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Large-Eddy Simulation of Flow and Pollution in Street Canyons

Explore pressure drop & ventilation performance in street canyons of different aspect ratios using Large-Eddy Simulation. Analyze pollutant removal effectiveness, friction factor, air & pollutant exchange rates. Research conducted with OpenFOAM 1.6, focusing on different flow regimes.

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Large-Eddy Simulation of Flow and Pollution in Street Canyons

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  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 • 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) 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 Gridpointand 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 PCH Turbulent PCH • Mean ACH • Turbulent ACH

  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), 1.6666(0.6), 1.25 (0.8), 1 (1), 0.5 (2) Top(symmetry) Back(periodic) 5h Front (periodic) z Outlet (periodic flow & open condition for pollutant) Inlet(periodic flow & zero concentration) y x 5h 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 Solid: LES Hollow: k-ε Circle: Mean Diamond: Turbulence Square: Total • Compared with k-ε turbulence results • ACH decreases with ARs • Turbulence >> Mean

  11. PCH • Compared with k-ε turbulenceand 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 = 0.0667

  13. AR = 0.0909

  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 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

  19. 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

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