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 2
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 GEAM - Torino - 07/11/2007 3
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 - 2007 GEAM - Torino - 07/11/2007 4
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 - 2007 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. 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 - 2007 GEAM - Torino - 07/11/2007 6
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 - 2007 GEAM - Torino - 07/11/2007 7
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 - 2007 GEAM - Torino - 07/11/2007 8
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 -0.4-0.2 0 0.2 0.4 60 2 1 40 0.6 1 0.8 0.5 20 0.6 0.4 0 z (mm) 0 0.4 0.3-20 -1 0.2-40 0.2 0.1-2 -2-1 0 1 2 U / Uext 0-60 -40-20 0 20 40 60 x (mm) 0 LMFA/ECL - 2007 GEAM - Torino - 07/11/2007 9
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 - 2007 GEAM - Torino - 07/11/2007 10
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 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 - 2007 GEAM - Torino - 07/11/2007 11
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 12
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 - 2007 GEAM - Torino - 07/11/2007 13
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 14
4. Wind tunnel experiments on urban district Experimental results Wind direction = 0 K 0 4 8 0 2 4 0 1 2 0 0.5 1 Wind tunnel concentration profiles (z = 0) y/h 24 12 0 0 12 24 36 48 x/h -12-24 RANS CFD calculations The plume is channelled by the main street Small transverse dispersion in perpendicular streets LMFA/ECL - 2007 GEAM - Torino - 07/11/2007 15
y/h 4. Wind tunnel experiments on urban district Experimental results Wind direction = 45 Wind tunnel concentration profiles (z = 0) RANS CFD calculations K 0 1 0 0.3 0 0.2 0 12 24 36 48 x/h 48 36 24 12 0 exchange mechanism at the intersections LMFA/ECL - 2007 GEAM - Torino - 07/11/2007 16
4. Preliminary comparison SIRANERISK / measur. Wind direction = 0 Ground level Concentration profiles Wind tunnel SIRANE Roof level LMFA/ECL - 2007 GEAM - Torino - 07/11/2007 17
4. Comparison SIRANE / measur. Wind direction = 15 Ground level Concentration profiles Wind tunnel SIRANE Roof level LMFA/ECL - 2007 GEAM - Torino - 07/11/2007 18
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 19
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 20