Large-Eddy Simulation of Flows and Pollutant Removal in Street
Canyons of Different Aspect Ratios
Tracy N.H. Chung & Chun-Ho LiuMarch 4, 2011
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Rundown• Objectives• Introduction• Equations
– Friction Factor– Air Exchange Rate (ACH)– Pollutant Exchange Rate (PCH)
• Model Description• CFD Results• Conclusions
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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
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Introduction• Flow regimes (Oke, 1988)
a) Isolated roughness regime (AR < 0.3)
b) Wake interference regime (0.3 < AR < 0.7) c) Skimming regime (0.7 < AR)
h
b
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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
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Friction Factor
• Friction Factor– A measure of the force to sustain a certain mean
flow
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21/
221 U
DexP
Uf
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ACH and PCH
Air Exchange Rate (ACH)•
– Mean ACH
– Turbulent ACH
Pollutant Exchange Rate (PCH)•
– Mean PCH
– Turbulent PCH
"ACHACHACH "PCHPCHPCH
dxwACH
dxwwACH ""21" dxcwPCH """
dxcwPCH
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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)
Inlet
(periodic flow & zero concentration)
Outlet
(periodic flow & open condition for pollutant)
Front (periodic)
Back(periodic)
Top(symmetry)
5h
h
b
z
x
y
5h
0.5h
Constant/Uniform Concentration
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Friction Factor
• Compared with experimental results– J.C. Han (1984)– Mainly focused on the isolated roughness regime
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ACH
• Compared with k-ε turbulence results• ACH decreases with ARs• Turbulence >> Mean
Solid: LESHollow: k-ε
Circle: MeanDiamond: TurbulenceSquare: Total
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PCH
• Compared with k-ε turbulence and experimental solutions• PCH does not have the same pattern with ACH• Turbulence >> Mean
Solid: LESHollow: k-ε
Circle: MeanDiamond: TurbulenceSquare: Total
AR = 0.0667
AR = 0.0909
AR = 0.25
AR = 0.5
AR = 1
AR = 2
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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
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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