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Investigation of the flow inside an urban canopy immersed into an atmospheric boundary layer using laser Doppler anemometry

Herpin, Sophie and Perret, Laurent and Mathis, Romain and Tanguy, Christian and Lasserre, Jean-Jacques Investigation of the flow inside an urban canopy immersed into an atmospheric boundary layer using laser Doppler anemometry. (2018) Experiments in Fluids, 59 (5). 2-24. ISSN 0723-4864

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Official URL: http://doi.org/10.1007/s00348-018-2532-1

Abstract

Laser Doppler anemometry (LDA) is used to investigate the flow inside an idealized urban canopy consisting of a staggered array of cubes with a 25% density immersed into an atmospheric boundary layer with a Reynolds number of δ+=32,300. The boundary layer thickness to cube height ratio (δ/h=22.7) is large enough to be representative of atmospheric surface layer in neutral conditions. The LDA measurements give access to pointwise time-resolved data at several positions inside the canopy (z=h/4, h/2, and h). Synchronized hot-wire measurements above the canopy (inertial region and roughness sublayer) are also realized to get access to interactions between the different flow regions. The wall-normal mean velocity profile and Reynolds stresses show a good agreement with available data in the literature, although some differences are observed on the standard deviation of the spanwise component. A detailed spectral and integral time scale analysis inside the canopy is then carried out. No clear footprint of a periodic vortex shedding on the sides of the cubes could be identified on the power spectra, owing to the multiple cube-to-cube interactions occuring within a canopy with a building density in the wake interference regime. Results also suggest that interactions between the most energetics scales of the boundary layer and those related to the cube canopy take place, leading to a broadening of the energy peak in the spectra within the canopy. This is confirmed by the analysis of coherence results between the flow inside and above the canopy. It is shown that linear interactions mechanisms are significant, but reduced compared to smooth-wall boundary-layer flow. To our knowledge, this is the first time such results are shown on the dynamics of the flow inside an urban canopy.

Item Type:Article
Audience (journal):International peer-reviewed journal
Uncontrolled Keywords:
Institution:French research institutions > Centre National de la Recherche Scientifique - CNRS (FRANCE)
Other partners > Ecole Centrale de Nantes (FRANCE)
Université de Toulouse > Institut National Polytechnique de Toulouse - Toulouse INP (FRANCE)
Université de Toulouse > Université Toulouse III - Paul Sabatier - UT3 (FRANCE)
Other partners > Université de Nantes (FRANCE)
Other partners > Dantec Dynamics (FRANCE)
Other partners > Université d'Angers (FRANCE)
Other partners > Université de La Rochelle (FRANCE)
Laboratory name:
Funders:
French National Research Agency
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Deposited On:06 Jul 2018 08:19

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