Understanding Fine Scale Processes in Intense Sediment Transport Regimes – SHEET-FLOW

The SHEET-FLOW project aims at developing a new generation of sediment transport models that address the role of turbulence-particle interactions (task 2 & 3) on a wide range of flow conditions. The development of this model requires the generation of new high resolution experimental datasets on well controlled steady and uniform flow conditions in open-channel flows (task 1). The model developed in task 2 and validated against data from task 1 will be used to investigate the role of turbulence-particle interactions on sediment fluxes and potential feedbacks on the flow dynamics.

We carried out the first measurement campaign with 3mm PMMA particles. The analysis is underway and the results obtained are in good agreement with our previous experiments. We started a systematic analysis of the data and a conference article has been written on the experimental protocol and the preliminary results.
Regarding task 2, we performed Large Eddy Simulations (LES) on the configurations of Kiger and Pan (2002) and Muste et al. (2005). The sensitivity analysis to the fluid-particle forces: drag, added mass and lift showed almost no effect of the added mass and lift forces (task 2.1). We developed a finite size correction model for the drag force which has been validated on the experimental data of the two configurations mentioned above. This work has been presetned at the international symposium THESIS 2019 (Newark, Delaware, USA, September 2019) and a research article is being finalized.

The understanding of turbulence-particle interactions gained during the projet together with the improved model will be critical to accurately upscale fine-scale processes in larger scale problems such as dunes evolution during river floods, wave driven sediment transport over sandbars or turbidity currents at the continental margins.

Mathieu, A., Chauchat, J., Bonamy, C., & Nagel, T. (2019). Two-Phase Flow Simulation of Tunnel and Lee-Wake Erosion of Scour below a Submarine Pipeline. Water, 11(8), 1727.

Sediment transport controls the morphological process in rivers, estuaries and coastal oceans. It is by nature a two-phase problem involving fluid-particle and particle-particle interactions covering the full range of particle concentration. At high concentration, transport is dominated by particle-particle interactions (intermittent collisions, enduring contacts) and at lower concentration, the transport becomes increasingly dominated by turbulent eddies, while those are also affected in return by the presence of particles. The conventional models are based on a single-phase approach in which sediment fluxes between the bed and the fluid are parameterized through empirical formulae. Predicted sediment fluxes may be wrong by a factor of ten. This shortcoming is partly due to the occurrence of sheet flows carrying highly concentrated suspended sediment above the bed.

The project SHEET-FLOW develops a two-phase flow approach that potentially incorporates most of the physical processes at stake. It relies on a close synergy between modelling and advanced acoustic instrumentation resolving velocities and concentration at the turbulent scales. This program will be applied to the well-controlled open-channel flows in the 10-m tilting flume facility available at LEGI (Grenoble, France). A wide range of flow conditions and sediment properties will be tested involving lightweight PMMA particles (1 and 3 mm size) and medium and coarse sands. The flow forcing will cover the sheet flow regime or intense bed-load transport corresponding to Shields number in 0.5 to 2. For these conditions, particles inertia is expected to influence sediment fluxes and turbulence damping mechanisms. Another important feature is the role of intermittent sediment bursts on the hydrodynamic roughness. All these processes will be investigated during the project.

The ultimate goal of the SHEET-FLOW project is to develop accurate Sub-Grid Scale (SGS) models for turbulence resolving two-phase flow simulations or Large Eddy Simulations (LES) of sediment transport in the sheet flow regime. These SGS models encompass fluid-particle forces and fluid and particle phases stress. Different modeling choices will be tested among which the Germano’s decomposition, the gradient diffusion model (istotropic and anisotropic) and the dynamical structure model. A priori analysis of high-reosolution LES simulations will be used to infer the functional dependencies of the sub-grid models on local filtered variables, such as velocity, concentration and their spatial gradients, as well as the grid size. The two-phase LES results will be used to investigate the fine-scale processes leading to turbulence modulation mechanisms, turbulent dispersion of particles and the interplay of intermittent turbulence and inter-granular interaction on the resulting transport dynamics under sheet flow conditions. The methodology will consist in investigating the turbulent kinetic energy budget and the relative contributions of the different stresses to the overall.

The LES results will be used to derive turbulence-averaged parameterizations for two-phase flow models. This last objective is critical to accurately upscale fine-scale turbulence-particle interaction processes in larger scale problems such as dunes evolution during river floods, wave driven sediment transport over sandbars or turbidity currents at the continental margins.