Investigation of local unstart in a hypersonic scramjet intake at a Mach number of 6

Abstract Understanding the mechanism of local unstart in a mixed compression scramjet intake is vital for its stable off-design operation. In the present work, the local unstart behavior of a hypersonic scramjet intake is studied at a Mach number of 6, and a flight realistic Reynolds number of 8.4 × 10 6 /m using a combination of experiments in a hypersonic wind tunnel and numerical computations. To study the effect of internal contraction ratio (ICR), experiments are performed on an intake model that is equipped with an all movable cowl in two modes: (a) cowl fixed at a particular ICR, and (b) cowl moved to a particular ICR during the steady test time of the wind tunnel. The intake exhibits started flow for ICR between 1.19 and 1.37, and locally unstarted flow for ICR ≥1.45. In the dynamic cowl experiments, the impingement of the forebody shock onto the inner cowl surface forms a leading-edge separation bubble, which moves upstream and detaches the cowl shock, leading to local unstart with subsonic spillage of mass near cowl lip. A shear layer emanating from the triple point of shock interaction divides this subsonic flow and the supersonic flow at the ramp side. The frequency content of the flow is different between the fixed and the dynamic cowl cases. Local unstart observed during fixed cowl experiments reveal low amplitude self-sustained oscillations around a frequency of 2.5 kHz. The dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) of numerical density contours reveal that the shear layer impingement on the cowl surface drives the self-sustained oscillations observed during local unstart.