Tidal Synchronization of Lee Vortices in Geophysical Wakes

Wake vortices in tidally modulated currents past a conical hill in a stratified fluid are investigated using large-eddy-simulation. The vortex shedding frequency is altered from its natural steady-current value leading to synchronization of wake vortices with the tide. The relative frequency (f*), defined as the ratio of natural shedding frequency (f(s,c)) in a current without tides to the tidal frequency (f(t)), is varied to expose different regimes of tidal synchronization. When f* increases and approaches 0.25, vortex shedding at the body changes from a classical asymmetric Karman vortex street. The wake evolves downstream to restore the Karman vortex-street asymmetry but the discrete spectral peak, associated with wake vortices, is found to differ from both f(t) and f(s,c), a novel result. The spectral peak occurs at the first subharmonic of the tidal frequency when 0.5 <= f* < 1 and at the second subharmonic when 0.25 <= f* < 0.5.