Patterns on a free surface caused by underwater topography: A laboratory-scale study

We study the link between bottom topography and its expression on a free-surface using Large-Eddy Simulations (LES) on the laboratory-scale. Free-surface patterns are presented for three configurations: neutral flow over wavy topography, stratified flow over wavy topography, and neutral flow over three-dimensional sinusoidal topography. The extent to which each configuration produces unique and identifiable surface patterns is explored. Our focus is on the fluid mechanics near the surface, for example, attachment and persistence of vortical structures, upwelling, and zones of convergence. Neutral flow over wavy topography creates a large number of powerful upwellings on the free surface. These upwellings appear to overwhelm the coherency of pre-existing vortices and vortex pairs. Consequently, the persistence of organized vortical motions on the free surface is reduced. In contrast, in stably stratified flow over a wavy boundary, upwellings are weakened, and more vortex pairs are observed. The surface signature of three-dimensional underwater topography shows elongated streaks in the streamwise direction. The above features allow these underwater topographies (at the depths presented) to be uniquely differentiated based solely on their surface signatures.