Author: Anikesh Pal

Publisher:

Published: 2016

Total Pages: 170

ISBN-13:

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. The first part of the research in stratified wakes concerns temporal/spatial simulations of turbulent self-propelled/towed wakes without including a body. Direct numerical simulations are performed to contrast the influence of the mean velocity profile with that of the initial turbulence on the subsequent evolution of velocity and density fluctuations in a stratified self-propelled wake. It is also verified that results of temporal simulations matches with that of the spatial simulations when the initial near-wake condition of the temporal approximation is chosen to match the inflow of the spatially evolving model. Typically, the wake of a body develops in the presence of external fluctuations, motivating a study of wake evolution under the influence of various intensities of external turbulence. The stratified wake was found to decay substantially faster than its unstratified counterpart for same intensity of the external turbulence. Theoretical arguments and additional simulations were performed to show that the level of external turbulence relative to wake turbulence is a key governing parameter in both stratified and unstratified backgrounds. The second part of this research focuses on flow past a sphere in a stratified fluid at a sub-critical Reynolds number of 3,700 and for a range of Froude numbers U/ND \in [0.025,1]. 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.