Vortex dynamics in tidally modulated stratified wakes*

Topographic features with steep slopes are sites of large energy conversion from the oscillating barotropic tide to internal waves and wake eddies. Large-eddy simulations of flow past an idealized conical obstacle with height h and bottom diameter D are undertaken to assess vortex dynamics initiated by a tidally modulated flow. The barotropic flow is composed of a uniform current (U_c) and a sinusoidal tidal modulation (U_tsin(2πf_tt)), where f_t is the tidal frequency. The topographic Froude number is O(0.1) so that an impinging steady current is driven laterally around the obstacle to generate eddies at a constant frequency f_s,c. The ratio f^*=f_s,c/f_t is varied to expose different regimes of tidal synchronization. The wake is found to change qualitatively such that far wake vortices are found at frequencies which differ from f_s,c and f_t. The frequency of wake vortices synchronize to the second subharmonic of the tidal frequency when 0.25 ≤ f^* < 0.5 (regime 1) and to its first subharmonic when 0.5 ≤ f^* ≤ 1 (regime 2). Qualitative changes also occur in the spatial organization of the near wake vortices due to flow separation initiated by tidal forcing. For instance, vortex shedding from the obstacle is laterally symmetric in regime 1 and strongly asymmetric in regime 2.