UC San Diego

Turbulent wakes are pervasive in man-made and natural environments. In the ocean and the atmosphere, these wakes interact with the background ambient stratification to give rise to a myriad of interesting phenomena, e.g., multistage decay of mean and turbulence, long-lived coherent structures, and the appearance of internal gravity waves, to name a few. With the rise in supercomputing power, high-fidelity numerical simulations have become an increasingly feasible way to investigate the phenomenology of these wakes. As these simulations become commonplace in research, there is an increased focus on the use of data-driven techniques to uncover the rich dynamics from the obtained datasets. This dissertation is an examination of turbulent wakes using data-driven techniques and numerical simulations.

In the first part, spectral proper orthogonal decomposition is used to investigate a turbulent disk wake database at $\Rey = 5 \times 10^{4}$ and $\Fro = \infty$, $10$, $2$. We first study the evolution of the vortex shedding mode and double helix mode in the unstratified wake ($\Fro = \infty$), building on and refining the previous experimental studies. Thereafter, the SPOD analysis of the stratified wakes is performed that uncovers two new results: (a) coherence originating at the body gets stronger and lives longer with progressively increasing stratification levels and (b) for $\Fro \gtrsim 2$, vortex shedding is the dominant mechanism of internal gravity wave generation.

In the second part of the work, large eddy simulations (LES) are used to investigate the flow past a prolate 6:1 spheroid. Firstly, high-resolution hybrid simulation is used to simulate the far wake of a 6:1 spheroid at 0-degree angle of attack and $\Rey = 10^{5}$, $\Fro = 2$ and $10$. The far wake is compared to the above-mentioned disk database. The spheroid wakes show differences in locations at which mean wake transitions take place. These differences are explained in light of energy budgets. Secondly, large eddy simulations of flow past a 6:1 spheroid at $\Rey = 5000$, $\Fro = \infty, 6, 1.9$, $1$, and a moderate angle of attack $\alpha = 10^{\circ}$ are carried out. Body forces, mean wake and vorticity dynamics, and flow spectra are analyzed in detail and presented in the dissertation.