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Dynamics of stratified flow past a sphere: simulations using body-inclusive numerical model.

Event Type: 
Date and Time: 
Friday, June 3, 2016 - 16:15
CTR Conference Room 103
Event Sponsor: 
Parviz Moin, Director of Center for Turbulence Research
Mr. Anikesh Pal, Department of Mechanical and Aerospace Engineering, University of California San Diego

Wakes of bluff bodies in a stratified environment are common in oceanic and atmospheric flows. Some examples are marine swimmers, underwater submersibles and flow over mountains and islands. Direct numerical simulations of flow past a sphere in a stratified fluid at a sub-critical Reynolds number (Re) of 3,700 and for a range of Froude numbers, Fr = U/ND ∈ [0.025,∞] are performed. The conservation equations are solved in a cylindrical coordinate system and an immersed boundary method is employed to represent the sphere. The prime objective of this investigation is to understand the statistical response of the near, intermediate and far wake of a sphere at sub-critical Re under the influence of buoyancy. It is observed that buoyancy leads to the inhibition of vertical motion resulting in faster decay of r.m.s. velocity in the vertical direction as compared to the horizontal r.m.s. velocity, collapse of the wake, propagation of internal gravity waves and the organization of the primarily horizontal flow into coherent vortical structures. Unprecedented with respect to previous studies, the time averaged turbulent kinetic energy budget is closed for the unstratified and stratified cases. A novel finding of this research is the regeneration of turbulent fluctuations in the near wake when the stratification increases beyond a critical level (Fr decreases beyond a critical value) which is in contrast to the previous results at lower Re that suggest monotone suppression of turbulence with increasing stratification. Vorticity evolution, energy spectra and the turbulence energy equation explain turbulence regeneration. Another objective of this study is to quantify the distinction between the body and turbulence generated internal waves, in terms of the amplitude, frequency, potential energy distribution and propagation angles. With a decrease in Fr, the body generation mechanism become stronger and waves exhibit upstream propagation.

Anikesh Pal is a Ph.D. candidate in the Department of Mechanical and Aerospace Engineering at University of California San Diego. He received his M.Tech from Indian Institute of Technology Kanpur, INDIA in Mechanical Engineering (Fluid and Thermal Sciences). His current research focuses on the dynamics of the flow past bluff bodies in stratified fluid. He is also interested in studying particle laden flows in stratified environment and their applications in geophysical flows.