Patryk Widera

Large eddy simulation of sediment transport in open-channel flow

Large Eddy Simulation (LES) is used to investigate the 3D-transport of sediment suspension in an open channel flow at a Reynolds number based on the frictional velocity.An Eulerian model was employed to represent the fluid and the sediment phases. The subgrid-scale fluid stresses were modeled with the Smagorinsky model, whereas subgrid-scale sediment fluxes were based on a gradient approach. To avoid tuning constants, a dynamic procedure was applied. A two-way coupling between sediment and fluid improved the computational description of sediment transport. Sediment concentration profiles and the statistics of sediment fluxes are presented and compared with the Rouse profile and the RANS solution. The results obtained agree well with the theory and with measurements. The LES results also indicate that the assumption of a constant Schmidt number, as is mostly assumed in RANS simulations is erroneous.

Study of the sediment transport over flat and wavy bottom using large-eddy simulation

In the present paper the sensitivity of the flow and of sediment transport to bottom roughness is studied. First, a thorough numerical investigation of smooth-bottom channel flow at Re τ = 395 is performed using large-eddy simulations (LESs). A dynamic version of the wall-adapted local eddy-viscosity (WALE) model is used for this study, whereas the subgrid-scale (sgs) diffusion stress is based on a gradient hypothesis. The computed results compare well with direct numerical simulation (DNS) data and experiments. Next, rough-bottom cases, with the bottom having a sinusoidal, wavy shape are considered. It was found that the wavy bottom has a strong influence on the flow field and that the sediment transport is highly sensitive to the bottom waviness, in particular for larger wave heights. It is shown that for light and low-concentrated sediment the Rouse theory is also valid in the case of a wavy bottom, mainly in the outer zone. Finally, it is found that the turbulent-Schmidt-number profiles are not very sensitive to the wave height.

Quality of dynamic variational multiscale models on distorted grids

The present paper focuses on a dynamic version of the variational multiscale model and investigates its performance in large eddy simulations (LES) of turbulent channel flow at Reynolds numbers (based on friction velocity) of 180, 395, and 590. The dynamic procedure was implemented for the variational multiscale model on the basis of the classical Smagorinsky model and the wall-adapting local eddy-viscosity (WALE) model. One base regular grid and three different distorted grids are employed in the evaluation. The dynamic multiscale models are compared with the direct numerical simulation (DNS) data and the conventional dynamic models. Next, the error components involved in LES are separated and the grid irregularity effect on the shortcoming of the subgrid scale models and the error of numerical approximation is assessed. A main outcome is that the dynamic multiscale models in comparison with the dynamic Smagorinsky and the dynamic WALE are less affected by grid distortion, leading to better behavior on the irregular grids.