Experimental investigation of flow unsteadiness characteristics in hypersonic backward facing steps of different heights

Experimental investigation of unsteadiness quantification and source identification in high-speed flows at Mach 4 and Mach 6 over a backward-facing step with step heights of h=8,10,12 mm is attempted. The experiments are conducted at a freestream Reynolds number of Re(1/m)≈16×106 using the Ludwieg tunnel present at the Hypersonic Experimental Aerodynamics Laboratory, Indian Institute of Technology Kanpur. To quantify the extent of unsteadiness, spectral analysis of the pressure fluctuations obtained at various streamwise locations of x/L=0.12,0.19,0.26,0.33,0.4,0.54 is performed. High-speed schlieren images are captured to characterize overall flow features and estimate the size of the separation bubble for each test case. The reattachment length scale Lr/h is found to be constant for a fixed upstream condition with an increment in Mach number resulting in a higher recirculation bubble extent. In order to obtain the qualitative flow feature attributing to the overall unsteadiness, thereby systematically characterizing the dominant unsteady flow features, modal analysis in terms of proper orthogonal decomposition and dynamic mode decomposition is carried out using the schlieren images. The coupling of the re-circulation bubble and initial segments of the separated shear layer, as well as lip shock, is found to bear low-frequency oscillations on the order of StLr∼O(10−2). However, the recompression shock is found to possess high frequency having StLr∼O(10−1). Nevertheless, as a distinguishable difference with a change in Mach number, it is observed that the subsonic boundary layer flow downstream of the reattachment location for Mach 4 is found to have low-frequency oscillations, whereas high-frequency unsteadiness is established for the Mach 6 case.