1. 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

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

3. 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

4. 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)

5. 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

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

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

8. LES Model Description
Domain of h=1, b=15 (AR=0.0667), 11 (0.0909), 4 (0.25), 3 (0.3333), 2 (0.5), (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

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

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

11. 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

12. AR =

13. AR =

14. AR = 0.25

15. AR = 0.5

16. AR = 1

17. AR = 2

18. Conclusions
The friction factor, ventilation and pollutant removal are investigated as a function of ARs
Friction factor attains its maximum at AR
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

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