The interaction of a hypersonic boundary layer on a flat plate with an impinging shock – an order of magnitude stronger than that required for incipient separation of the boundary layer – near sharp and blunt leading edges (with different bluntness radii from 2 to 6 mm) is investigated experimentally, complemented by numerical computations. The resultant separation bubble is of length comparable to the distance of shock impingement from the leading edge, rather than the boundary layer thickness at separation; it is termed large separation bubble. Experiments are performed in the IISc hypersonic shock tunnel HST-2 at nominal Mach numbers 5.88 and 8.54, with total enthalpies 1.26 and $1.85~\text{MJ}~\text{kg}^{-1}$ respectively. Schlieren flow visualization using a high-speed camera and surface pressure measurements using fast response sensors are the diagnostics. For the sharp leading edge case, the separation length was found to follow an inviscid scaling law according to which the scaled separation length $(L_{sep}/x_{r})M_{er}^{3}$ is found to be linearly related to the reattachment pressure ratio $p_{r}/p_{er}$; where $L_{sep}$ is the measured separation length, $x_{r}$ the distance of reattachment from the leading edge, $M$ the Mach number, $p$ the static pressure and the subscripts $r$ and $e$ denote the conditions at the reattachment location and at the edge of the boundary layer at the shock impingement location respectively. However, for all the blunt leading edges $(L_{sep}/x_{r})M_{er}^{3}$ was found to be a constant irrespective of Mach number and much smaller than the sharp leading edge cases. The possible contributions of viscous and non-viscous mechanisms towards the observed phenomena are explored.

Understanding start–unstart behavior of intakes in hypersonic Mach numbers is essential for seamless operation of scramjet engines. We consider a high compression ratio intake (CR = 40) at a Mach number of M = 6 in this work. Start–unstart characteristics are studied in a hypersonic wind tunnel at a flight realistic Reynolds number (Re = 8.7 × 106/m, M = 6). A flap provided at the rear end of the isolator simulates the effect of backpressure for throttling ratios in the range of 0–0.69. Experiments are conducted in two modes: (a) with the flap fixed at a particular throttling ratio and (b) the flap moved to a particular throttling ratio after the started flow has been established. Unsteady pressure measurements and time-resolved Schlieren visualization are undertaken. Modal analysis of pressure (using fast Fourier transform) and Schlieren images (using dynamic mode decomposition) are carried out. The intake shows started behavior for throttling ratios up to 0.31 and a dual behavior, where it remains started in dynamic flap runs but unstarted in fixed flap runs for throttling ratios of 0.35 and 0.42. The intake exhibits a staged evolution to a large amplitude oscillatory unstart for throttling ratios of 0.55 and 0.69, with frequencies of 950 Hz and 1100 Hz, respectively. For the first time, a staged evolution (5 stages) to a subsonic spillage oscillatory unstart of a hypersonic intake is detailed using corroborative evidence from both time-resolved Schlieren and pressure measurements. A precursor to the final large amplitude oscillatory unstart is identified, and the flow mechanism for sustained oscillations is explained.

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