Influence of the plan density of a cube-based canopy on the structure of the lower atmospheric boundary layer
Résumé
An experimental investigation of the structure of the boundary-layer developing over an urban-like rough wall is performed considering wall configurations that represent three idealised urban terrains. Both stereoscopic particle image velocimetry and two-point hot-wire data are investigated through one- and two-dimensionnal spectral analysis. It is shown that the flow characteristics within the inertial layer appear to be independent from the wall configuration. This region is populated with coherent structures of the same type as those found in smooth-wall flows, namely very large-scale motions and large-scale motions, the later showing self-similar features. These coherent structures, whose footprint is mainly visible on the streamwise and spanwise velocity components, also appear to be present in the roughness sublayer. Within the roughness sublayer, the influence of the canopy density shows in the characteristics
of the wall-normal velocity component w. The most energetic scales in the auto-spectra of w and in the co-spectra between u and w are indeed of the same order of magnitude as the roughness obstacle size and decrease with increasing density. Instead of varying progressively with the packing density, they appear to be constrained by the presence of the canopy for the densest investigated configurations consistently with the change of the flow regime reported in the literature.
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