LES, DES and RANS simulations of Langmuir circulation in shallow water

Wind and wave-driven Langmuir circulation (LC) in the upper ocean mixed layer has been extensively studied via field measurements and numerical simulations. However, studies of LC under the influence of coastal boundaries and its interactions with other coastal ocean processes are lacking. Large-eddy simulations (LES) of LC in inner coastal shelves have been limited to periodicity over horizontal directions, characteristic of a water column unaffected by coastal boundaries. As such, these simulations have been performed with pseudo-spectral solvers employing a highly accurate spectral discretization in the horizontal directions. These simulations may be deemed as "single water column LES", in which the vertical turbulent mixing induced by the cells is well-resolved, but their interaction with lateral boundaries and lateral flows are left unresolved. To address this deficiency, it is necessary to extend eddy-resolving simulations to non-spectral discretizations capable of handling non-periodic boundary conditions in lateral directions. A second-order accurate finite volume discretization will be used employing LES, DES (detached eddy simulation) and RANS (Reynold-averaged Navier-Stokes) methodologies, characterized by different forms of subgrid-scale modeling and near-wall treatment. It is found that careful consideration must be given to these components, as various combinations of the subgrid-scale model and the near-wall treatment can lead to excessive damping of the cells near the bottom of the water column. This adverse numerical effect can prevent the simulation from (1) accurately capturing the interaction between LC with the bottom boundary layer and (2) accurately predicting the lateral length scales of LC. The best performing subgrid-scale models and near-wall treatments will be identified. Performance of the simulations will also be assessed against recently published data of full-depth LC obtained from field measurements.