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

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, whi...

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Veröffentlicht in:Environmental fluid mechanics (Dordrecht, Netherlands : 2001) Netherlands : 2001), 2020-08, Vol.20 (4), p.739-763
Hauptverfasser: Jaroslawski, Thomas, Savory, Eric, Perret, Laurent
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Sprache:eng
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Zusammenfassung: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, λ p  = 25%), 1 h spaced two-dimensional bars ( λ p  = 50%, corresponding to the skimming-flow regime) and 3 h spaced two-dimensional bars ( λ p  = 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.
ISSN:1567-7419
1573-1510
DOI:10.1007/s10652-019-09721-w