Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation
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Author(s)
Li, Xian-Xiang
Norford, Leslie Keith
Britter, Rex E
Date Issued
February 2014
Journal
Environmental Fluid Mechanics
Publisher
Springer Netherlands
Citation
Li, Xian-Xiang, Rex E. Britter, and Leslie K. Norford. “Transport Processes in and above Two-Dimensional Urban Street Canyons under Different Stratification Conditions: Results from Numerical Simulation.” Environmental Fluid Mechanics 15.2 (2015): 399–417.
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Author's final manuscript
Abstract
Thermal stratification (neutral, unstable and stable) plays an important role in determining the transport processes in and above urban street canyons. This paper summarizes the recent findings of the effect of thermal stratification on the transport of momentum, heat, and pollutants in the two-dimensional (2D) urban street canyons in the skimming flow regime. Special attention is paid to the results from large-eddy simulations (LESs), while other experimental and numerical results are referred to when necessary. With increasing Richardson number, Ri, the drag coefficient of the 2D street canyon as felt by the overlying atmosphere decreases in a linear manner. Under neutral and stable stratification, a nearly constant drag coefficient of 0.02 is predicted by the LESs. Under unstable stratification, the turbulent pollutant transport is dominated by organized turbulent motions (ejections and sweeps), while under stable stratification, the unorganized turbulent motion (inward interactions) plays a more important role and the sweeps are inhibited. The unstable stratification condition also enhances the ejections of turbulent pollutant flux, especially at the leeward roof-level corner, where the ejections dominate the turbulent pollutant flux, outweighing the sweeps. With increasing Ri, both the heat (area active scalar source) and pollutant (line passive scalar source) transfer coefficients decrease towards a state where the transfer coefficients become zero at Ri≈0.5. It should be noted that, due to the limit of the 2D street canyon configuration discussed in this paper, great caution should be taken when generalising the conclusions drawn here.
MIT Department
Massachusetts Institute of Technology. Department of Architecture
Massachusetts Institute of Technology. Department of Urban Studies and Planning
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DOI of Published Version
https://doi.org/10.1007/s10652-014-9347-2
