High-resolution large-eddy simulation of turbulent mixing of a river plume

Abstract:

A non-hydrostatic sigma-coordinate numerical model (NHWave) is applied to study the structure of a river plume, and the vertical mixing due to shear instabilities. A 3D large-eddy simulation approach is used with the aim to resolve the flow turbulence in the stratified ambient fluid at high Reynolds number. The domain is of depth 10m, length 500m and width 25m, and initially quiescent containing saltwater of salinity 26 psu. Fresh water plume is sent from the left boundary with a range of internal Froude number. Simulation resulting using Standard Smagorinsky closure demonstrates that the model is able to predict shear instabilities although it could not resolve the secondary instability at high Reynolds number. The characteristic length scale of the shear instabilities is around 10 m, which is consistent with field observation of Connecticut River plume using a 4-channel broadband echo sounder (Geyer et al. 2010, Geophy. Res. Lett., 37, L22607). The mixing efficiency and dissipation rate are obtained from the numerical simulation results, and these results are used to investigate and evaluate the Richardson-number-dependent parameterization of the mixing process. The model can also provide the information on fine structures of surface elevation variations, which enables us to correlate the surface signature with the turbulent billow underneath. The model therefore may be useful to help interpret surface signatures observed using various remote sensing techniques. Supported by Office of Naval Research.