Turbulence in rough-wall boundary layers: universality issues

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