Large-Eddy Simulation of Flows and Pollutant Removal in Street Canyons of Different Aspect Ratios Tracy N.H. Chung & Chun-Ho Liu March 4, 2011

Rundown Objectives Introduction Equations Model Description Friction Factor Air Exchange Rate (ACH) Pollutant Exchange Rate (PCH) Model Description CFD Results Conclusions

Objectives Study the pressure drop characteristics across the street canyon Determine the ventilation performance with respect to building-to-street-width (aspect) ratios (AR) h/b Investigate the pollutant removal performance with respect to aspect ratios

Introduction Flow regimes (Oke, 1988) h b a) Isolated roughness regime (AR < 0.3) h b b) Wake interference regime (0.3 < AR < 0.7) c) Skimming regime (0.7 < AR)

Computational Fluid Dynamics (CFD) Large-Eddy Simulation (LES) OpenFOAM 1.6 Open source code One subgrid-scale TKE equation Parallel Computing in Gridpoint and HPCPower2 (HKU) The job is divided into smaller parts to speedup Multiple processors work together in parallel such that the execution is speeded up by a multiple of the number of processors

Friction Factor Friction Factor A measure of the force to sustain a certain mean flow

ACH and PCH Air Exchange Rate (ACH) Pollutant Exchange Rate (PCH) Mean ACH Turbulent ACH Mean PCH Turbulent PCH

LES Model Description Domain of h=1, b=15 (AR=0.0667), 11 (0.0909), 4 (0.25), 3 (0.3333), 2 (0.5), 1.6666(0.6), 1.25 (0.8), 1 (1), 0.5 (2) Top (symmetry) Back (periodic) 5h Front (periodic) Outlet (periodic flow & open condition for pollutant) Inlet (periodic flow & zero concentration) 5h z x y 0.5h h b Constant/Uniform Concentration

Friction Factor Compared with experimental results J.C. Han (1984) Mainly focused on the isolated roughness regime

ACH Compared with k-ε turbulence results ACH decreases with ARs Turbulence >> Mean Solid: LES Hollow: k-ε Circle: Mean Diamond: Turbulence Square: Total

PCH Compared with k-ε turbulence and experimental solutions PCH does not have the same pattern with ACH Turbulence >> Mean Solid: LES Hollow: k-ε Circle: Mean Diamond: Turbulence Square: Total

AR = 0.0667

AR = 0.0909

AR = 0.25

AR = 0.5

AR = 1

AR = 2

Conclusions The friction factor, ventilation and pollutant removal are investigated as a function of ARs Friction factor attains its maximum at AR 0.0909 Ventilation is better with a wider street width Pollutant removal is not in line with the ventilation performance and AR 0.5 is found to be the most effective Turbulence is more significant in ventilation and pollutant removal than mean

Acknowledgment This project is partly supported by the General Research Fund of the Hong Kong Research Grant Council HKU 715209E Thank you!! Q & A Session