Effects of Buildings Layout on the Flow and Pollutant Dispersion in Non-uniform Street Canyons ZHANG Yunwei, PhD candidate May 11, 2010, Xi an, Shannxi Province, China E_mail: zhangyunwe@gmail.com 1
Contents: Introduction Methods Results Conclusions 2
Introduction Simplifications in street canyon studies: Simplification Real street canyon: non-uniform A street canyon model in wind tunnel study: uniform 3
Introduction What s the previous studies focus on? Wind flow (velocity magnitude, direction); Aspect ratio (build-height height-to to-street street-width); Building roof shape; Vehicle induced turbulence; Tree plantings and so on. 4
Introduction Studies of non-uniform street canyons No study on building layout effects Field measurements: Li X.L. et al. (2007), vertical variations of PM 2.5 and CO, step-up notch. Brown M.J. et al., horizontal nature of wind flow, with focus on inflow wind Direction effect. Schatzmann M. et al., temporal variation on point.. Wind tunnel simulations: Schatzmann M. et al., pollutant temporal variation on point. Klein P.K. and Rotach M. W., vertical profiles of wind on different points.. 5 Numerical simulations: Baik J.J. and Park R.S. simulated the flows in stepup/ down notch..
Introduction What s the goal? Try to study the effects of building layout on wind flow and pollutant dispersions in non-uniform street canyons. Including: establishing the non-uniform street canyon model; determining boundary conditions; studying building layout effects on CFD simulations. 6
Methods: Non-uniform street canyon model: y x street Tall building (H2=45m) Low building (H1=30m) Computational domain z b2 b1 y 1.5H1 street H2 H1 (a) (b) Figure 1. Schematic diagram of structures of non-uniform street canyon model; (a) top view, (b) side view of the computational domain. 7
Methods: Character of the non-uniform street canyon: Buildings are of different heights; Containing step-up and step-down notch simultaneously. 8
Methods: Studying cases: building layout structures y x Wind direction street street street street (a) apart; (b) adjacent; (c) overlapped; (d) uniform. Figure 2. Different building layout structures of street canyons studied. 9
Methods: Numerical methods Large-eddy simulation for turbulence flow and scalar transport for pollutant dispersion. The resolved-scale dynamic equations of the mathematical model were solved by the Finite Volume Method (FVM), with the SIMPLE algorithm used to deal with the implicit dependence of velocity and pressure. Boundary conditions Inlet-and-outlet boundary: symmetrically periodic (inlet velocity at y=l/2+ Δy corresponding to outlet velocity at y=l/2-δy, where Δy is the distance from the grid point to the middle plan in y direction. The top boundary is set to be free. Non-slip wall conditions are used at all solid walls, with wall functions. A line source located at the center of the canyon is used to represent the vehicle emissions. 10
Methods: Inlet-and-outlet boundary: symmetrically periodic condition y x outlet Wind direction Δy Δy L middle plane of the computational domain in y direction (y=l/2). inlet street Periodic boundary condition: u(i, inlet, y, z) u(i, outlet, y, z) The current symmetrically periodic condition: u(i, inlet, L/2 - Δy, z) u(i, outlet, L/2 + Δy, z) u(i, inlet, L/2 + Δy, z) u(i, outlet, L/2 - Δy, z) i range (1,2,3) to present the wind velocity in (x, y, z) directions. 11
Results: Statistics are carried out when then flow fully developed for 1000 time steps (20s). Pollutant concentrations are normalized by: C = C U ref Q / L H 1 where C* is the normalized pollutant concentrations, C is the resolved pollutant concentration, U ref is the reference velocity, and Q is the pollutant emission rate (in μg. s -1 ), L is the street length. 12
Results: Wind flow Simulated by the adjacent case. Figure 3. Typical stream line in non-uniform street canyons. 13
Results: H2 H1 1 20 18 16 14 12 10 8 6 4 2 1 45 40 35 30 25 20 15 10 5 1 25 22 19 16 13 10 7 4 1 z y b2 b1 b2 b1 b2 b1 (a) apart; (b) adjacent; (c) overlapped. Figure 4. Velocity vector and normalized pollutant concentration contours near the leeward wall. 14
Results: 1 20 18 16 14 12 10 8 6 4 2 1 45 40 35 30 25 20 15 10 5 y x (a) apart; 1 25 22 19 16 13 10 7 4 1 (b) adjacent; Tall buildings Low buildings (c) overlapped. Figure 5. Velocity vector and normalized pollutant concentration contours at the pedestrian level. 15
Results: Normalized concentration 70 60 50 40 30 20 10 average concentration maximum concentration 0 uniform case apart case adjacent case overlap case 1 2 3 4 Figure 6. Average and maximum normalized concentrations at the pedestrian level. 16
Conclusions: Spirally circulation flows dominate inside non-uniform street canyons. Pollution is mitigated in non-uniform street canyons. In urban planning, building layouts along the street canyon should be considered, where regional prevailing wind transverse the street canyon. Tall buildings apart with each other along the street is the best choice. 17
Thank you! 18