Pollutant dispersion in urban geometries

Size: px

Start display at page:

Download "Pollutant dispersion in urban geometries"

Kimberly Ashlynn Joseph
5 years ago
Views:

1 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 et d Acoustique Pollutant dispersion in urban geometries p. 1/29

2 Index 1. Introduction 2. Experimental set-up and techniques 3. Overview of experimental results. The model SIRANE 5. Comparison between SIRANE and experiments 6. Conclusions Pollutant dispersion in urban geometries p. 2/29

3 1. Introduction The study focuses on pollutant dispersion in a densely packed district Garbero V., 28, "Pollutant dispersion in urban canopy: study of the plume behaviour through an obstacle array", PhD thesis, Politecnico di Torino - Ecole Centrale de Lyon Pollutant dispersion in urban geometries p. 3/29

4 1. Introduction Approach of the study The study has been conducted in an idealized urban geometry: obstacle array simulating a simplified urban district The array represents a street-network: domain made up of streets inter-connected via intersections where the interaction between the flows developing in the different region is limited Pollutant dispersion in urban geometries p. /29

5 1. Introduction Mass transfer processes in a street-network advection along the street dispersion at street intersection dispersion at the canopy-atmosphere interface Aim of the study Flow field and dispersion have been investigated in order to define the dependence of these processes on geometrical layout wind direction Pollutant dispersion in urban geometries p. 5/29

6 2. Experimental set-up and techniques Atmospheric wind tunnel of the Laboratoire de Mécanique des Fluides et d Acoustique, Ecole Centrale de Lyon Working section: 1 m long, 2.5 m high, 3.7 m wide U = 5 m/s. Re 1 5 Dynamical similarity of the fully developed turbulent flow Pollutant dispersion in urban geometries p. 6/29

7 2. Experimental set-up and techniques Reality H = 2 m L = 1 m Model H = 5 mm L = 25 mm The street network is simulated at scale factor 1: The pollutant was released from a point source placed within an intersection at z = H/2 and x = 1δ Pollutant dispersion in urban geometries p. 7/29

8 2. Experimental set-up and techniques Different array layouts and different incident wind directions have been taken into account L θ L W y W x H Configuration 1: W x = W y = H θ = o, 1 o, 25 o, 5 o Pollutant dispersion in urban geometries p. 8/29

9 2. Experimental set-up and techniques Different array layouts and different incident wind directions have been taken into account L θ L W y W x H Configuration 2: W x = 2H, W y = H θ = o Pollutant dispersion in urban geometries p. 8/29

10 2. Experimental set-up and techniques Different array layouts and different incident wind directions have been taken into account L θ L W y W x H Configuration 3: W x = H, W y = 2H θ = o Pollutant dispersion in urban geometries p. 8/29

11 2. Experimental set-up and techniques Flow field measurements were performed by means of Hot Wire Anemometer vertical profiles above the array z=2h z=h/2 Concentration measurements were performed by means of Flame Ionisation Detector horizontal profiles within (z = H/2) and above (z = 2H) the array, vertical profiles Garbero V. et al., "Experimental study of pollutant dispersion within a network of streets", to be submitted to Boundary Layer Meteorology Pollutant dispersion in urban geometries p. 9/29

12 3. Overview of Experimental results External velocity field A neutrally stratified urban boundary layer was simulated U u * 15 1 z-d δ.6. z-d δ.6. z-d δ z-d 1 δ U u = 1 k ln ( ) z d z σ u /u * δ =.8 m; u =.23 m/s; z =.75 mm. d = 5 mm σ v /u * σ w /u * Wind tunnel data Raupach (1991) Pollutant dispersion in urban geometries p. 1/29

13 3. Overview of Experimental results Mean concentration profiles within and above the array at increasing distances from the source z=h/2 exp z=2h exp 36 K σy/η y/h Pollutant dispersion in urban geometries p. 11/29

14 . The model SIRANE The urban dispersion model SIRANE has been developed at LMFA. It is an operational model for air quality management in urban areas. SIRANE is based on a decomposition of the domain in 2 regions: External atmosphere Urban canopy Soulhac L., 2, "Modélisation de la dispersion atmosphérique à l interieur de la canopée urbaine", PhD thesis, Ecole Centrale de Lyon Pollutant dispersion in urban geometries p. 12/29

15 . The street-network model Topology of the street network: nodes and segments Pollutant budget in each street Exchange at intersections Pollutant dispersion in urban geometries p. 13/29

16 . Flow and dispersion in the urban canopy Budget of pollutant mass in the street Q + Q E HWU street C street + Q s = Q source strength Q E pollutant fluxes at street intersection Q s turbulent flux through the street-atmosphere interface HWU street C street flux of pollutants through the downstream section of the street Pollutant dispersion in urban geometries p. 1/29

17 . Flow and dispersion in the urban canopy Advection along the street axis (infinite street) U street = u H f(θ ) h(h/w,z i /W), f(θ ) = cos(θ ) Soulhac L. et al., 29, "Flow in a street canyon for any external wind direction", Boundary Layer Meteorology, vol. 126, n. 3, u H cos(θ ) component of the velocity at roof level that is parallel to the street axis; H/W street aspect ratio; f(θ ) = cos(θ )+[α+(1 α)cos(θ )] z i /W adimensional roughness of the street walls. α empirical parameter correction for street of finite length Pollutant dispersion in urban geometries p. 15/29

18 . Flow and dispersion in the urban canopy Mass transfer velocity at the street-atmosphere interface Experimental study u d = α ( u U ; W H ), u d.2u (W/H = 1) U velocity difference across the shear layer at the street canyon top Salizzoni P. et al., "Street canyon ventilation and atmospheric turbulence", accepted by Atmospheric Environment SIRANE parametric model u d = σ w 2π Q s = LWσ w 2π (C street C ext ) Q s turbulent flux across the street-atmosphere interface C street and C ext concentration within and above the street; LW exchange surface; σ w vertical fluctuating velocity at roof level. Pollutant dispersion in urban geometries p. 16/29

19 . Flow and dispersion in the urban canopy Exchange model for the intersection The averaging exchange fluxes over wind direction fluctuations P i,j (θ) = f(θ θ )P i,j (θ)dθ f(θ θ ) = 1 σ θ 2π exp[ 1 2 (θ θ σ θ ) 2 ] Soulhac L. et al., 29, "Flow and dispersion in street intersections", Atmospheric Environment, vol. 3, n. 18, Pollutant dispersion in urban geometries p. 17/29

20 . Flow and dispersion above the urban canopy Gaussian model in neutral boundary layer Q C(x, y, z) = exp [ (y y c) 2 ] 2πU m σ y σ z 2σy 2 { exp [ (z z c) 2 ] + exp [ (z + z c) 2 ]} 2σ 2 z 2σ 2 z U m is the mean plume advection velocity σ y and σ z are the plume dispersion parameters y c is the plume deflection z c is the effective height of release above the ground Pollutant dispersion in urban geometries p. 18/29

21 11. SIRANE set-up Mean plume advection velocity U m (x) = C(x,y,z)U(z)dy dz C(x,y,z)dy dz = C(z)U(z)dz C(z)dz Mean plume dispersion parameters σ y = σ v t (1 + t/2tl,y ) σ z = σ w t (1 + t/2tl,z ) σ v and σ w are the fluctuating velocity components T l is the lagrangian time-scale and gives a measure of the maximum time correlation Pollutant dispersion in urban geometries p. 19/29

22 . Flow and dispersion above the urban canopy Mass fluxes at the canopy-atmosphere interface Turbulent fluxes at the top of the street canyons Vertical fluxes at street intersections Each vertical flux is modelled as a point or linear source Pollutant dispersion in urban geometries p. 2/29

23 . SIRANE set-up Model parameters An ongoing work is sensibility analysis on the model parameters Pollutant dispersion in urban geometries p. 21/29

24 5. Comparison between SIRANE and experiments The mean concentrations are expressed in a standard dimensionless form: K = CU HLH Q The moments of the horizontal and vertical concentration distributions are calculated: y c = z c = + y K(y)dy + K(y)dy σy 2 = + z K(z)dz + σz 2 = K(z)dz + (y y c) 2 K(y)dy + K(y)dy + (z z c ) 2 K(z)dz + K(z)dz Pollutant dispersion in urban geometries p. 22/29

25 5. Configuration 1 - θ = 2.5 o Within the canopy 1.5 z=h/2 exp z=h/2 sirane z=h/2 exp x=h/2 sirane K σy/η y/h Pollutant dispersion in urban geometries p. 23/29

26 5. Configuration 1 - θ = 2.5 o Above the canopy.3 z=2h exp z=2h sirane z=2h exp z=2h sirane K 2 σy/η y/h Pollutant dispersion in urban geometries p. 23/29

27 5. Configuration 1 - θ = 1 o Within the canopy. z=h/2 exp z=h/2 sirane z=h/2 exp z=h/2 sirane 36 K σ y /Η y/h Pollutant dispersion in urban geometries p. 2/29

28 5. Configuration 1 - θ = 1 o Above the canopy.3 z=2h exp z=2h sirane 8.3 z=2h exp z=2h sirane 36 K σ y /Η y/h Pollutant dispersion in urban geometries p. 2/29

29 5. Configuration 1 - θ = 25 o Within the canopy.15 z=h/2 exp z=h/2 sirane K 1 8 z=h/2 exp z=h/2 sirane σ y /Η y/h Pollutant dispersion in urban geometries p. 25/29

30 5. Configuration 1 - θ = 25 o Above the canopy.15 z=2h exp z=2h sirane K.3 8 z=2h exp z=2h sirane 2 6 σ y /Η y/h Pollutant dispersion in urban geometries p. 25/29

31 16. Configuration 1 - θ = 5 o Within the canopy.25 z=h/2 exp x=h/2 sirane K 2 1 σy/η z=h/2 exp z=h/2 sirane y/h Pollutant dispersion in urban geometries p. 26/29

32 16. Configuration 1 - θ = 5 o Above the canopy.25 z=2h exp x=2h sirane K σy/η z=2h exp z=2h sirane y/h Pollutant dispersion in urban geometries p. 26/29

33 5. Configuration 2 - θ = 2.5 o Within the canopy z=h/2 exp z=h/2 sirane z=h/2 exp z=h/2 sirane 28 K σ y /Η Pollutant dispersion in urban -12geometries p. 27/29 y/h

34 5. Configuration 2 - θ = 2.5 o Above the canopy z=2h exp z=2h sirane K z=2h exp z=2h sirane 28 σ y /Η Pollutant dispersion in urban -12geometries p. 27/29 y/h

35 5. Configuration 3 - θ = 2.5 o Within the canopy z=h/2 exp z=h/2 sirane K 2 3 z=h/2 exp z=h/2 sirane 2 σ y /Η Pollutant dispersion in urban geometries p. 28/29

36 5. Configuration 3 - θ = 2.5 o Above the canopy.3 z=2h exp z=2h sirane 8 36 z=2h exp z=2h sirane K 2 σ y /Η Pollutant dispersion in urban geometries p. 28/29

37 6. Conclusions SIRANE is able to simulate the main aspects of the dispersion within a street-network Problems in simulating the lateral diffusion for θ = 2.5 o Main disagreement observed in the far-field profiles within the canopy for θ 25 o Pollutant dispersion in urban geometries p. 29/29

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

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