Challenges in turbulence modeling and simulation of free-surface flows

Free-surface wave flows are of much importance in ocean engineering applications pertaining to ships, coastal barrier systems, offshore platforms, wave energy converters, and fixed/floating offshore wind turbines. Numerical Wave Tanks (NWTs) and Numerical Tow Tanks (NuTTs) which are used for engineering design depend on adequately accurate numerical simulations of turbulence effects in these free-surface flows. The steep gradients of density and velocity at the air-water interface pose several challenges to turbulence modeling and simulation. In this work, we examine the importance of (i) the realizability characteristics of the RANS (Reynolds-Averaged Navier-Stokes) turbulence model, and (ii) the initial state of kinetic energy and eddy viscosity at the interface. Unrealizable turbulence models and inappropriate initial viscosity levels can lead to significant errors. The physics underlying the extreme sensitivity to these model/simulation features at the air-water interface is explicated. The utility of scale-resolving simulations (SRS) is also investigated.