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Dr. Sanjiv Ramachandran

Postdoctoral fellow Dr. Sanjiv Ramachandran.

Education

Sanjiv Ramachandran obtained his undergraduate degree in aerospace engineering from the Indian Institute of Technology, Mumbai. For his graduate studies, he went to Pennsylvania State University where he achieved a masters degree in aerospace engineering as well as a doctoral degree in meteorology. He is currently employed at the University of Massachusetts, Dartmouth as a postdoctoral research associate in the Physics department.

Sanjiv's doctoral dissertation dealt with understanding some of the issues that arise in the modeling of "subgrid" (finer than the grid resolution) turbulence in Large Eddy Simulation (LES) of the atmospheric boundary layer when the grid resolution is comparable to the energy-containing eddies. LES is most reliable when there is a wide separation of scales between the grid resolution and the large, energy-containing eddies. When such a separation of scales is absent, LES can be sensitive to the underlying subgrid model as a significant fraction of the total turbulent stresses (and fluxes) now reside at the subgrid scales. Using the conservation equations for the subgrid stresses and fluxes, Sanjiv's thesis explored the dependence of various terms in these equations as a function of scale and discussed their implications for subgrid modeling of under-resolved turbulence.

Current Research

Presently, Sanjiv is studying the effects of subgrid mixing on simulations that resolve both  oceanic mesoscales and submesoscales, features that are O(1-10km) and evolve on inertial time-scales. The submesoscales are of interest as they play host to frontal instabilities that can significantly enhance the rate of restratification of the mixed layer, beyond that caused due to geostrophic adjustment alone. The large vertical velocities associated with submesoscale instabilities also has important implications for the transport of nutrients from deeper down into the mixed layer. Comparing simulations using an anisotropic Smagorinsky model (ASM) with those using constant lateral and vertical subgrid coefficients, we find the choice of subgrid model affects the evolution of submesoscale instabilities non-trivially. In particular, the efficiency of extraction of APE, the spectral fluxes and the eddy kinetic energy budgets can vary considerably across subgrid models and across different subgrid constants for a fixed model. Attached is a plot showing how the efficiency of extraction of APE evolves in time for different levels of subgrid dissipation  . The run 'KX1KZ4' corresponds to a constant lateral subgrid viscosity of K_x = 1 m^2/s and an analytically prescribed vertical viscosity that is 10^(-5) m^2/s in the interior and 4 x 10^(-3) m^2/s within the Ekman layer. For the run 'KX1KZ1', K_x = 1 m^2/s while the vertical viscosity within the Ekman layer is 1 x 10^(-3) m^2/s. We are also investigating the effects of baroclinicity below the mixed layer on the buoyancy and tracer fluxes. Preliminary results show significant enhancement of fluxes below the mixed layer in the presence of deep baroclinicity. Ongoing work is focused on understanding the  spatial structure of these fluxes.

Conferences and Poster Presentations

Ramachandran, S., Tandon, A . and Mahadevan, A., "Effect of subgrid-scale mixing on the evolution of submesoscale instabilities," Ocean Sciences Meeting,  Salt Lake City, Utah, 20-24  February 2012

 Ramachandran, S., Tandon, A. and Mahadevan, A., "Submesoscale-resolving simulations using an anisotropic Smagorinsky model," 64th annual meeting, APS Division of Fluid Dynamics, Baltimore, Maryland, November 2011

 Ramachandran, S. and Wyngaard, J., "Subgrid modeling using conservation equations," John C. Wyngaard symposium on atmospheric turbulence and boundary layers, State College, Pennsylvania, June 2010

Publications

Ramachandran, S., A. Tandon and A. Mahadevan 2013, Effect of subgrid-scale mixing on the evolution of forced submesoscale instabilities, Ocean Modelling (In Press)

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