On the onset of postshock flow instabilities over concave surfaces

This work reports a numerical investigation of shock focusing phenomena over concave surfaces. The study focuses on the effects of Reynolds and Mach numbers on the detailed behavior of flow features related to shear-layer instabilities and jet formation in the post-shock region. Computations are done for four incident-shock Mach numbers covering subsonic and transonic flow regimes and a wide range of Reynolds numbers. The simulations reveal a number of interesting wave features starting from early stage of shock interaction and transition from inverse-Mach reflection to transitioned regular reflection followed by very complex flow patterns at focusing and post focusing stages. Different subsequent flow characteristics develop as a result of multiple shock/shear layer interactions. During the later stage of the flow interaction, a formation of two opposing jets is predicted by the simulation in accordance with the experiments. It is shown that the formation of primary opposing jets as well as the development of Kelvin-Helmholtz instabilities can be hindered for low Mach and Reynolds numbers. However, for high flow regimes a second pair of opposing jets appears and develops far from the wall, exhibiting similar features as the primary pair of opposing jets at moderate Mach numbers. Two new bifurcations in flow patterns are observed at this stage which promote further development of vortex structures and shear-layer rollup. (C) 2014 AIP Publishing LLC.