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

  • Sophie Herpin
  • Laurent Perret
  • Romain Mathis
  • Christian Tanguy
  • Jean-Jacques Lasserre
Research Article

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 \(\delta ^+=32{,}300\). The boundary layer thickness to cube height ratio (\(\delta /h=22.7\)) 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.

Notes

Acknowledgements

The authors wish to thank the financial support of the French National Research Agency through the Research Grant URBANTURB ANR-14-CE22-0012-01, as well as Vincent Jaunet and Frank Kerhervé, assistant professors at prime laboratory in Poitiers, France, for useful advices on LDA spectra algorithms.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Sophie Herpin
    • 1
    • 2
    • 3
  • Laurent Perret
    • 1
    • 4
  • Romain Mathis
    • 5
  • Christian Tanguy
    • 6
  • Jean-Jacques Lasserre
    • 6
  1. 1.Laboratoire de recherche en HydrodynamiqueEnergétique et Environnement Atmosphérique (LHEEA UMR CNRS 6598) Ecole Centrale de NantesNantes Cedex 3France
  2. 2.Laboratoire de mécanique de Lille (LML)-Ecole Centrale de Lille, Université Lille I- Sciences et technologies, CNRS: FRE3723Arts et Métiers ParisTech-Bâtiment M6 Bvd Paul LangevinVilleneuve D Ascq CedexFrance
  3. 3.AGROCAMPUS OuestAngers CedexFrance
  4. 4.Institut de Recherche en Sciences et en Technologies de la Ville (IRSTV)-FR CNRS 2488-Ecole Centrale de Nantes Rue MassenetNantes Cedex 3France
  5. 5.Institut de Mécanique des Fluides de Toulouse, IMFT, Université de ToulouseCNRSToulouseFrance
  6. 6.DANTEC Dynamics SASNozayFrance

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