The paper presents a numerical methodology to assess the natural ventilation. UrbaWind is an automatic computational fluid dynamics code. It was developed to model the wind in urban environments. The turbulence modelling, namely the dependence of turbulence length on the distance from wall, and the model constants were calibrated in order to reproduce with good agreements flow separation around buildings walls and pressure coefficient field on façades. Numerical results match well with the experiments: separation patterns and pressure field on walls in dense urban areas. Examples are presented at the end of the paper in order to show the advantages of the methodology for urban designers as they need pressure coefficients to assess the air change rate of buildings...

1. Wind-induced pressure coefficients
on buildings dedicated to air change
rate assessment with CFD tool in
complex urban areas
Stephane.SANQUER, Guillaume.CANIOT
@meteodyn.com

2. 1. INTRODUCTION
2. NUMERICAL APPROACH
3. VALIDATION
4. PRESSURE FIELD ON A BUILDINGS WALLS IN A COMPLEX URBAN AREA
5. CONCLUSION
0. CONTENTS

3. Air Flow inside the building depends on external condition.
Wind Mappings/Pressure on the wall are the key parameters to optimize the master plan.
Cp DataBase useful for simple building shape otherwise not accurate for
complex with neighboring buildings
Numerical method will help designers : Computation Fluid Dynamics
1. INTRODUCTION

4. 2. NUMERICAL APPROACH
Equations :
RANS equation :
Turbulence Equation k-L :
  0'' 






























 iji
i
j
j
i
jij
ij
Fuu
x
u
x
u
xx
P
x
uu





 
































j
j
i
j
j
i
Tk
ik
T
i
i x
u
x
u
x
u
P
x
k
ku
x
Tt Lk 2/1
 
TL
k
C
2/3
 
𝐿 𝑇 = 𝜅 𝐷 𝑊 𝐶𝐿
Turbulence viscosity :
Dissipation rate :
Turbulence Length Scale
09.001.0  CDepends on thermal stability (Yamada and Arritt)
𝐶𝐿 ?
Two parameters to tune : 𝐶𝐿 𝑎𝑛𝑑 𝐶𝜇

5. 3. VALIDATION
Turbulence model Reference XR/b XR_TOP/b
k-e (Standard) Tominaga el al. 2.7 No separation
k-e (Modified) Tominaga el al. 3 to 3.2 0.52 to 0.58
Differential stress model Mochida et al. 4.2 >1
LES Tominaga el al. 1 to 2.1 0.50 to 0.62
k-l (CL=0.20, Cmu=0.09) UrbaWind 1.7 0.25
k-l (CL=0.15, Cmu=0.09) UrbaWind 2.5 0.60
k-l (CL=0.10, Cmu=0.09) UrbaWind 2.5 0.70
k-l (CL=0.15, Cmu=0.01) UrbaWind 1.5 0.60
k-l (CL=0.10, Cmu=0.01) UrbaWind 2 0.70
Experiment Meng and Hibi 1.42 0.52
Flow Separation :
Dimension : 20 x 20 x 40 m
b

6. 3. VALIDATION
Pressure coefficient on buildings :
-1.5
-1.0
-0.5
0.0
0.5
1.0
0 1 2 3
CFD (UrbaWind)
0
3
2
1
Cp
Wind tunnel and
full scale results range
Mean results
Experimental values from
the Silsoe 6m Cube (NZ)

7. 4. PRESSURE FIELD ON A BUILDINGS WALLS IN A COMPLEX
URBAN AREA
Sensibility of geometry of the building and wind direction
Cp=0 Cp=0.5
Cp=0.6 Cp=0.6
Impact of geometry
Impactofwinddirection

8. 4. PRESSURE FIELD ON A BUILDINGS WALLS IN A COMPLEX
URBAN AREA
-1.5
-1.0
-0.5
0.0
0.5
1.0
0 1 2 3
0°-Detached
45°-Detached
0°- Urban
45° - Urban
0
3
2
1
Cp
0° 45°
CFD (DCp) 0° 45°
Detached 1.0 0.7
Urban 0.1 0.2
Table (DCp) 0° 45°
Urban 0.45 0.35
Influence of complex urban environment
DCp over estimated using table.
WINDa UCpAQ 

9. 4. PRESSURE FIELD ON A BUILDINGS WALLS IN A COMPLEX
URBAN AREA
H H H
H
H
M
L
L
L L
H
H
H HM
Influence of complex urban environment
H = High (DCp≥0.39) / M = Medium (0.17≤ DCp<0.39) / L = Low (DCp<0.17) Level of Natural Ventilation
Formulae or tables difficult to apply in a complex urban environment

10. 5. CONCLUSION
Pressure Coefficient are sensible to wind direction, building shape, building environment.
Be careful when apply formulae or use table.
CFD tools can deal with complex area.

11. Thanks you