Large-Eddy Simulation of Flow over Two-Dimensional Obstacles: High Drag States and Mixing

Abstract

A three-dimensional large-eddy simulation (LES) model was used to examine how stratified flow interacts with bottom obstacles in the coastal ocean. Bottom terrain representing a 2D ridge was modeled using a finite-volume approach with ridge height (4.5 m) and width (?30 m) and water depth (?45 m) appropriate for coastal regions. Temperature and salinity profiles representative of coastal conditions giving constant buoyancy frequency were applied with flow velocities between 0.16 and 0.4 m s?1. Simulations using a free-slip lower boundary yielded flow responses ranging from transition flows with relatively high internal wave pressure drag to supercritical flow with relatively small internal wave drag. Cases with high wave drag exhibited strong lee-wave systems with wavelength of ?100 m and regions of turbulent overturning. Application of bottom drag caused a 5?10-m-thick bottom boundary layer to form, which greatly reduced the strength of lee-wave systems in the transition cases. A final simulation with bottom drag, but with a much larger obstacle height (16 m) and width (?400 m), produced a stronger lee-wave response, indicating that large obstacle flow is not influenced as much by bottom roughness. Flow characteristics for the larger obstacle were more similar to hydraulic flow, with lee waves that are relatively short in comparison with the obstacle width. The relatively strong effect of bottom roughness on the small obstacle wave drag suggests that small-scale bottom variations may be ignored in internal wave drag parameterizations. However, the more significant wave drag from larger-scale obstacles must still be considered and may be responsible for mixing and momentum transfer at distances far from the obstacle source.