Wakes of underwater hills : Vortex dynamics, Form drag and Turbulence

Wakes of three-dimensional topographies in the abyssal ocean are sites of enhanced flow variability and mixing. To investigate the attendant vortex dynamics, internal waves and turbulence, a three-phased approach is adopted. In the first phase, a steady current impinges on a conical abyssal hill in a stratified environment. The sensitivity of the flow to boundary conditions (slip and partial-slip instead of no-slip) on the flat wall and on the obstacle surface is examined. Significant changes occur in the structure of lee vortices and wake turbulence when the boundary condition (BC) is changed. For instance, the boundary layer on the flat bottom in the no-slip case suppresses the unsteady behavior of the separated boundary layer behind the hill. In contrast, unsteady shedding of vortices from the body in a high Reynoldsnumber flow is captured by slip and partial-slip BCs. In the second phase, tidal modulations are superimposed on the steady flow to study the changes in vortex dynamics in the abyssal hill wake. The strength of the tidal modulations relative to the mean flow (R) and the tidal excursion number (Ex) are varied to reveal tidal synchronization of wake vortices. The ratio of the natural shedding frequency to the tidal frequency (f*) varies from 0.1 to 1 when Ex is varied, at R = 1. Wake vortices are observed at the subharmonics of the tidal frequency when 0.25 ≤ f* ≤ 1. Even weak tidal modulations (R ∼ O (0.1)) can alter the frequency of these wake vortices. Qualitative changes are observed in the spatial organization of the vortices, which influences form drag and dissipation. In the third phase, a statically unstable disturbance (originating from internal wave propagation) over a slope with inclination β and background buoyancy frequency N is considered. The energy exchange occurs between four energy reservoirs, namely the mean and turbulent components of kinetic energy (KE) and available potential energy (APE). When β is non-zero, a mean flow is initiated at a frequency of N sin β accompanied by an oscillatory energy exchange between the mean KE and APE reservoirs. The energy transfer between the mean and turbulent reservoirs of KE and APE is explored