at inducing indoor airflows than a static pressure of equal value as openings convert static pressure back into velocities. It is reasonable to consider that all design variables are interdependent and have an influence on interior airflow, but it is also useful to observe the influence of a design variable on its own to take its effects into consideration.
*Centre de Thermique de I’INSA de Lyon/Equipe Equipement de I’Habitat, Bltiment 307, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex ,France. tFaculty of Engineering, Architectural Department, University of Sam Ratulangi, Jl. Kampus Unsrat Bahu, Manado 95115, Indonesia. 1
2
J. Kindanyen
Fig.
et al.
I. Schematic view of simulation
with CFD
Thus, in this study, in order to analyse the effects of roof shape for different wind incidences, it was necessary to assume that all other parameters were constant.
EXPERIMENTAL
METHODS
Simulation with CFD In this study, we use FLUENT [9] software, and the standard k-c turbulence model is implemented to account for turbulent flow. The how and boundary conditions are isothermal. The reference wind velocity is 5.843 m/s at a reference height of 4.25 m with a reference turbulent intensity of 22%. The simulations were carried out with very rough mesh cells. According to Borth and Suter’s study [6], and as a result of the conclusions of our previous study [7], rough mesh cells are suitable for simulation with CFD. A coarse grid is sufficient to provide a good qualitative prediction of the fluid flow. Figure 1 shows a schematic view of the simulation and the cell dimension set; the building model was placed at the centre of the floor of the cube (3 1.20 x 3 1.20 x 12.00 m). This cube was subdivided into x-, y- and =-directions with grid sizes as shown in Table 1. The cell dimension distribution in the building core and its environment stayed constant for each test except for roof areas. The CFD results of average velocity coefficients (C,), local average velocity coefficients and coefficients of spatial variation (C,,,), as well as maximum C, (C,“,,J for
Table Length Min. 0.20
(x) Max.
I .20
1.Variation
of grid dimension
(m)
Width O_) Min. Max.
Height Min.
0.20
0.50
1.20
(z) Max. 1.20
Fig. 2. Dimensions of basic model (m).
each model were calculated when convergence had been reached, or when the global absolute residual iteration was small enough (less than 1E - 03 approximately). For our calculations we used the results of velocity magnitude at slices of 1.35m height. Building models For this study, simulations were carried out on 10 tested models. Window sizes were identical at inlet and outlet, 1.50m high and 3.78 m wide, corresponding to a wall porosity of 30%. This window size remained constant for all models. For this purpose the tested models had the same dimensions: 7.20 x 7.20 x 2.70 m (Fig. 2). The roof shapes were selected to provide a representative sample of room shapes and architecture; Model 3a has the same shape as Model 3, but the windward and leeward sides are inverted. The total height (ht) of each model is shown in Fig. 3.
Effects
of Roof Shapes on Wind-Induced
3
Air Motion
ht = 2.90 m
I
Model 4
Model 3 & 3a
Model 1 (basic model)
Model 5
Model 6
Model 9
Model IO
I
Fig. 3. Roof configurations.
Processing of results For each model tested, the following three non-dimensional indoor air motion parameters were computed, based on the 36 points measured according to grid distribution:
I
-cp cl1 i2
41516,
a b
(1)
3
- - m+u_-_’ c --e d
e
i
1
I / _ / ! I
6 Va = l/6 J, Vi
... 6 Vf = l/6 J, Vi
Fig. 4. Plan of indoor air velocity distribution at a height of 1.35m. where C, = average velocity coefficient C, = coefficient of spatial variation C “mrr= maximum local average velocity coefficient V, = mean velocity at interior location i (m/s) L’, = mean outdoor reference free-stream velocity at the height of4.25 m (5.843 m/s) as (VJ VJ = standard deviation of C, n = number of points measured in the model (36). C, is the measurement of the relative force of the interior air movement in the horizontal plane, which is representative of the occupied space of the room, in this case 1.35 m above floor level. We used the coefficient of spatial variation at the 36 measurement points to analyse the uniformity of indoor airflow, which is an indicator of airflow homogeneity, a low value of C,, indicating a uniform flow, and a high value being indicative of a greater spatial unevenness for the interior velocity distribution.
points estabFigure 4 shows the grid of measurement lished for the distribution of indoor inflow by using the average velocity per line (position). RESULTS AND DISCUSSION Importance of wind direction For each tested model, we observed that wind direction had a great influence on the interior airflow pattern: the highest C, was generally found for the wind direction of 0” (4 = 0). Except for Models 1, 3 and 3a, the highest velocity coefficient of these models was not found for a wind angle of 0” but for a wind incidence of 30”. The C, of each model for 4 = 90”, of course, is the lowest of all wind directions. Comparison of C, for all the models shows that Model 6 has the highest average indoor velocity coefficient for 4 = 0” (C, = 0.6214) and at the same time the lowest C,
J. Kindangen et al.
4 (a)
07
$
0.6
m Model i
Q
0.5
0
0.4
. Model 2 !-J Mode1 3 Model 3a Model 4
q Model 5
imModel 30”
45
60”
90”
I~Model10
wind dlrectlons
(b)
0.9
E
0.8
8
jmModel9
. Model 1 . Model 2 D Model 3 m Model 3i
n Model 4 l-~ Model 5
n Model 6 pi Model 7 . Model 8 30
45” wlnd dimctlons
60”
90”
w Model 9
q Model 1(
Fig. 5. (a) Effects of roof shape on C, for different wind directions. (b) Coefficients of spatial variation for all the models. (c) Maximum C, for all the models.
for 4 = 90” (C, = 0.0411) (Fig. 5a). The mean spatial variation coefficient of Model 6 for all wind angles is 0.5724; this corresponds to an average uniformity of indoor airflow. As shown in Fig. 5b, the difference of spatial variation coefficient for all models and for wind angles of 0, 30, 45 and 60” is not very significant. However, for a wind incidence of 90’ the difference of C,, for each model is remarkable. For this wind angle, the lowest C,, is found for Model 10 and the highest C,, for Model 7. The coefficient of spatial variation for Models 4 and 10 is lower than the mean C,, for all models and wind directions. Figure 5c shows the distribution of maximum C, for all the models and for different wind directions. The maximum C,, of Model 6 is the highest of all models and of all wind directions, corresponding to 0.948. However. when wind incidence is 60” Model 10 has the highest maximum C, of all models, likewise for a wind angle of 90 One rather surprising finding was that the C, of Model
6 is higher for winds between 0 and 30” from the normal, but not for other wind incidences (4 = 45, 60 and 90’). For a wind angle of 45 , C, of Model 6 is lower than that of Models 5, 9 and IO. Likewise for 4 = 60 ‘, the C, of Model 6 is lower than that of Models 3a, 4, 5, 9 and 10, and for d, = 90’ the C, of Model 6 is the lowest of all models. As noticed, in particular for wind angles of 0 and 30’) Model 6 is the most favourable model of all tested models to improve interior airflow. A comparison of kinetic energy of turbulence and of static pressure for the basic model and Model 6 as shown in Appendix A can explain why Model 6 has the highest C, only for wind incidences of 0 and 30’ A “momentum flow” component is present for small wind angles ($<30’) at the inlet. Model 6 has a more influential shape which increases the suction pressure. As was found by Ernest et al. [3], the suction pressure is more effective at inducing airflow through the model than the corresponding force of the positive pressure front on the windward side. We also noticed that the indoor airflow pattern for different wind directions, especially the pattern of indoor airflow direction, is identical for all models (Appendix 9). In other words, roof shapes induce interior air velocity, but have little intluence on pattern of interior airflow direction. In this case, the configurations of windows or openings were entirely responsible for the different interior airflow direction patterns. Casrs of’ohliyue wind direction Figure 5c shows the distribution of maximum C, for all the models and for different wind directions. One can see that there are special cases for Models 1, 2, 3, 3a and 8, whose maximum C, values for wind directions of 30, 45 and 60’ are higher than those found when the wind is perpendicular to the model (4 = 0”). However, a higher maximum C, value does not always produce a higher C,; this is the case of Models 2 and 8. Consequently, the highest C, for both these models is not found for wind directions when they have higher maximum C,. (4 = 30, 45 and 60’ ). but when wind direction is 0’ We noticed that most models (except for Models 1. 3 and 3a) have higher C,. when the wind is perpendicular to the model (4 = 0 ‘). As described, the coefficient of velocity for Models 1, 3 and 3a for oblique wind directions (I$ = 30” and/or 4 = 45 ) is higher than that for 4 = 0’. Therefore, it is difficult to draw the conclusion that when the windows are located in opposite walls, higher indoor velocities are obtained when the wind is at an angle with the windward windows, as was found by Givoni [l, 21. The present study, which in aggregate proved inconclusive about the effects of wind direction, suggested that wind direction cannot be studied independently of other design variables. Between
inlet
and outlet
Figure 6a-e show the distribution of average indoor air velocity between inlet and outlet in the centre section (Appendix C). When the inflow goes through the inlet its velocity is slowed down, while it accelerates near the outlet. This proves that airflow, for all models and for all wind angles, suffers a little kinetic energy loss. As was found by Murakami et ul. [8], airflow. when going into a
5
Effects of Roof Shapes on Wind-Induced Air Motion ,
,
position
position
zmnou,o.r_ (e) 6
of indoor
a
position
position
Fig. 6. Distribution
I
I
-D_ ------I Model 1 +
Model 2
-n_
Model 3a
Model 3 +
+
Model 4 +
Model 5
-_t
Model 6 -_t
Model 7
+
Model 6 +
Model 9
+
Model 10
air velocity at a height of 1.35 m in centre section for (a) 0, (b) 30, (c) 45, (d) 60 and (e) 90”.
room through an open window, retains a large part of its mean kinetic energy. A major part of the preserved energy is directly convected outside the room through the leeward window without interior dissipation, On the average, for the whole model and all wind directions, air velocity in d and e positions decreases to about 63% of inlet air velocity; after it has passed these positions it increases about 43%. A great dissipation of
air velocity between inlet and outlet is suffered by a wind incidence of 60”, on average about 45%, then followed by wind angles of 45 and 30” with 24 and 22% respectively. The air velocity at the inlet and the outlet for 4 = 0” was near identical, likewise for C$= 90”, while for Models 5, 6, 7 and 10 for a wind angle of 0” the outlet air velocity was higher than that of the inlet, and for Models 1, 3, 3a, 5, 6, 7, and 10 for a wind angle of 90”.
J. Kindangen et al
6
0”
30”
45”
60”
0”
90
-D_ Model I+
45”
60”
90
wind directions
wind directions
I
30”
+
Model 2
Model I+
Model 3 -_c
Model 4
Fig. 8. Effects of windward and leeward overhangs on a singlesloped roof building.
by about 37%. For wind angles of 30,45, 60 and 90” the C, of these models remains nearly identical. Therefore, for a single-sloped roof, the influence of an overhang on the windward and leeward sides is such that the indoor airflow only improves for a wind direction of 0”. 0”
30”
45”
60’
90” Importance
wind directions + r
Model 1 +
Model 5 -_t
Model 6
Fig. 7. Etfects of eaves on (a) a fat roof building and (b) a prism roof building.
For 4 = 90“, Model 10 has a greater outlet air velocity than that of the inlet, about 21% difference. This model also has at the same time the highest maximum C, and the lowest C,,; in this case, it certainly has the highest average velocity coefficient. Influence
qfoverhang
By comparing Model 1 with Model 2, and then Model 5 with Model 6, as shown in Fig. 7, the following can be seen. The effect of eaves on the flat roof building is really only significant for a wind incidence of O’, about 39% of amplification. In addition it becomes less significant for some other incidences of wind, 10% and 7% of amplification (30 and 45’) and for the last two incidences of wind (60 and 90”) values are nearly equal. The overhang of prism roof buildings such as Models 5 and 6 also has a significant influence on C, only for wind within 0” from normal (about 22%). Overhangs are adequate for protection against rain and glare, one of the problems facing buildings in hot humid regions. Figure 8 shows the average velocity coefficients for the single-sloped roof model with an overhang on windward and leeward sides, as compared to the model without eaves on either side. For a normal orientation (4 = 0”) the C, of the model having eaves on both sides increases
of roof height Among the models tested, those with a higher roof height have a higher C,; this means that roof height is one of factors that can improve indoor airflow. However, it is not the case for (p = 90’; by comparing Models 6 and 7, which have the same form but different total height, it is shown that there is an anomaly where the effect of roof height is the biggest for $J = 0” but the opposite for 4 = 90”. If the buildings have identical height (Models 6, 9 and lo), Model 6, with a double-sloped roof and overhangs on four sides, is more likely to induce a higher air motion inside the building. This is only true for a wind direction of O”, and for a wind direction of 30” air motion will decrease and Model 10 will gradually become better. Indeed, indoor airflows through large openings could be induced by mean pressures, momentum flow, or a combination of both. As expected, the pressure on the windward side when this side has more exposed projected area is greater than that when it has less exposed projected area. Consequently, we can presume that the building orientation of 6&90’ for Model 10, which has a larger exposed projected area of the windward side than that of Models 6 and 9, has a higher average positive pressure zone and a higher C,. This is shown in Fig. 9. In adddition, comparison of Models 3 and 3a shows that when the higher end of the roof is facing the wind the exposed projected area of the windward side has more influence on C, than when the lower end of the roof is facing the wind. On the contrary. if we compare Models 9 and 10, with the same roof profile, semi-cylindrical in shape but with different orientation or position, it can be seen that the C, of Model 9 for wind incidences of 0 and 30’ is lower than the C, of Model 10, while for wind incidences of 60 and 90- the reverse is true. They both have the same C, value for oblique wind angle (d = 45“). Unlike the
Effects of Roof Shapes on Wind-Induced Air Motion
0”
30”
45”
60”
90”
wind directions
Fig. 9. CLmparison
of Models 6, 9 and 10.
previous case, this comparison shows that the influence of streamlined form, allowing augmentation of the momentum-flow, has greater influence on C, than the influence of average positive pressure increasing on the windward side.
CONCLUSIONS Simulation using CFD to calculate average indoor velocity coefficients, uniformity of indoor airflow and maximum C,, has been carried out on a large set of roof configurations. The main results of this study are as follows. Increasing the average velocity coefficient induced by the shape form and its orientation together helps to improve airflow inside buildings. Influence of eaves on increasing of C, for flat roofs is quite significant. Overall,
the overhang of each model has a more significant influence for 4 I 30”. The height of the roof is also one of the important factors for interior airflow which, for large wind angles (60 5 4 I 90”) at the inlet, acts to increase the exposed projected area on the windward side, followed by the positive pressure of the windward side and then the C,. In particular for pyramidal, prism-shaped and semicylindrical roofs, for small wind angles (4530”) at the inlet, they induce an increase in the momentum flow contribution when they form a streamlined section to freestream flow. It appears that in order to promote indoor air motion, it is better to augment the strength of the suction zone rather than that of the positive pressure zone. This study has also investigated distribution of airflow into a room having cross-ventilation. It has been shown that the airtlow through an open window into the room still preserves a large part of its mean kinetic energy when it remains inside the room. A major part of its preserved energy is directly convected outside the room through the leeward window without interior dissipation. This study aims to find the best way to improve interior airflow in the design stage of naturally ventilated buildings. However, the interior airflow also depends on other design parameters to be taken into consideration, such as type of windows or openings, building layout, site planning, etc. These results also help to establish a data base for the C, as a function of the roof shape configurations; future work in this domain is needed to address the following issues. l
l l
Other architectural parameters such as natural ventilation in multi-room buildings, type of windows or openings, etc. Site plan, ground type or topography effects. Integration in a tool of building thermal comfort assessment in a humid tropical climate.
REFERENCES
1, B. Givoni, L’Homme, I’Architecture et le Climal, Edition du Moniteur, 2. 3. 4.
5. 6.
7.
8. 9.
7
Paris (1978). B. Givoni, Laboratory study of the effect of window size and location on indoor air motion. Architectural Science Reaiew 8, 4245 (1965). D. R. Ernest, F. S. Bauman and E. A. Arens, The prediction of indoor air motion for occupant cooling in naturally ventilated building. ASHRAE Transactions 97(l), 539-552 (1991). D. R. Ernest, F. S. Bauman and E. A. Arens, The effects of external wind pressure distribution on wind-induced air motion inside buildings. Journal of Wind Engineering and Industrial Aerodynamics 414,2539-2550 (1992). G. Gouin, Contribution Aerodynamique a 1’Etude de la Ventilation Naturelle de 1’Habitat en Climat Tropical Humide. These de Docteur, Universitt de Nantes (1984). J. Borth and P. Suter, Influence of mesh refinement on the numerical prediction of turbulent air flow in rooms. In Proceedings of Roomvent 1994, Air Distribution in Rooms, Fourth International Conference, Volume 1, pp. 1388148, Krakow, Poland (1994). J. 1. Kindangen, Modelisation Aeraulique en Climat Tropical Humide, d’un BPtiment Largement Ouvert sur 1’Exttrieur. Memoire de DEA, CETHIL/EEH, Institut National des Sciences Appliquees de Lyon (1994). S. Murakami, S. Kato, K. Mizutani and Y. D. Kim, Wind tunnel test on velocityypressure field of cross-ventilation with open windows. ASHRAE Transactions 97(l), 525-538 (1991).
FLUENT version 4.31, Fluent, Inc., New Hampshire (1993).
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8
APPENDIX A
Fig. Al. Distribution
of static pressure
and kinetic energy of turbulence model) and Model 6.
Apppendix B starts on.fhcing payr
in centre section for Model
I (basic
EfJkcts of Roof Shapes on Wind-Induced Air
JbfOhm
_. _ ._ ._ -_ -______.. ____-_.-1 _ I_ APPENDIX B
._- -
A_-.
-
.
-
~*
.-
_
-----
_
._
__ .-
-
-
.-
._
-_.
._ . _ ..I .
.
.
.
.
.
.
.
.._.-
_
_
_
-
60”
Fig, A2. Indoor
airflow pattern for different wind angles at a height of 1.35m for the basic model. There is the same tendency of indoor airflow pattern for all the models.
J. Kindangen et al.
10
APPENDIX C
Basic model
Model 3
Model 3a
Model 4
Model 5
Effects of Roof Shapes on Wind-Induced
Model 7
Model 6
Model 8
Model 9
Model 10 Fig. A3. Flow pattern
11
Air Motion
for all the models in centre section