DOI: 10.25549/usctheses-c40-378230
title: Laboratory and numerical experiments on the near wake of a moving body in a stratified fluid

description: The wake behind a moving body in a stratified ambient has been of continued interest due to its geophysical and naval applications. While in many cases, the wake is initially turbulent, it eventually evolves into a state dominated by low Froude number dynamics and frequently contains persistent pattern information. Much empirical evidence has been collected on the late wake, but less on the near wake where the effect of stratification is in debate and how it evolves into the late stage is unclear. This thesis tackles this problem by investigating the near wake generated by a grid and a sphere in a linearly stratified background over a wide range of Reynolds number and Froude number, using both experimental and numerical approaches. The results not only provide a quantitative description of the near wake behavior, but also can serve as a benchmark for the stratified flow community in related research. ❧ First, a porous circular grid is used as a generator of turbulent wakes. Proper optically-based measurement of the early-stage flow in the vertical center plane is made possible by using refractive index matching techniques. Temporally averaged results show the interplay between internal wave motions and Kelvin-Helmholtz-generated vortices, which occur predominantly at the wake edge due to the strong vertical shear. The stratified turbulence, originating from Kelvin-Helmholtz instabilities, develops up until a non-dimensional time Nt≈10, after which the flow starts to be taken over by low Froude number dynamics. The near wake, however, shows certain dependence on both Reynolds number and Froude number. Therefore, the evidence does not support universality in this stage, which may only be possible when a certain constraint is imposed on the Froude number range. ❧ The dynamic mode decomposition (DMD) technique, is then applied to analyze flow patterns and their energy spectra for the same data set. The lee wave and vortices of different length scales are captured successfully using DMD. A mode energy spectrum is constructed to track the evolution of the vortices, showing a clear dependence on Froude number. We also observe the energy transfer to larger scales in the near wake, as the initial shear-triggered roll-ups diffuse and pair with neighbors. The temporal DMD modes and spatial DMD modes are compared from multiple angles. This whole chapter also serves as a detailed example of the application of DMD to time-resolved PIV data for stratified flow, and confirms the utility of DMD in identifying dynamics at different scales. ❧ In the last part, a sphere is used to study the stratification effects on the near wake structure of a bluff body, with Froude number greater than 0.5 and Reynolds number less than 1000. Simulation results provide full-field information, and experimental results serve as a benchmark check. The wakes are found to undergo similar structural transitions across different Reynolds numbers when decreasing Froude number, except that such a similar transition occurs at a lower Froude number when the Reynolds number is larger. Possible empirical parametrization of centerline defect velocity is explored, and a model of damped oscillation superimposed on power law appears quite successful in characterizing the strongly stratified cases.