Tidally modulated uniform flow past a conical obstacle: a LES study

In the ocean, tides and currents occur in conjunction. Large eddy simulations (LES) are undertaken to examine the interaction of a tidally modulated current ($U_c$ and $U_t$ being the amplitude of current and tidal flow respectively) with a conical obstacle. The M2 tidal frequency, ω, and the length scales of the obstacle, namely the height $h$ and diameter $D$, are kept constant. In the presence of strong background stratification (Froude number $Fr_c=U_c/Nh$ less than 1), the velocity ratio $R=U_t/U_c$ is systematically varied (by varying $U_t$ while $U_c$ is constant). There is upstream blocking and a downslope jet, which flows downward from the obstacle peak, forms on the lee side. The jet velocity is strongest when the barotropic velocity reaches $U_m = U_c + U_t$, which is the phase with the large vertical displacement of isopycnals in the lee. As the barotropic flow velocity reduces from $U_m$, these isopycnals relax, engendering overturns and mixing. Over a tidal cycle, shear and convective instabilities in the $R > 1$ regime show more temporal variability when compared to the $R\ll 1$ regime. For example, when $R$ is greater than 1, large convective overturns develop in the lee and break down into turbulence at maximum velocity phase. Analysis of vortex dynamics reveal the formation of vortex dipoles during flow reversal, triggered by strong lateral flow and acceleration of fluid in the recirculation zone. The characteristics of the internal wave field are also altered on varying $R$.