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