wind resource assessment in urban areas for sustainable development

Building Simulation

Conference - 2015

WIND ASSESSMENT IN URBAN AREA WITH CFD TOOLS APPLICATION TO NATURAL VENTILATION POTENTIAL

AND OUTDOOR PEDESTRIAN COMFORT

Stéphane Sanquer, Guillaume Caniot, Sachin Bandhare

Meteodyn, FRANCE

CONTEXTE AND OBJECTIVES

From an urban designer point of view, the knowledge of the urban climatology and especially the wind flow around buildings is crucial in many applications such as:

• Air quality and thermal behaviour inside buildings since heat exchange depends on Air change rate;

• Wind energy production from small wind turbines ;

• Wind pedestrian comfort in outdoor spaces and in opened indoor spaces exposed to wind.

CFD tools dedicated to calculating wind flow inside built environment give advantages to understand and interpret the wind in any urban area. Wind mappings become useful to urban designers to optimize the master plan: position and orientation of buildings

(Jansen et al. 2013, Fadl and Karadelis 2013, Szücz 2013)

IBPSA Building Simulation Conference , Hyderabad, India, Dec. 7-9, 2015


The aim of this work is to present a methodology that allows the quick assessment of urban master plan without carrying out complex long computations.

• The natural ventilation potential based on pressure coefficients mappings 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



The human comfort index is based on wind speeds exceedance statistics Quick computations for urban designer : Which model ? Validation cases to show the performances of the CFD tool (UrbaWind) Applications to explain the approach and their practical advantages


NUMERICAL APPROACH (UrbaWind)


RANS equations : Turbulence Equation k-L :

0''

iji

i

j

j

i

jij

ijFuu

x

u

x

u

xx

P

x

uu

j

j

i

j

j

iTk

ik

Ti

i x

u

x

u

x

uP

x

kku

x

Tt Lk 2/1

TL

kC

2/3

𝐿𝑇 = 𝜅 𝐷𝑊𝐶𝐿

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

VALIDATION : flow separation around buildings


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=0.10, Cmu=0.01) UrbaWind 2 0.70

Experiment Meng and Hibi 1.42 0.52

Dimensions : 20 x 20 x 40 m b

CFD : Tominaga et al., 2008 Experiments : Meng & Hibi, 1996

VALIDATION : pressure on building walls


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 (Richards et al., 2007)

VALIDATION : pressure on buildings


Sensibility of geometry of the building and wind direction

Cp=0 Cp=0.5

Cp=0.6 Cp=0.6

Impact of geometry

Imp

act

of

win

d d

irec

tio

n

VALIDATION : pressure on buildings


-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

Tables (DCp) 0° 45°

Urban 0.45 0.35

Influence of complex urban environment

DCp over estimated using table.

WINDa UCpAQ

VALIDATION : wind speed in urban area


Nigata experiments : Yoshie et al. 77 points at h=2m =Cv(CFD)-Cv(exp) Mean error = 0.04 Standard deviation=0.14 50% ||<0.1 RANS underestimates the wind speed in the wake compared to experiment made with thermistor anemometers High value of are in low speed area or closed to flow separation zone => small change of position leads to important speed variation.

Application : Natural ventilation potential


Threshold on Cp : example of warm tropical climate

WINDBD

a

WINDa

UCpV

AACH

UCpAQ

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50

T(i

nd

oor)-

T(o

utd

oor)

( C

)

ACH (1/h)

Roof without insulation

Roof insulation

Roof and NE wall insulation

Indoor Thermal comfort

Givoni diagram : Thermal dynamic

simulation in an office at La Réunion Island

during the summer (Garde R., 2006)



H H H

H

H

M

L

L

L L

H

H

H H M

Threshold on Cp : example of warm tropical climate

Guidelines for Natural ventilation potential in tropical warm climate (Gandemer et al.) Level of natural

ventilation efficiency

DCp range

ACH too low DCp < 0.2

ACH enough 0.2 ≤ DCp < 0.4

ACH good 0.4≤ DCp < 0.6

ACH very good 0.6≤ DCp

WINDa UCpAQ

UrbaWind UrbaWind



Level of natural ventilation efficiency

DCp range

ACH too low DCp < 0.2

ACH enough 0.2 ≤ DCp < 0.4

ACH good 0.4≤ DCp < 0.6

ACH very good 0.6≤ DCp

Threshold on Cp : example of warm tropical climate Réunion Island (Indian Ocean) French Guiana (South America)

Application : Pedestrian comfort


Pedetrian Wind comfort assessment on Seguin Island in Paris

UrbaWind

Green : suitable for stationary position Yellow : unconfortable

Frequency of exceeding the comfort threshold

Conclusions


• Numerical methodology to assess the wind pedestrian comfort and the natural ventilation in any complexe urban area

• Purposes : To develop an Automatic, Quick and Accurate CFD Tools (UrbaWind) to urban designers

• Turbulence modeling : k-l to urban flows => reproduce the flow separation

• Validation with experimental data

• Wind and pressure field used to assess the comfort and the natural ventilation potential

wind resource assessment in urban areas for sustainable development

Presentations & Public Speaking