Investigation of sheet-flow processes based on novel acoustic high-resolution velocity and concentration measurements

Abstract

International audienceA new dataset of uniform and steady sheet flow experiments is presented in this paper. An Acoustic Concentration and Velocity Profiler (ACVP) is used to measure time-resolved profiles of collocated 2C velocity (u,w) and sediment concentration and to measure the time evolution of the bed interface position. Ensemble averaging over eleven similar experiment realisations is done to evaluate the mean profiles of streamwise velocity, concentration , sediment flux and Reynolds shear stress. The repeatability, stationarity and uniformity of the flow are carefully checked for a Shields number θ ≈ 0.5 and a suspension number of S = 1.1. The mean profile analysis allows to separate the flow into two distinct layers: a suspension layer dominated by turbulence and a bed layer dominated by granular interactions. The bed layer can be further subdivided into a frictional layer capped by a collisional layer. In the suspension layer, the mixing length profile is linear with a strongly reduced von Karman parameter equal to 0.225. The Schmidt number is found to be constant in this region with a mean value of σ s = 0.44. The present results are then interpreted in terms of existing modelling approaches and the underlying assumptions are discussed. In particular, the well-known Rouse profile is shown to predict the concentration profile adequately in the suspension layer provided that all the required parameters can be evaluated separately. However, the strong intermittency of the flow observed in the bed layer under the impact of turbulent large-scale coherent flow structures suggests the limitations of averaged steady two-phase flow models

SedFoam-2.0: a 3-D two-phase flow numerical model for sediment transport

“…where κ is the von Karman constant and the exponent 1.66 has been proposed by Chauchat (2017) based on matching experimental data from Revil-Baudard et al (2015). This model is only working in the 1-D configuration for which the direction of gravity is y.…”

“…In this subsection the model results are compared with experimental results from Revil-Baudard et al (2015) and Sumer et al (1996) for turbulent sheet flows; the goal of these test cases is to validate the numerical implementation of different turbulence models (mixing length and k − ω) and to calibrate the free parameter B.…”

“…The first case is based on the experimental configuration of Revil-Baudard et al (2015) for unidirectional sheet flows. The particles are non-spherical lightweight PMMA particles with density ρ a = 1190 kg m −3 and diameter d = 3 ± 0.5 mm.…”

“…3 for volume-averaged velocity profiles (panels a, d), defined as u = αu a + βu b , sediment concentration profile (panels b, e), and Reynolds shear stress profile (panels c, f). The numerical results are compared with the measurements reported in Revil-Baudard et al (2015). Different combinations of granular stress model and turbulence model are presented.…”

“…Amongst these different configurations, the µ(I ) rheology coupled with the mixing length model (panels a, b, c) corresponds to the model proposed by Revil-Baudard and Chauchat (2013) and Chauchat (2017), and the kinetic theory coupled with the k − ε (panels d, e, f) corresponds to the model proposed by and Cheng et al (2017a). The mixing length model has been calibrated based on Revil-Baudard et al (2015) data using the µ(I ) rheology (Chauchat, 2017) and the von Karman constant is reduced to κ = 0.225 to model a significant turbulence damping under sheet-flow conditions. Also, the rheological parameters of the µ(I ) rheology are identical to the ones proposed by Chauchat (2017) and they are summarized in Table 9.…”