Wake Vortices and Dissipation in a Tidally Modulated Flow Past a Three-Dimensional Topography

Large eddy simulations are employed to investigate the role of tidal modulation strength on wake vortices and dissipation in flow past three-dimensional topography, specifically a conical abyssal hill. The barotropic current is of the form U-c + U-t sin(Omega(t)t), where U-c and U-t are the mean and oscillatory components, respectively, and Omega(t) is the tidal frequency. A regime with strong stratification and weak rotation is considered. The velocity ratio R = U-t/U-c is varied from 0 to 1. Simulation results show that the frequency of wake vortices reduces gradually with increasing R from its natural shedding frequency at R = 0 to Omega(t)/2 when R >= 0.2. The ratio of R and the excursion number, denoted as (S) over bar controls the shift in the vortex frequency. When 0.4 <= (S) over bar <= 2, vortices are trapped in the wake during tidal deceleration, extending the vortex shedding cycle to two tidal cycles. Elevated dissipation rates in the obstacle lee are observed in the lateral shear layer, hydraulic jet, and the near wake. The regions of strong dissipation are spatially intermittent, with values exceeding O(U-c(3)/D) during the maximum-velocity phase, where D is the base diameter of the hill. The maximum dissipation rate during the tidal cycle increases monotonically with R in the downstream wake. Additionally, the normalized area-integrated dissipation rate in the hydraulic response region scales with R as (1 + R)(4). Results show that the wake dissipation energetically dominates the internal wave flux in this class of low-Froude number geophysical flows.