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

Farm laws and the Agrarian crisis in India

In the aftermath of the repeal of the three farm laws and the end of the historic farmers struggle on the borders of the nation's capital, Delhi bear an interesting phenomenon that needs to be studied and analysed. This paper studies some of the significant academic interventions and analysis of the laws and how they would affect specifically the small and middle farmers. Through the course of this paper, the attempt is not to create a binary between the amendments made to the existing laws and the pre-amendment state protections. One of the motivations of the paper is also to understand the need for state driven bulwarks to protect the interests and lives of the farmers. In the process, it has been necessary to understand the need to not only retain the prior provisions that reign in the heavy hand of the corporations but the ways in which their potential can be maximised in favour of the farmers. This paper involves close reading and analysis of research by political scientists and sociologists

Instabilities, Turbulence and Mixing from Internal Waves at bottom topography

This study of nonlinear processes and turbulence associated with internal gravity waves has three phases. In the first phase, the reflection problem is studied with simulations involving laboratory-scale slopes whose inclination varies between critical (resonant case with slope angle equal to wave propagation angle) and somewhat off-critical values. We start with three-dimensional direct numerical simulation (DNS) and find strong nonlinearity in the response if the off-criticality is not too large: harmonics in the radiated wave field and cyclical near-bottom turbulence. Subharmonic frequencies are found slightly away from the interaction region in some cases, prompting a followup study with two-dimensional simulations where the underlying mechanism is identified as parametric subharmonic instability (PSI) of the reflected wave beam: the sum of the frequencies and the sum of the wave numbers of the daughter subharmonic waves equal the corresponding quantities for the reflected wave. This is the first demonstration that the reflected wave formed when a small-amplitude linear wave is incident on a slope becomes sufficiently nonlinear so as to suffer PSI.In the second phase, DNS and large eddy simulation (LES) are employed to study the turbulent dissipation and mixing brought about by convective overturns in a stratified, oscillatory bottom layer underneath internal waves. The phasing of turbulence, the onset and breakdown of convective overturns, and the pathway to irreversible mixing are quantified. $L_T$ decreases during the convective instability while $L_O$ increases, which implies that $L_T$ is not linearly related to $L_O$. Thus, the instantaneous Thorpe-inferred dissipation rates are quite different from the actual values. However, the ratio of their cycle-averaged values is found to be O(1).In the third phase, a novel modeling technique called SOMAR-LES has been developed wherein a three-dimensional, body-conforming Large Eddy Simulation (LES) model that resolves turbulence scales is coupled with the large-sale Stratified Ocean Model with Adaptive Refinement (SOMAR) to accurately represent small scale turbulence as well as its effect on flow evolution at large scale. Numerical simulations are performed with coupled SOMAR-LES to examine the flow at model Kaena ridge, a steep supercritical generation site

Improvement in sealing effectiveness of air curtains using positive buoyancy

Air curtains are commonly employed in building applications to facilitate aerodynamic sealing against the exchange flow that occurs through an open doorway due to the density differences owing to buoyancy. Such situations often prevail due to temperature gradients across a doorway of an air-conditioned building, e.g., during the summer season in an Indian subcontinental situation. In the present study, we numerically investigate the performance of `positively buoyant' air curtains. In such installations, the density of the jet fluid is larger than the density of the fluid contained within the building space. Using the two-dimensional Reynolds-averaged Navier-Stokes (2D RANS) formulation, we compute the temperature distribution in the flow domain and estimate the associated sealing effectiveness for various values of positive jet buoyancy and operating velocities of the air curtain. These estimates of sealing effectiveness are compared with that of a neutrally buoyant air curtain to assess the influence of positive buoyancy. We report an increase in sealing effectiveness of up to 10%, whereas its peak value improves by about 5%.Comment: Fluid Mechanics and Fluid Power Conference 202

Multiscale modeling of internal waves and turbulence at rough, realistic topography with SOMAR-LES

The Stratified Ocean Model with Adaptive Refinement (SOMAR) is a modeling framework with the flexibility of adaptive mesh refinement (AMR) at localized regions with high gradients. Several test cases including the lock-exchange problem, solitary wave propagation, and internal tide generation have been previously considered to validate the method. Local refinement of the grid allows the solver to achieve highly accurate results with substantial reduction in computational cost. Recently, SOMAR-LES has been developed wherein a three-dimensional, body-conforming, Large Eddy Simulation (LES) model that resolves turbulent scales is coupled with SOMAR to accurately represent small scale turbulence as well as its effect on flow evolution at large scales. The coupling is two-way: the LES is driven with large scale forcing, and SOMAR receives feedback in the form of an eddy viscosity, diffusivity, and sub-grid scale fluxes. This novel multi-scale modeling technique is applied to study the near- and far-field baroclinic response when the oscillating barotropic tide interacts with underwater topography. Numerical simulations are currently being performed with SOMAR-LES to examine the flow at Kaena ridge, a steep supercritical generation site, where the topographic length scales are of O(100 km), and the barotropic forcing corresponds to a small outer excursion number (Ex ~ 0.01) and small Froude number (Fr ~ 0.006). The SOMAR-LES results will be used to quantify baroclinic energy conversion and internal wave properties such as the radiated wave flux and modal composition.https://jdc.jefferson.edu/jchsfposters/1000/thumbnail.jp