High resolution simulation of the turbulent wake behind a sphere in a stratified fluid /
- Author(s): De Stadler, Matthew Bronson;
- et al.
The wake of a bluff body is significantly modified by the presence of a stable density stratification. Buoyancy effects introduce a complex coupling between kinetic and potential energy which results in a significantly longer wake lifetime, internal wave radiation, and long lived coherent structures. This dissertation presents results obtained from high resolution numerical simulations of stratified turbulent wakes. The dissertation is divided into two parts. The first part of the dissertation uses the well established temporal approximation to simulate from the near wake to the far wake. In this part of the dissertation, the effect of the Prandtl number on a stratified turbulent wake was considered. For 0.2 < Pr < 7 the qualitative behavior of the wake is the same as Pr = 1. There are differences in a number of small scale features but these changes are small relative to the significantly higher computational cost. The role of momentum imbalance in the stratified wake of a propelled body was also considered. Adding a small to moderate amount of excess momentum, 40% or less, to a self- propelled wake does not produce qualitatively different behavior. Flow statistics retain the character of a self- propelled wake but with larger quantitative values. The second part of the dissertation involved development and application of a new computational tool to simulate spatially-evolving flow past a sphere. Flow past a weakly heated sphere is considered to demonstrate the suitability of the present scheme to accurately capture flow past a body. The most significant contribution of the present dissertation is the simulation of flow past a sphere at a Froude number of 3. The present case of flow past a sphere in a stable density stratification is the first spatially evolving numerical simulation to capture the early to intermediate development of the stratified turbulent wake. The wake evolution occurs in 3 stages : a very weakly stratified near wake (Nt < 2), a transition region (2 < Nt < 5) and a more strongly stratified region corresponding to the non-equilibrium regime for Nt > 5. Buoyancy effects lead to strong anisotropy in the velocity field in the wake beginning shortly after Nt = 2