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Accueil > Équipes > Equipe MEIGE : Modélisation, Expériences et Instrumentation pour la Géophysique et l’Environnement > Diffusion scientifique > Séminaires internes

Experimental investigation of sediment transport in sheet-flow conditions

Thibaud Revil-Baudard, 28/11/2013

Abstract :

In order to better understand the dominant physical mechanisms in intense bed-load transport (sheet flow) and the effect of the mobile layer on the turbulence (damping in the dense layer and modification in the dilute suspended layer), experiments have been performed over fixed rough bed and in sheet-flow conditions. These experiments have been performed in the LEGI open channel using an acoustic profiler together with high frequency digital images obtained from the side wall. The acoustic profiler allows to get colocalized streamwise and wall-normal components of velocity with a spatial resolution of 3mm and a sampling frequency of 75Hz over the entire water column. The digital images are acquired at 150Hz and are analyzed using a videotrajectography technique to get the mean particles velocity profile (collab. P. Snabre). This data set permits to get statistically converged mean velocity, turbulent Reynolds shear stress and turbulent intensities profiles.

Concerning the results, we first present an analysis of the mean profiles bringing out the principal mechanisms acting from the fixed bed to the dilute suspension zone. Our results show that a logarithmic layer is observed and that the associated mixing length is highly affected by the presence of the mobile bed (reduction of the Von Karman constant by a factor of two). Also, below this logarithmic layer the momentum diffusion coefficient scales with the particle diameter and strongly increases downward down to the fixed bed showing that the inter granular stresses dominate in this layer. Second, a quadrant analysis of the velocity fluctuations is presented. Sweeps strongly dominate ejections close to the bed in sheet-flow conditions whereas they have more or less the same intensity over fixed rough bed conditions. Also, interactions are much more important in sheet-flow conditions compared with fixed bed ones (factor of two) leading to a decrease of the Reynolds stresses.