Large-Eddy Simulation of Flow and Scalar Transport in a Modeled Street Canyon

Chun-Ho Liu National Center for Atmospheric Research, Boulder, Colorado*

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Mary C. Barth National Center for Atmospheric Research, Boulder, Colorado*

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Abstract

This study uses large-eddy simulation (LES) to illustrate the flow and turbulence structure and to investigate the mechanism of passive scalar transport in a street canyon. Calculations for a modeled street canyon with building-height-to-street-width ratio of unity at Reynolds number equal to 12 000 are conducted. When the approaching wind is perpendicular to the street axis, the calculation produces a primary vortex in the street canyon, similar to previous studies. An evaluation of the LES results with wind-tunnel measurements reveals good agreement for both mean and turbulence parameters of the flow and scalar fields. The computed primary vortex is confined to the street canyon and is isolated from the free stream flow such that the removal of a scalar emitted at the street level is accomplished by turbulent diffusion at the roof level. It is determined from the calculations that very little scalar is removed from the street canyon, and 97% of the scalar is retained. The scalar mixing at the roof level occurs primarily on the leeward side of the street canyon. In addition to the primary vortex, three secondary vortices are located in the corners of the street canyon at which scalar mixing is enhanced. An examination of additional simulations shows how the location of the scalar source affects the distribution of the scalar.

Corresponding author address: Chun-Ho Liu, Department of Mechanical Engineering, The University of Hong Kong, 7F Haking Wong Bldg., Pokfulam Rd., Hong Kong. liuchunho@graduate.hku.hk.

Abstract

This study uses large-eddy simulation (LES) to illustrate the flow and turbulence structure and to investigate the mechanism of passive scalar transport in a street canyon. Calculations for a modeled street canyon with building-height-to-street-width ratio of unity at Reynolds number equal to 12 000 are conducted. When the approaching wind is perpendicular to the street axis, the calculation produces a primary vortex in the street canyon, similar to previous studies. An evaluation of the LES results with wind-tunnel measurements reveals good agreement for both mean and turbulence parameters of the flow and scalar fields. The computed primary vortex is confined to the street canyon and is isolated from the free stream flow such that the removal of a scalar emitted at the street level is accomplished by turbulent diffusion at the roof level. It is determined from the calculations that very little scalar is removed from the street canyon, and 97% of the scalar is retained. The scalar mixing at the roof level occurs primarily on the leeward side of the street canyon. In addition to the primary vortex, three secondary vortices are located in the corners of the street canyon at which scalar mixing is enhanced. An examination of additional simulations shows how the location of the scalar source affects the distribution of the scalar.

Corresponding author address: Chun-Ho Liu, Department of Mechanical Engineering, The University of Hong Kong, 7F Haking Wong Bldg., Pokfulam Rd., Hong Kong. liuchunho@graduate.hku.hk.

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