Boundary-Layer Meteorology

, Volume 18, Issue 4, pp 373–397

A wind-tunnel study of turbulent flow close to regularly arrayed rough surfaces

  • M. R. Raupach
  • A. S. Thom
  • I. Edwards
Article

DOI: 10.1007/BF00119495

Cite this article as:
Raupach, M.R., Thom, A.S. & Edwards, I. Boundary-Layer Meteorol (1980) 18: 373. doi:10.1007/BF00119495

Abstract

Recent observations of flux-gradient anomalies in atmospheric flow close to forests, and similar rough surfaces, prompted a wind-tunnel investigation in which cross-wire anemometry was used to study the vertical development and horizontal variability of adiabatic flow over five regularly arrayed rough surfaces, encompassing a 32-fold range of roughness concentration λ. The roughness elements were cylinders, 6 mm in both height and diameter.

Below a layer in which the velocity profile is semi-logarithmic, two surface influences upon the mean velocity field can be distinguished: wake diffusion and horizontal inhomogeneity. The wake diffusion effect causes non-dimensional vertical velocity gradients to be smaller than in the semi-logarithmic region; at least for elements with aspect ratios l/h ≲ 1, it is governed by the transverse dimension l of the roughness elements, and is observed when z > h + 1.5l (where z is height above the underlying surface, and h is the height of the roughness elements). A simple diffusivity model successfully describes the horizontally averaged velocity profiles in the region of wake influence, despite conceptual disadvantages. The horizontal inhomogeneity of the flow is negligible when z > h + D (D being the inter-element spacing), and does not entirely mask the wake diffusion effect except over very sparsely roughened surfaces (λ ≲ 0.02). A criterion for negligibility of both effects, and hence for applicability of conventional turbulent diffusivity theory for momentum, is z > h + 1.5D. These results are compared with atmospheric data, and indicate that wake diffusion may well cause some underestimation of the zero-plane displacement d over typical vegetated surfaces.

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Copyright information

© D. Reidel Publishing Co 1980

Authors and Affiliations

  • M. R. Raupach
    • 1
  • A. S. Thom
    • 2
  • I. Edwards
    • 3
  1. 1.CSIRO Division of Environmental MechanicsCanberraAustralia
  2. 2.Department of MeteorologyUniversity of EdinburghEdinburghUK
  3. 3.Royal Naval Engineering CollegePlymouthUK

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