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Abstract Largeeddy simulations (LES) are employed to investigate the role of timevarying 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 πf 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 is varied in a parametric study, which reveals states of high timeaveraged form drag coefficient. There is a twofold amplification of the form drag coefficient relative to the notide ( U t = 0) case when lies between 0.5 and 1. The spatial organization of the nearwake vortices in the high drag states is different from a Kármán vortex street. For instance, the vortex shedding from the obstacle is symmetric when and strongly asymmetric when . The increase in form drag with increasing stems from bottom intensification of the pressure in the obstacle lee which we link to changes in flow separation and nearwake vortices.

Abstract Direct numerical simulations are performed to compare the evolution of turbulent stratified shear layers with different density gradient profiles at a high Reynolds number. The density profiles include uniform stratification, twolayer hyperbolic tangent profile and a composite of these two profiles. All profiles have the same initial bulk Richardson number ( $$Ri_{b,0}$$ R i b , 0 ); however, the minimum gradient Richardson number and the distribution of density gradient across the shear layer are varied among the cases. The objective of the study is to provide a comparative analysis of the evolution of the shear layers in term of shear layer growth, turbulent kinetic energy as well as the mixing efficiency and its parameterization. The evolution of the shear layers in all cases shows the development of Kelvin–Helmholtz billows, the transition to turbulence by secondary instabilities followed by the decay of turbulence. Comparison among the cases reveals that the amount of turbulent mixing varies with the density gradient distribution inside the shear layer. The minimum gradient Richardson number and the initial bulk Richardson number do not correlate well with the integrated TKE production, dissipation and buoyancy flux. The bulk mixing efficiency for fixed $$Ri_{b,0}$$ R i b ,more »

Direct numerical simulations are performed to investigate a stratified shear layer at high Reynolds number ( $Re$ ) in a study where the Richardson number ( $Ri$ ) is varied among cases. Unlike previous work on a twolayer configuration in which the shear layer resides between two layers with constant density, an unbounded fluid with uniform stratification is considered here. The evolution of the shear layer includes a primary Kelvin–Helmholtz shear instability followed by a wide range of secondary shear and convective instabilities, similar to the twolayer configuration. During transition to turbulence, the shear layers at low $Ri$ exhibit a period of thickness contraction (not observed at lower $Re$ ) when the momentum and buoyancy fluxes are countergradient. The behaviour in the turbulent regime is significantly different from the case with a twolayer density profile. The transition layers, which are zones with elevated shear and stratification that form at the shearlayer edges, are stronger and also able to support a significant internal wave flux. After the shear layer becomes turbulent, mixing in the transition layers is shown to be more efficient than that which develops in the centre of the shear layer. Overall, the cumulative mixing efficiency ( $E^C$ )more »

Abstract 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(Ω_{t}t ), whereU _{c}andU _{t}are the mean and oscillatory components, respectively, and Ω_{t}is the tidal frequency. A regime with strong stratification and weak rotation is considered. The velocity ratioR =U _{t}/U _{c}is varied from 0 to 1. Simulation results show that the frequency of wake vortices reduces gradually with increasingR from its natural shedding frequency atR = 0 to Ω_{t}/2 whenR ≥ 0.2. The ratio ofR and the excursion number, denoted as, controls the shift in the vortex frequency. When , 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 during the maximum‐velocity phase, where D is the base diameter of the hill. The maximum dissipation rate during the tidal cycle increases monotonically withR in the downstream wake. Additionally, the normalized area‐integrated dissipation rate in the hydraulic response region scales withR as (1 +R )^{4}. Results show that themore » 
Turbulence and mixing in a nearbottom convectively driven flow are examined by numerical simulations of a model problem: a statically unstable disturbance at a slope with inclination $\unicode[STIX]{x1D6FD}$ in a stable background with buoyancy frequency $N$ . The influence of slope angle and initial disturbance amplitude are quantified in a parametric study. The flow evolution involves energy exchange between four energy reservoirs, namely the mean and turbulent components of kinetic energy (KE) and available potential energy (APE). In contrast to the zeroslope case where the mean flow is negligible, the presence of a slope leads to a current that oscillates with $\unicode[STIX]{x1D714}=N\sin \unicode[STIX]{x1D6FD}$ and qualitatively changes the subsequent evolution of the initial density disturbance. The frequency, $N\sin \unicode[STIX]{x1D6FD}$ , and the initial speed of the current are predicted using linear theory. The energy transfer in the sloping cases is dominated by an oscillatory exchange between mean APE and mean KE with a transfer to turbulence at specific phases. In all simulated cases, the positive buoyancy flux during episodes of convective instability at the zerovelocity phase is the dominant contributor to turbulent kinetic energy (TKE) although the shear production becomes increasingly important with increasing $\unicode[STIX]{x1D6FD}$ . Energy that initially resides whollymore »

Abstract 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. Whenf ^{*}increases and approaches 0.25, vortex shedding at the body changes from a classical asymmetric Kármán vortex street. The wake evolves downstream to restore the Kármán vortex‐street asymmetry but the discrete spectral peak, associated with wake vortices, is found to differ from bothf _{t}andf _{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.