Development and validation of an operational model for air quality predictions SIRANE Salizzoni P., Garbero V., Soulhac L.,

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1 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.,

buildings and by the street geometry; Mass exchange between the recirculating region within the

2 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 GEAM - Torino - 07/11/2007 2

3 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 GEAM - Torino - 07/11/2007 3

1. Flow in the street 2. Exchanges at each intersection 3.

4 Decomposition of the domain Mechanisms controlling the dispersion in a district External flow Dispersion over the roof level RANS CFD calculations Urban canopy 1. Flow in the street 2. Exchanges at each intersection 3. Exchange between street canyons and the external flow LMFA/ECL GEAM - Torino - 07/11/2007 4

5 Flow in the external atmosphere Monin-Obukhov similarity theory Monin-Obukhov length : L MO = κ ρc u 3 p * ( ) g T H 0 0 Velocity profile : ( ) u z u z d + z z d + z z κ * quartier 0,quartier quartier 0,quartier 0,quartier = ln ψm ψ m z 0,quartier LMO LMO with 2 ( ) ( ) ( ) ( ) ψm ζ = 2 ln 1+ x 2 +ln 1+ x 2-2arctan x + π 2 si LMO < 0 (cas instable) 1 4 avec x = ( 1 16ζ) ψm ( ζ ) = 0 si LMO = 0 (cas neutre) ψ m ( ζ ) = 5 ζ si LMO > 0 (cas stable) + profiles of temperature and turbulence σ v et σ w LMFA/ECL GEAM - Torino - 07/11/2007 5

Dispersion in the external atmosphere Processes to take into account : Diffusive flux from streets Advective vertical flux from intersections External sources (eg.

6 Dispersion in the external atmosphere Processes to take into account : Diffusive flux from streets Advective vertical flux from intersections External sources (eg. industry) ( ) ( y y ) 2 Q 1 = ( s,eff ) + ( bat ˆ + s,eff ) + ( + s,eff ) S,eff s c x,y,z exp pdfz z H pdfz z 2H H pdfz z 2h H 2 2π U 2 m σ σ y y Gaussian plume model LMFA/ECL GEAM - Torino - 07/11/2007 6

7 Dispersion in the urban canopy Geometrical description of a district Representation of urban canopy Simplification of building geometry Pollutant budget in each street Exchange at intersections LMFA/ECL GEAM - Torino - 07/11/2007 7

8 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 ( Cstreet ) = + + d HWL. dt QS QI,in QH,turb QI,out In fluxes Out fluxes Q H,turb Q I,out Q I,in Q S LMFA/ECL GEAM - Torino - 07/11/2007 8

9 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 (m 2 /s 2 ) show the decoupling between the canyon flow and the overlying atmospheric boundary layer flow (right). x / H z (mm) U / Uext x (mm) 0 LMFA/ECL GEAM - Torino - 07/11/2007 9

10 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 C 0 u d Turbulent exchange at the interface C 1 C 2 u d σ WL Q = C C 2π ( ) w H,turb street street,ext M q LMFA/ECL GEAM - Torino - 07/11/

3. Exchange model at street intersections 15 45 30 RANS CFD calculations Calculation of exchange fluxes as a function of wind direction P i,j (θ) Averaging fluxes

11 3. Exchange model at street intersections RANS CFD calculations Calculation of exchange fluxes as a function of wind direction P i,j (θ) Averaging fluxes over wind direction fluctuations with f ( ) ( ) ( ) P θ = f θ θ P θ dθ i,j 0 0 i,j ( ) 2 1 1θ θ exp σ π 0 θ θ 0 = 2 2 σ θ θ LMFA/ECL GEAM - Torino - 07/11/

12 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 GEAM - Torino - 07/11/

mm L x = L y = 5H S x = S y = H Model scale 1:400 Reality : H = 20 m L x = L y =

13 4. Wind tunnel experiments on urban district Experimental setting Building geometry Lx Sx Ly Sy Establishment of the urban boundary layer y x 7 m z H x Model : H = 50 mm L x = L y = 5H S x = S y = H Model scale 1:400 Reality : H = 20 m L x = L y = 100 m S x = S y = 20 m District studied LMFA/ECL GEAM - Torino - 07/11/

4. Wind tunnel experiments on urban district Experimental setting Concentration measurements with Flame Ionisation Detector: Lateral profiles (y-direction) at

14 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 y x LMFA/ECL GEAM - Torino - 07/11/

4. Wind tunnel experiments on urban district Experimental results Wind direction = 0 K 0 4 8 0 2 4 0 1 2 0 0.

15 4. Wind tunnel experiments on urban district Experimental results Wind direction = 0 K Wind tunnel concentration profiles (z = 0) y/h x/h RANS CFD calculations The plume is channelled by the main street Small transverse dispersion in perpendicular streets LMFA/ECL GEAM - Torino - 07/11/

16 y/h 4. Wind tunnel experiments on urban district Experimental results Wind direction = 45 Wind tunnel concentration profiles (z = 0) RANS CFD calculations K x/h exchange mechanism at the intersections LMFA/ECL GEAM - Torino - 07/11/

17 4. Preliminary comparison SIRANERISK / measur. Wind direction = 0 Ground level Concentration profiles Wind tunnel SIRANE Roof level LMFA/ECL GEAM - Torino - 07/11/

18 4. Comparison SIRANE / measur. Wind direction = 15 Ground level Concentration profiles Wind tunnel SIRANE Roof level LMFA/ECL GEAM - Torino - 07/11/

level The model seams to represent the main features of the concentration field

19 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 GEAM - Torino - 07/11/

20 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 GEAM - Torino - 07/11/

Pollutant dispersion in urban geometries

Pollutant dispersion in urban geometries V. Garbero 1, P. Salizzoni 2, L. Soulhac 2 1 Politecnico di Torino - Department of Mathematics 2 Ecole Centrale de Lyon - Laboratoire de Méchaniques des Fluides