The Spanwise Variation of Roof-Level Turbulence in a Street-Canyon Flow
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The effect of upstream roughness and canyon width on turbulent street-canyon flow is investigated, using wind-tunnel measurements made in a horizontal plane at near roof level of a street canyon and stereoscopic particle image velocimetry. Three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR = W/h = 1 and 3) are used; the arrays consist of three-dimensional cubes (plan area density, λp = 25%), 1h-spaced two-dimensional bars (skimming flow, λp = 50%) and 3h-spaced two-dimensional bars (wake-interference flow, λp = 25%). Understanding the spanwise structure of the flow and how it interacts with large-scale structures is necessary to reliably predict the mean pollutant transport in the lateral direction along the canyon and to further investigate the three-dimensional behaviour of turbulent street-canyon flows. The mean turbulent statistics are presented, whilst two-point correlations and integral length scales are computed for the different configurations. The results show a significant effect of upstream roughness on these quantities. The total turbulent kinetic energy and shear stress are found to be highest for the wake-interference flow regimes and lowest for the skimming-flow regimes. It is found that the three-dimensional upstream roughness configurations result in a significantly weaker correlation in the spanwise direction at canyon roof level, with a similar trend observed in the spanwise integral length scales. The shear-layer thickness is found to be related to the magnitude of the correlations near roof level of the street canyon.
KeywordsBoundary layer Particle image velocimetry Spanwise turbulence correlations Street canyon Wind tunnel
We thank Mr. Thibaut Piquet for his technical support during the experimental program, the Douglas Muzyka Scholarship (The University of Western Ontario, Canada) and the National Sciences and Engineering Research Council (NSERC) of Canada for providing funding. Financial support of the French National Research Agency through the research Grant URBANTURB No. ANR-14-CE22-0012-01 is also acknowledged.
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