Fine Scale Turbulent and Granular Processes in Intense Sediment Transport Regimes

Abstract

<p>Intense sediment transport regimes are important in river and coastal geomorphology as they are responsible for important morphological evolution occurring during extreme climatic events. Under such transport conditions, a complex interplay between suspended load, dominated by turbulence-particle interactions, and bed-load, dominated by particle-particle interactions, is taking place. Both turbulence and granular processes are interacting with each other in the so-called four-way coupling corresponding to a modification of fluid turbulence due to granular interactions and vice-versa.</p><p> </p><p>In order to better understand these fine-scale turbulent and granular processes in intense sediment transport regime we acquired new experimental data at the lab in a 10m long open-channel flume.  Two plastic sediment sizes have been used separately, 1 and 3mm in diameter, they are introduced at the upstream end of the flume using a sediment feeder system combining a hopper and a conveyor belt. A single profile is measured at 3m from the outlet in the centerline of the flume using the Acoustic Concentration and Velocity Profiler (ACVP) having a 1.5mm vertical resolution and 78Hz temporal resolution. The acoustic system allows to concurrently measure the mean velocity and the concentration profiles as well as second order statistics.  The experimental dataset contains 3 flow regimes for each particle size and 4 sediment load for each flow condition ranging from no sediments to almost saturated (or capacity) flow conditions. Ultimately, we acquired 86 runs with some redundancy to evaluate the repeatability of the experiments. In terms of dimensionless numbers, we cover the following  range of Shields number 𝜃 ∈[0.3 ; 1.5] and suspension number w<sub>s</sub>/u<sub>*</sub> ∈[0.4 ; 1.3] where w<sub>s </sub>stands<sub></sub>for the settling velocity of the individual particles and u<sub>* </sub>stands for the bed friction velocity. All flow conditions are in the subcritical fully turbulent hydraulically rough regime.</p><p> </p><p>This extensive dataset is further used to develop and validate a two-phase flow Eulerian-Eulerian model. The new experimental data combined with Eulerian-Lagrangian simulations (CFD-DEM) provide a strong guideline to establish constitutive relations for granular stress models as well as for turbulence models. We propose an empirical modification of the kinetic theory of granular flows to account for particle-particle friction essentially through a modification of the radial distribution function and by adding a dependency of the restitution coefficient to particle friction. A two-equation turbulence model, k-omega SST, is used for the boundary layer. The key terms that are the most uncertain in this problem are the fluctuating energy transfer terms between the fluid (Turbulent Kinetic Energy) and the particles (granular temperature). The objective of the present contribution is to try to elucidate this question using the combined experimental, theoretical and numerical approach presented above. Preliminary results are very encouraging and a synthesis of this work will be presented at the conference.</p>