Turbulence in tidal flow over rough three-dimensional topography

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This project examines how tidal flows in the ocean interior, particularly near underwater mountains, seamounts and ridges, contribute to the generation of turbulent and other complex flows. These flows contribute to mixing of heavier, cold bottom waters with overlying lighter, warmer oceanic waters. The project uses specialized high resolution numerical simulations to describe and predict turbulent motions over idealized conical mounts. These simplified geometries are used to represent rough underwater topography, like that found in the oceans. The flows investigated, and the water temperature distribution they create, play an important role in setting weather and climate. Also, they establish the environment for marine life, and determine the conditions faced by manned and unmanned submerged vehicles, usually used for defense and ocean exploration. Results from this project will be utilized by the scientific community engaged to observations of fine-scale processes in the environment and their parameterization in numerical large-scale models. A graduate student and a postdoctoral researcher will be supported and they will gain exposure to ocean sciences, fluid mechanics, and computational science in the context of a problem of great societal interest. Ocean tides constitute a large reservoir of energy that can be converted to internal gravity waves and turbulence. Tidally modulated flows encounter submerged, three-dimensional (3D), small-scale topographic features of order 1-10 km within larger scale, nominally two-dimensional ridges, islands and headlands. These flows generate propagating internal gravity waves at the oscillation frequency and at higher harmonics, along with transient downslope jets and lee waves. The shed vortices generated interact among themselves and with the body leading to complex turbulent motions. This project will identify the relative roles of the wake vortices (sub-mesoscale eddies), formed by flow around the three-dimensional features, and the nonlinear internal wave field using high-resolution simulations. High-resolution 3D large eddy simulations (LES) of flow over a steep (supercritical slope) conical obstacle in the stratified flow regime of low topographic Froude number will be performed. The simulations and analyses aim to characterize and understand the dynamics as a function of the following non-dimensional parameters: the relative strength of mean and oscillating velocity components, the ratio of the flow excursion length to a characteristic along-flow topography length, and the shape of the horizontal cross-section of the obstacle. The simplified setting used in the simulations will allow to advance knowledge of the combined dynamics of the vortices, wake and the nonlinear internal-wave near field, and of the consequences for stratified turbulence at 3D topography. Preliminary results have shown the generation of interacting eddies, complex 3D nonlinear internal wave fields and turbulence occurring in multiple vertical layers.