High drag states in tidally modulated stratified wakes

TL;DR

This study uses large eddy simulations to explore how tidal currents influence the form drag and vortex behavior of submerged topography in stratified, rotating fluids, revealing high drag states linked to specific tidal frequencies.

Contribution

It identifies the conditions under which tidal modulation significantly amplifies form drag and alters vortex shedding patterns in stratified wakes.

Findings

High form drag occurs when the ratio of natural shedding frequency to tidal frequency is between 0.5 and 1.

Vortex shedding symmetry varies with the frequency ratio, from symmetric to strongly asymmetric.

Increased form drag is due to pressure intensification and flow separation changes in the wake.

Abstract

Large eddy simulations (LES) are employed to investigate the role of time-varying currents on the form drag and vortex dynamics of submerged 3D topography in a stratified rotating environment. The current is of the form $U_c+U_t \sin(2\pi f_t t)$, where $U_c$ is the mean, $U_t$ is the tidal component and $f_t$ is its frequency. A conical obstacle is considered in the regime of low Froude number. When tides are absent, eddies are shed at the natural shedding frequency $f_{s,c}$. The relative frequency $f^*=f_{s,c}/f_t$ is varied in a parametric study which reveals states of high time-averaged form drag coefficient. There is a two-fold amplification of the form drag coefficient relative to the no-tide ($U_t=0$) case when $f^*$ lies between 0.5 and 1. The spatial organization of the near-wake vortices in the high drag states is different from a K\'arm\'an vortex street. For instance, the vortex shedding from the obstacle is symmetric when $f^*=5/12$ and strongly asymmetric when $f^*=5/6$. The increase in form drag with increasing $f^*$ stems from bottom intensification of the pressure in the obstacle lee which is linked to changes in flow separation and near-wake vortices.