SOMAR-LES: A framework for multi-scale modeling of turbulent stratified oceanic flows

A new multi-scale modeling technique, SOMAR-LES, is presented in this paper. Localized grid refinement gives SOMAR (the Stratified Ocean Model with Adaptive Resolution) access to small scales of the flow which are normally inaccessible to general circulation models (GCMs). SOMAR-LES drives a LES (Large Eddy Simulation) on SOMAR's finest grids, forced with large scale forcing from the coarser grids. Three-dimensional simulations of internal tide generation, propagation and scattering are performed to demonstrate this multi-scale modeling technique. In the case of internal tide generation at a two-dimensional bathymetry, SOMAR-LES is able to balance the baroclinic energy budget and accurately model turbulence losses at only 10% of the computational cost required by a non-adaptive solver running at SOMAR-LES's fine grid resolution. This relative cost is significantly reduced in situations with intermittent turbulence or where the location of the turbulence is not known a priori because SOMAR-LES does not require persistent, global, high resolution. To illustrate this point, we consider a three-dimensional bathymetry with grids adaptively refined along the tidally generated internal waves to capture remote mixing in regions of wave focusing. The computational cost in this case is found to be nearly 25 times smaller than that of a non-adaptive solver at comparable resolution. In the final test case, we consider the scattering of a mode-1 internal wave at an isolated two-dimensional and three-dimensional topography, and we compare the results with Legg (2014) numerical experiments. We find good agreement with theoretical estimates. SOMAR-LES is less dissipative than the closure scheme employed by Legg (2014) near the bathymetry. Depending on the flow configuration and resolution employed, a reduction of more than an order of magnitude in computational costs is expected, relative to traditional existing solvers

Estimating pressure and internal-wave flux from laboratory experiments in focusing internal waves

Instantaneous measurements of pressure and wave flux in stratified incompressible flows are presented for the first time using combined time-resolved particle image velocimetry (PIV) and synthetic schlieren (SS). Corrections induced by variations of the refractive index in this strongly density-stratified fluid are also considered. The test case investigated here is a three-dimensional geometry consisting of a Gaussian ring-type topography forced by an oscillating tide representative of geophysical applications. Density and pressure are reconstructed from SS or PIV in combination with linear theories and combined SS-PIV. We perform a direct comparison between the experimental results and three-dimensional direct numerical simulations of the same flow conditions and control parameters. In particular, we show that the estimated velocity or density and the hence wave flux from linear theory solely based on SS or PIV can be flawed in regions of focusing internal waves. We also show that combined measurements of SS and PIV are capable of circumventing these limitations and accurately reproduce the results computed from the DNS

Insights from a 3-D temperature sensors mooring on stratified ocean turbulence

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