Free surface algorithms for 3D numerical modelling of reservoir flushing

River morphodynamics and sediment transportSedimentation in reservoir

3D Modelling of the Flow Distribution in the Delta of Lake Oyeren (Norway)

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Resolving large bed roughness elements with an unstructured hexahedral grid

River hydrodynamicsBed roughness and flow resistanc

Calculation of primary and secondary flow and boundary shear stresses in a meandering channel

Turbulent flow in a meandering channel is computed with two Computational Fluid Dynamics (CFD) codes solving the Navier–Stokes equations by employing different turbulence closure approaches. The first CFD code solves the steady Reynolds-Averaged Navier–Stokes equations (RANS) using an isotropic turbulence closure. The second code is based on the concept of Large Eddy Simulation (LES). LES resolves the large-scale turbulence structures in the flow and is known to outperform RANS models in flows in which large-scale structures dominate the statistics. The results obtained from the two codes are compared with experimental data from a physical model study. Both, LES and RANS simulation, predict the primary helical flow pattern in the meander as well as the occurrence of an outer-bank secondary cell. Computed primary as well as secondary flow velocities are in reasonably good agreement with experimental data. Evidence is given that the outer-bank secondary cell in a meander bend is the residual of the main secondary cell of the previous bend. However, the RANS code, regardless of the turbulence model employed, overpredicts the size and strength of the outer-bank secondary cell. Furthermore, only LES is able to uphold the outer-bank second secondary cell beyond the bend apex until the exit of the bend as turbulence anisotropy contributes to its persistence. The presence of multiple secondary cells has important consequences for the distribution of shear stresses along the wetted perimeter of the channel, and thereby the sediment transport in meandering channels. Consequently, even though LES is expected to compute the bed-shear stresses along the wetted perimeter of the channel with a higher degree of accuracy than the RANS model, comparisons between LES and RANS computed wall shear stresses agree well. These findings are useful for practitioners who need to rely on RANS model predictions of the flow in meandering channels at field scale

Evaluation of low Reynolds number turbulence models for an open-channel flow over a rough bed using LES data

There are quite a few studies investigating low Reynolds number turbulence models for predicting the near-wall flow over smooth walls. However, analogous work over rough walls is sparse despite the fact that many flows of practical interest, particularly in geophysical flows and hydraulic engineering, are over rough walls. This paper presents a priori and a posteriori evaluations of low Reynolds number Reynolds-averaged Navier-Stokes (RANS)–based modeling approaches for flows over rough walls with the help of high-resolution large-eddy simulation (LES) data. The LES data allow the calculation of the vertical distribution of the turbulent kinetic energy, the dissipation rate, the specific dissipation rate, and the turbulent eddy viscosity, so that the assumptions inherent of RANS-based turbulence models can be tested a priori. A posteriori testing of the models through RANS simulations showed that the three low Reynolds turbulence models evaluated in this paper provide a fairly accurate prediction of the velocity profile, turbulent eddy viscosity, and wall shear stress