Flow Separation and Control

We investigate unsteady dynamics in hypersonic sphere wakes using direct numerical simulations (DNS). High-speed sphere wakes exhibit distinct features including inner viscous and outer inviscid regions separated by a dividing streamline, which converges downstream to form a recompression shock at the wake neck. The recirculation zone behind the sphere, bounded by this shock, generates complex unsteady phenomena including neck oscillations, shear layer instabilities, vortex shedding, and shock-shear layer interactions. Our DNS computations reveal the coupling between recirculation bubble unsteadiness and recompression shock oscillations at the neck. The simulations capture broadband and periodic fluctuations in base pressure and wake structure consistent with experimental observations. We examine how shock oscillations interact with shear layer instabilities at the neck region, analyzing the coherent structures and their energy budgets that drive this coupled unsteadiness. The role of compressibility effects, including shock motion and density fluctuations on far-wake turbulence evolution is quantified with physical parameters such as Mach and Reynolds numbers. Spectral analysis reveals dominant frequencies and the coherent structures associated with both shock oscillation and shear layer modes. The study provides insights into the fundamental mechanisms governing hypersonic wake dynamics.