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Low relative-submergence effects in a rough-bed open-channel flow

Author(s): Maxime Rouzes; Frédéric Yann Moulin; Emma Florens; Olivier Eiff

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Keywords: Double-averaging; logarithmic law; rough-wall boundary-layers; roughness sublayer; stereoscopic PIV; turbulent open-channel flows; relative submergence

Abstract: Multi-plane stereoscopic PIV measurements were performed in an open-channel flume fitted with cubes to investigate very low submergence ratios, h/k={1.5,2,3}, where h is the water depth and k the roughness height. The spatial standard deviation of the mean flow components reveals that the extent of the roughness sublayer increases drastically with the decrease in h/k to span the entire water column for the lowest h/k investigated. Despite this, the logarithmic law is still observed on the double-averaged velocity profiles for all h/k, first with a fixed von Kármán constant κ and, second, via the indicator function where κ is a free parameter. Also, the longitudinal and vertical normal stresses indicate a universal boundary layer behaviour independent of h/k. The results suggest that the logarithmic and wake-defect laws can still be applied at such low h/k. However, the lateral normal stress depends on h/k in the range investigated as well as on the geometry of the roughness pattern.


Year: 2019

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