4. Wind tunnel experiments on urban district

Ecole Centrale de Lyon
Laboratoire de Mécanique des Fluides et d'Acoustique
Development and validation of an
operational model for
air quality predictions
SIRANE
Salizzoni P., Garbero V., Soulhac L.,
Flow and dispersion in urban areas
• Flow and dispersion in the lower
part of the atmospheric boundary
layer, where the flow dynamics are
typically determined by the size
and the density of the buildings and by the street geometry;
• Mass exchange between
the recirculating region within
the street canyons and the
external flow and within
the canopy itself.
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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In order to model flow and dispersion in urban areas :
two choices are available
1. a complete reconstruction of the urban geometry within
the computational domain and the solution of the system
of the governing equations by means of CFD codes;
2. a parameterization of momentum and mass exchange
processes taking place in the lower part of the boundary
layer and in the urban canopy, by means of simplified
operational models
SIRANE – (L. Soulhac).
LMFA/ECL - 2007
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Decomposition of the domain
Mechanisms controlling the
dispersion in a district
External flow
Dispersion over the roof level
Urban canopy
RANS CFD calculations
1. Flow in the street
2. Exchanges at each intersection
3. Exchange between street
canyons and the external flow
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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Flow in the external atmosphere
Monin-Obukhov similarity theory
• Monin-Obukhov length :
LMO =
−ρCpu3*
κ ( g T0 ) H0
• Velocity profile :
u( z) =
z − dquartier + z0,quartier
z − dquartier + z0,quartier
z
u*
− ψm
− ψm 0,quartier
ln
κ
z0,quartier
LMO
LMO
(
ψm ( ζ ) = 2 ln (1 + x ) 2 +ln 1 + x 2
with
)
2 -2arctan ( x ) + π 2 si LMO < 0 (cas instable)
avec x = (1 − 16ζ )
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ψm ( ζ ) = 0 si LMO = 0 (cas neutre)
ψm ( ζ ) = −5ζ si LMO > 0 (cas stable)
• + profiles of temperature and turbulence σv et σw
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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Dispersion in the external atmosphere
Processes to take into account :
• Diffusive flux from streets
• Advective vertical flux from intersections
• External sources (eg. industry)
c ( x,y,z ) =
1 ( y − ys )
exp −
σ2y
2
2π Um σ y
QS,eff
2
× pdfz ( z − Hs,eff ) + pdfz ( z − 2Hbatˆ + Hs,eff ) + pdfz ( z − 2h + Hs,eff )
Gaussian plume
model
LMFA/ECL - 2007
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Dispersion in the urban canopy
Geometrical description of a district
Representation of urban canopy
Simplification of
building geometry
Pollutant budget in each street
LMFA/ECL - 2007
Exchange at intersections
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Mass balance within the box
• Analytical model for the average velocity in each street
(Soulhac et al., 2007, Boundary Layer Meteorology)
• Budget of pollutant mass in the street
d (HWL.Cstreet )
dt
= QS + QI,in − QH,turb + QI,out
In fluxes
Out fluxes
QI,out
QH,turb
QI,in
LMFA/ECL - 2007
QS
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2. Mass exchange between the street and the external
atmosphere - Experimental studies (Salizzoni, 2006).
The mean velocity (m/s) field shows the typical recirculating street canyon flow (left).
The differences in the turbulent kinetic energy levels (m2/s2) show the ‘decoupling’
between the canyon flow and the overlying atmospheric boundary layer flow (right).
-0.4
-0.2
x/H
0
0.2
0.4
60
2
1
0.6
40
0.8
1
0.5
20
0.6
z (mm)
0.4
0
0
0.3
0.4
-20
0.2
-1
0.2
-40
0.1
-2
0
-2
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-1
0
U / Uext
1
2
-60
-40
-20
0
20
x (mm)
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40
60
0
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2. Mass exchange between the street and the external
atmosphere - Model
Concentration gradient diffusion approach;
the transfer velocity is calculated from the external boundary layer properties
and the canyon geometry
C0
ud
C2
C1
• Turbulent exchange at the interface
ud
QH,turb =
σ w WL
2π
(C
street
− Cstreet,ext )
Mq
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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3. Exchange model at street intersections
15°
45°
30°
RANS CFD
calculations
Calculation of exchange fluxes as a
function of wind direction
Pi,j(θ)
Averaging fluxes over wind direction fluctuations
Pi,j ( θ0 ) = f ( θ − θ0 ) Pi,j ( θ ) dθ
with f ( θ − θ0 ) =
LMFA/ECL - 2007
1 θ − θ0
exp −
2 σθ
2π
1
σθ
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4. Wind tunnel experiments on urban district
Valeria Garbero – PhD Thesis
Study of turbulent dispersion from a point source in an urban district
Influence of wind direction
Atmospheric wind tunnel of the Ecole Centrale de Lyon
Dimensions of the test section: 14m x 2.5 m x 3.7m
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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4. Wind tunnel experiments on urban district
Experimental setting
Building geometry
Lx
Sx
Ly
Sy
Establishment of the
urban boundary layer
7m
y
x
z
H
x
Model :
H = 50 mm
Lx = Ly = 5H
Sx = S y = H
Reality :
Model scale
1:400
LMFA/ECL - 2007
H = 20 m
Lx = Ly = 100 m
Sx = Sy = 20 m
District studied
GEAM - Torino - 07/11/2007
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4. Wind tunnel experiments on urban district
Experimental setting
Concentration measurements with Flame Ionisation Detector:
• Lateral profiles (y-direction) at different distance from the source
• Variation of the wind direction
Source located in
an intersection
0°
X
X
15°
30°
45°
y
x
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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4. Wind tunnel experiments on urban district
Experimental results
Wind direction = 0°
RANS CFD calculations
•
The plume is channelled by the
main street
•
Small transverse dispersion in
perpendicular streets
LMFA/ECL - 2007
x/H
-24
-12
y/H 0
12
24
0
0
4
12
8
0
2
K
24
4
0
1
36
2
48
0 0.5 1
Wind tunnel concentration profiles (z = 0)
GEAM - Torino - 07/11/2007
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4. Wind tunnel experiments on urban district
Experimental results
Wind direction = 45°
RANS CFD calculations
36
24
x/H
0.3 0
LMFA/ECL - 2007
0
12
y/H 24
36
48
0
0
12
1
0
K
0.2
48
Wind tunnel concentration profiles (z = 0)
exchange mechanism at the
intersections
GEAM - Torino - 07/11/2007
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4. Preliminary comparison SIRANERISK / measur.
Wind direction = 0°
Ground level
LMFA/ECL - 2007
Concentration profiles
Wind tunnel
–
SIRANE
Roof level
GEAM - Torino - 07/11/2007
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4. Comparison SIRANE / measur.
Wind direction = 15°
Ground level
LMFA/ECL - 2007
Concentration profiles
Wind tunnel
–
SIRANE
Roof level
GEAM - Torino - 07/11/2007
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4. Comparison SIRANE / measur.
Wind direction = 30°
Ground level
Concentration profiles
Wind tunnel
–
SIRANE
Roof level
• The model seams to represent the main features of the concentration field
• Necessity to parameterize the different exchange coefficients to compare more precisely
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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5. Conclusions and perspectives
Conclusions
• Experimental studies has been conducted in order to
understand dispersion mechanisms through a group of
obstacles
• An operational dispersion model, SIRANE, has been developed
to predict concentration in each street of an urban domain
• A comparison between model and experiments shows that
SIRANE describes the main characteristics of the plume
dispersing within district
LMFA/ECL - 2007
GEAM - Torino - 07/11/2007
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