Investigation of Bluff Body Wakes in Incompressible and Compressible Flows via Spectral Element and Discontinuous Galerkin Spectral Element Method Approaches

This thesis focuses on the turbulent bluff body wakes in incompressible and compressible flows. An incompressible wake flow past an axisymmetric body of revolution at a diameter-based Reynolds number Re=5000 is investigated via a direct numerical simulation. It is followed by the development of a compressible solver using a split-form discontinuous Galerkin spectral element method framework with shock capturing. In the study on incompressible wake flows, three dominant coherent vortical motions are identified in the wake: the vortex shedding motion with the frequency of St=0.27, the bubble pumping motion with St=0.02, and the very-low-frequency (VLF) motion originated in the very near wake of the body with the frequencies St=0.002 and 0.005. The very-low-frequency motion is associated with a slow precession of the wake barycenter. The vortex shedding pattern is demonstrated to follow a reflectional symmetry breaking mode, with the detachment location rotating continuously and making a full circle over one vortex shedding period. The VLF radial motion with St=0.005 originates as m = 1 mode, but later transitions into m = 2 mode in the intermediate wake. Proper orthogonaldecomposition (POD) and dynamic mode decomposition (DMD) are further performed to analyze the spatial structure associated with the dominant coherent motions. Results of the POD and DMD analysis are consistent with the results of the azimuthal Fourier analysis. To extend the current incompressible code to be able to solve compressible flows, a computational methodology is developed using a high-order approximation for the compressible Navier-Stokes equations with discontinuities. The methodology is based on a split discretization framework with a summation-by-part operator. An entropy viscosity method and a subcell finite volume method are implemented to capture discontinuities. The developed high-order split-form with shock-capturing methodology is subject to a series of evaluation on cases from subsonic to hypersonic, from one-dimensional to three dimensional. The Taylor-Green vortex case and the supersonic sphere wake case show the capability to handle three-dimensional turbulent flows without and with the presence of shocks. It is also shown that higher-order approximations yield smaller errors than lower-order approximations, for the same number of total degrees of freedom.