Turbulence in rough-wall boundary layers: universality issues |
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Authors: | Mohammad Amir Ian P Castro |
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Institution: | (1) School of Engineering Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, UK;(2) Present address: Department of Engineering, University of Aberdeen, Aberdeen, AB24 3FX, UK; |
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Abstract: | Wind tunnel measurements of turbulent boundary layers over three-dimensional rough surfaces have been carried out to determine
the critical roughness height beyond which the roughness affects the turbulence characteristics of the entire boundary layer.
Experiments were performed on three types of surfaces, consisting of an urban type surface with square random height elements,
a diamond-pattern wire mesh and a sand-paper type grit. The measurements were carried out over a momentum thickness Reynolds
number (Re
θ) range of 1,300–28,000 using two-component Laser Doppler anemometry (LDA) and hot-wire anemometry (HWA). A wide range of
the ratio of roughness element height h to boundary layer thickness δ was covered (0.04 £ h/d £ 0.400.04 \leq h/\delta \leq 0.40). The results confirm that the mean profiles for all the surfaces collapse well in velocity defect form up to surprisingly
large values of h/δ, perhaps as large as 0.2, but with a somewhat larger outer layer wake strength than for smooth-wall flows, as previously
found. At lower h/δ, at least up to 0.15, the Reynolds stresses for all surfaces show good agreement throughout the boundary layer, collapsing
with smooth-wall results outside the near-wall region. With increasing h/δ, however, the turbulence above the near-wall region is gradually modified until the entire flow is affected. Quadrant analysis
confirms that changes in the rough-wall boundary layers certainly exist but are confined to the near-wall region at low h/δ; for h/δ beyond about 0.2 the quadrant events show that the structural changes extend throughout much of the boundary layer. Taken
together, the data suggest that above h/δ ≈ 0.15, the details of the roughness have a weak effect on how quickly (with rising h/δ) the turbulence structure in the outer flow ceases to conform to the classical boundary layer behaviour. The present results
provide support for Townsend’s wall similarity hypothesis at low h/δ and also suggest that a single critical roughness height beyond which it fails does not exist. For fully rough flows, the
data also confirm that mean flow and turbulence quantities are essentially independent of Re
θ; all the Reynolds stresses match those of smooth-wall flows at very high Re
θ. Nonetheless, there is a noticeable increase in stress contributions from strong sweep events in the near-wall region, even
at quite low h/δ. |
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