Numerical simulation of breaking wave generated sediment suspension and transport process based on CLSVOF algorithm

The sediment suspension and transport process under complex breaking wave situation is investigated using large eddy simulation （abbreviated as LES hereafter） method. The coupled level set （LS） and volume of fluid （VOF） method is used to accurately capture the evolution of air？water interface. The wall effect at the bottom is modeled based on the wave friction term while the complicate bottom boundary condition for sediment is tackled using Chou and Fringer’s sediment erosion and deposition flux method. A simulation is carried out to study the sediment suspension and transport process under periodic plunging breaking waves. The comparison between the results by CLSVOF method and those obtained by the LS method is given. It shows that the latter performs as well as the CLSVOF method in the pre-breaking weak-surface deformation situation. However, a serious mass conservation problem in the later stages of wave breaking makes it inappropriate for this study by use of the LS method and thus the CLSVOF method is suggested. The flow field and the distribution of suspended sediment concentration are then analyzed in detail. At the early stage of breaking, the sediment is mainly concentrated near the bottom area. During the wave breaking process, when the entrapped large-scale air bubble travels downward to approach the bottom, strong shear is induced and the sediment is highly entrained.     

Comparison of laminar model,RANS, LES and VLES for simulation of liquid sloshing

Liquid sloshing in storage tank is a fundamental problem of great engineering importance. Sloshing motion can be laminar or turbulent. However, the necessity for inclusion of turbulence in CFD simulation of sloshing flows has not yet been established. In this paper, three roll–induced sloshing cases are studied to assess the merits and shortcomings of the laminar model and three most–commonly used turbulence models (RANS k–ε, LES and Very LES). To overcome the deficiencies in the RANS and LES, the new Very LES (VLES) model, which combines the RANS k–ε and LES, is developed in this paper. The free surface profiles are reconstructed by a coupled Level–Set and Volume–of–Fluid (CLSVOF) method. To the authors’ knowledge, the comprehensive and systematical assessment of the effect of turbulence on sloshing simulation has not been reported in the literature. The numerical results are evaluated using experimental measurements from Delorme and Souto−Iglesias. The present study indicates that the inclusion of an appropriate turbulence model has a profound influence on the simulations of violent and non–violent sloshing flows. The VLES and LES models can provide accurate predictions of free surface profiles and impact pressures, whereas the laminar flow assumption and the RANS model cannot adequately capture the energy dissipation in the sloshing simulation and lead to the inaccurate flow predictions.