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Roof-level large- and small-scale coherent structures in a street canyon flow

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Abstract

The characteristics of large- and small-scale turbulent motions at roof-level in a street canyon flow were experimentally investigated along with their spatio-temporal organization and their mutual interaction in this region. Quadrant analysis was conducted to identify sweep and ejection events, whilst a spanwise spatial filter was used to identify the large-scale motions in the flow. The present analysis was conducted for six configurations; three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR = W/h = 1 and 3). The upstream roughness arrays consisted of three-dimensional cubes (plan area density, λp = 25%), 1h spaced two-dimensional bars (λp = 50%, corresponding to the skimming-flow regime) and 3h spaced two-dimensional bars (λp = 25%, corresponding to the wake-interference flow regime). It was found that the roughness configuration has a strong effect on the size of the quadrant events in the street canyon, with the wake-interference flow regimes producing significantly larger sweep and ejection events than the skimming-flow regimes. Upstream wake-interference flow regimes were found to have a significantly greater temporal correlation than in the skimming-flow regimes. Large-scale streamwise and spanwise velocity components were found to have large spatio-temporal correlation integral scales, in agreement with the large-scale structures existing in the overlying boundary layer. It was also confirmed that there is a coupling between large- and small-scales at roof-level of the street canyon. While sweep and ejection events were found to be due to the dynamical contribution of the small-scales, their occurrence is shown to be coupled with that of large-scale low- and high-momentum regions, respectively.

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Acknowledgements

The authors would like to thank Mr. Thibaut Piquet for his technical support during the experimental program, Dr. Karin Blackman the Douglas Muzyka Scholarship (The University of Western Ontario, Canada) and the Natural 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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Authors and Affiliations

  1. Department of Mechanical and Materials Engineering, University of Western Ontario, London, Canada

    Thomas Jaroslawski & Eric Savory

  2. LHEEA, UMR CNRS 6598, Centrale Nantes, Nantes, France

    Laurent Perret

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  1. Thomas Jaroslawski
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  2. Eric Savory
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  3. Laurent Perret
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Correspondence to Thomas Jaroslawski.

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Jaroslawski, T., Savory, E. & Perret, L. Roof-level large- and small-scale coherent structures in a street canyon flow. Environ Fluid Mech 20, 739–763 (2020). https://doi.org/10.1007/s10652-019-09721-w

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