Numerical investigations of internal solitary waves: the evolution of instability in the bottom boundary layer and the wave-vortex-induced particle motion

Abstract

Internal waves are a phenomenon that occurs in stratified oceans. These waves play a vital role in the oceanic ecosystem, e.g., influencing the distribution of nutrients. I have conducted high-resolution simulations of internal solitary waves of depression as part of my thesis to gain further insight into the physical mechanisms these waves govern. I have numerically recreated a well-known laboratory experiment of internal waves to obtain the behavior concerning instability in the bottom boundary layer beneath the wave. During the study, I have established a threshold criterion regarding instability in the bottom boundary layer. I obtained good agreement with laboratory experiments for the transition to instability, where long-standing computational attempts have been unsuccessful. The instability occurs much earlier for waves of considerably smaller amplitude than previously predicted. In the presence of vortices in the bottom boundary layer, Lagrangian tracer particles were seeded, and traced, to represent sediment motion. A different part of this thesis involves analyzing data obtained by a new method for measuring the two-dimensional water flow in the field near ice. The new method uses air bubbles as tracers to capture water velocity and has been validated in the laboratory with good agreement with analytical solutions.