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- Title
Wake Vortices and Dissipation in a Tidally Modulated Flow Past a Three‐Dimensional Topography.
- Authors
Puthan, Pranav; Pawlak, Geno; Sarkar, Sutanu
- 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 Uc + Ut sin(Ωtt), where Uc and Ut are the mean and oscillatory components, respectively, and Ωt is the tidal frequency. A regime with strong stratification and weak rotation is considered. The velocity ratio R = Ut/Uc is varied from 0 to 1. Simulation results show that the frequency of wake vortices reduces gradually with increasing R from its natural shedding frequency at R = 0 to Ωt/2 when R ≥ 0.2. The ratio of R and the excursion number, denoted as S‾ $\overline{S}$, controls the shift in the vortex frequency. When 0.4≤S‾≤2 $0.4\le \overline{S}\le 2$, 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 OUc3/D $\mathcal{O}\left({U}_{c}^{3}/D\right)$ during the maximum‐velocity phase, where D is the base diameter of the hill. The maximum dissipation rate during the tidal cycle increases monotonically with R in the downstream wake. Additionally, the normalized area‐integrated dissipation rate in the hydraulic response region scales with R as (1 + R)4. Results show that the wake dissipation energetically dominates the internal wave flux in this class of low‐Froude number geophysical flows. Plain Language Summary: The interaction of ocean currents with submerged obstacles generates both internal wave motions and turbulent eddies. Modulation of ocean currents by oscillating tidal motions leads to changes in the distribution of energy between waves and eddies with important implications for energy dissipation and subsequently for ocean mixing. We present high‐resolution three‐dimensional simulations of the interaction of a tidally modulated current with a model submerged hill where the magnitude of the tidal flow relative to the steady current is varied. Results show that relatively small tidal currents can modify the process of eddy formation in the wake of the obstacle, changing the eddy shedding frequency from its natural value. The shedding rate decreases as tidal magnitude is increased until the interval synchronizes to twice the tidal period. We find significant loss of kinetic energy in the wake of the obstacle, along its sides and near its crest, with maximum values occurring when the peak tidal current coincides with the steady flow. Energy dissipation increases sharply with increasing tidal current magnitude. The relative amount of energy transported away by internal wave motions increases, though it remains small in comparison with the loss of energy associated with eddy motions near the obstacle. Key Points: Weak tidal modulations can alter the wake‐vortex frequency of an abyssal hill wake, a mechanism governed by the tidal strength parameterThe downstream wake and the hydraulic response region are prominent dissipation "hot‐spots"The local dissipation at the hill can substantially exceed the radiated wave flux in the low‐Froude number regime
- Subjects
THREE-dimensional flow; TIDAL currents; OCEANIC mixing; LARGE eddy simulation models; OCEAN currents; INTERNAL waves; VORTEX shedding; ROTATIONAL motion
- Publication
Journal of Geophysical Research. Oceans, 2022, Vol 127, Issue 8, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2022JC018470