The turbulent flow inside an idealized urban street canyon with an aspect ratio of one is studied by means of large-eddy simulation. The Regional Atmospheric Modelling System is configured to simulate the turbulent flow in a neutrally stratified atmosphere with the initial wind perpendicular to the street canyon axis. The mean velocity components, resolved-scale turbulent kinetic energy (RS-TKE), the skewness and kurtosis of the resolved-scale velocity components (u along the canyon and w vertically) are compared with wind-tunnel measurements. The comparison indicates that a reasonable agreement is achieved. The simulation slightly underestimates the intensity of the primary eddy. It is found that distribution of the RS-TKE is very asymmetric: high in the vicinity of the downstream wall, and uniformly low in the vicinity of the upstream wall. The analyses of skewness and kurtosis indicate that there is a layer just below the rooftop in the canyon where ejection events dominate. Quadrant analysis of resolved-scale velocity fluctuations, u′ and w′, under the rooftop at the centre of the canyon reveals that the exchange of momentum across the canyon top is contributed unevenly by different events. Weak ejection events dominate the frequency of occurrences, but fewer strong sweep events contribute the majority of the total momentum transfer. The features of momentum transfer are further investigated by analysing the spatial–temporal variations of u′, w′,and u′w′ at the roof level. It is found that the variation of these variables is highly intermittent and is associated with multi-scale turbulent events. The period of eddies containing high RS-TKE is attributed to the Kelvin–Helmhotz instabilities. These results improve our understanding of the turbulent structure in street canyon flow. Copyright © 2004 Royal Meteorological Society
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