Time-evolution of the Volume- Averaged Concentration Within Urban - PowerPoint PPT Presentation

Time-evolution of the Volume- Averaged Concentration Within Urban Street Canyons Validation and Parametrization of the One-box Model B. Conan, L. Perret, LHEEA Centrale Nantes – CNRS, Nantes, France E. Savory Dpt. Mech. Eng, Western University, London, ON, Canada August 2018 10th ICUC / 14th SUE 1

Ventilation of urban canopy Urban air quality is a major health issue for our generation and – the next, Understanding and modeling mechanisms of turbulent transport – of passive scalar in urban environment is still a scientifjc challenge, Simplifjed approaches such as “box models” are used in – operational conditions, Fine studies of simplifjed street canyon ventilation contributes to – understanding the physics and evaluating simplifjed approaches. August 2018 10th ICUC / 14th SUE 2

Box models for dispersion modeling dC c ( t ) =− 1 τ c [ C C ( t )− C abl ] Caracteristic time τ c depends on: – dt Mean fmow ● Canyon geometry ● C c volume-averaged canyon concentration Inlet conditions ? ● (assumes well mixed pollutant in the canyon) What parameter to evaluate τ c ? C abl concentration of the incoming ABL August 2018 10th ICUC / 14th SUE 3

Exchange velocity dC c ( t ) τ c [ C C ( t )− C abl ] =−¯ =− 1 E h [ C C ( t )− C abl ] dt In many studies, E = α U e is used (Johnson, 1973; Berkowicz, 2000; Soulhac, 2000 ; Salizzoni, 2009) Image from Weitbrecht (2008). In the study of « dead-zones » in river groynes Weitbrecht (2008) used E , the exchange velocity as a parameter for the emptying: W W / 2 E ( t ) = 1 W ∫ | w ( x, z = h,t ) | dx h −W / 2 Weitbrecht (2008) observed that E/U e has a linear relation with the hydraulic radius R D : R D = hW ( h + W ) August 2018 10th ICUC / 14th SUE 4

Application to street canyons & infmuence of infmow conditions Inspired by Weitbrecht (2008), Perret et al. (2017) applied the exchange velocity to urban street canyons to study the infmuence of infmow conditions on E. Perret (2017). Blackman (2015) August 2018 10th ICUC / 14th SUE 5

Application to street canyons & infmuence of infmow conditions Perret et al. (2017) proposed a modifjed hydraulic radius including d parameter to account for infmow conditions in the estimation of E: R H = ( h + h − d ) W ~ R H = hW ( h + h − d )+ W h + W Closed symbols: W/h = 1, open symbols: W/h = 3. Upstream roughness: 1 h bars (red squares), 3 h bars (blue circles) and 25% cubes (purple triangles). Data for river groynes of Weitbrecht et al. (2008) (brown plus signs) and canonical cavities of Chang et al. (2006) (orange stars). August 2018 10th ICUC / 14th SUE 6

Questions Does the 3D space-average concentration in a urban ● street canyon follow an exponential decay? Is the exchange velocity E a wise candidate to ● parametrize exponential decay? August 2018 10th ICUC / 14th SUE 7

Canyon dispersion simulation Numerical set-up (LES) W = 1 h (x, y, z) => (6h, L = 4h, H = 4h) ● W = 0.5 h (x, y, z) => (4.5h, L = 4h, H = 4h) ● (Δx, Δy, Δz) = (h/48, h/48, h/48) up to z = h/2 ● Cyclic boundary conditions in x and y (except for C in x) ● Sc = 0,71 ● Re = 6 500 ● C = 1 inside one canyon ● C = 0 elsewhere ● 7 to 15 M cells ● Scalar release after 160T ● August 2018 10th ICUC / 14th SUE 8

Canyon dispersion simulation - fmow validation u κ/ u ∗ √ ( | u' w ' | )/ u ∗ √ ( k )/ u ∗ - fmow inside the canyon August 2018 10th ICUC / 14th SUE 9

Exponential decay is observed August 2018 10th ICUC / 14th SUE 10

U e parameters works well for the confjgurations tested Adaptation for 3D W / 2 L / 2 ● E ( t ) = 1 W L ∫ ∫ | w ( x, y ,z = h,t ) | dx dy −W / 2 − L / 2 Results for W=0.5h (black square) and W=1h (black triangle) ● August 2018 10th ICUC / 14th SUE 11

Conclusions – Time-evolution of the volume-average concentration in a canyon follows an exponential decay for W/h=0.5h W/h=1 ratio, – The exchange velocity proposed by Weitbrecht (2008), and adapted in 3D seems to work well for street canyons of 1h and 0.5h, → is it true for higher W/h ratios? August 2018 10th ICUC / 14th SUE 12

Acknowledgement & References This study was performed with the fjnancial support from the French National Research Agency through research grant URBANTURB no. ANR-14-CE22-0012-01. This work was granted access to the HPC resources of supercomputer LIGER under the allocation 2017-E1703020 from Ecole Centrale de Nantes Berkowicz, R., Nov 2000. Ospm - a parameterised street pollution model. Environmental Monitoring and Assessment 65 (1), 323–331. Blackman, K., Perret, L., Savory, E., Aug 2015. Efgect of upstream fmow regime on street canyon fmow mean turbulence statistics. Environmental Fluid Mechanics 15 (4), 823–849. Johnson, W., Ludwig, F ., Dabberdt, W., Allen, R., 1973. An urban difgusion simulation model for carbon monoxide. Journal of the Air Pollution Control Association 23 (6), 490–498. Perret, L., Blackman, K., Fernandes, R., Savory, E., 2017. Relating street canyon vertical mass-exchange to upstream fmow regime and canyon geometry. Sustainable Cities and Society 30 (Supplement C), 49 – 57. Salizzoni, P ., Soulhac, L., Mejean, P., 2009. Street canyon ventilation and atmospheric turbulence. Atmospheric Environment 43, 5056– 5067. Soulhac, L., 2000. Modêlisation de la dispersion atmosphêrique á l’intêrieur de la canopêe urbaine. Ph.D. thesis, Ecole Centrale de Lyon. Weitbrecht, V., Socolofsky, S. A., Jirka, G. H., 2008. Experiments on mass exchange between groin fjelds and main stream in rivers. Journal of Hydraulic Engineering 134 (2), 173–183. August 2018 10th ICUC / 14th SUE 13