Physical insights into unstart dynamics of a hypersonic mixed compression intake

Abstract

Hypersonic air-breathing cruise vehicles powered by supersonic combustion ramjet engines are the potential candidate for future space and defense applications. The air intake of the scramjet engine is a vital component that uses shock waves to compress the air to pressure and temperatures suitable for supersonic combustion. Understanding the unstart dynamics of such intakes is of prime importance for the seamless operation of scramjet intakes. While the unstart dynamics in supersonic intakes are studied widely by various researchers, only a few such studies are reported in hypersonic intakes. The mechanisms associated with the same are not clearly understood. In the current work, a design optimization framework is established by coupling (a) oblique-shock theory and Non-dominated Sorting Genetic Algorithm II (NSGA-II) and (b) Computational fluid dynamics (CFD) and NSGA - II to minimize total pressure loss and maximize intake exit temperature of planar mixed compression intake at a design Mach number of 6. The ramp and cowl angles constitute the design space. The intake with maximum exit temperature is chosen to study its unstart dynamics using a combination of experiments in a hypersonic wind tunnel (M = 6 and Re = 8.86 × 106/m) and unsteady numerical investigations using the open-source suite SU2. The intake model is equipped with a movable cowl and flap to study the internal contraction and throttling induced unstart. Simultaneous pressure measurements and schlieren flow visualization are carried out to study unsteady flow physics associated with intake unstart. The dynamic content in the flow is analyzed using Fast Fourier Transform (FFT) and spectrogram of the unsteady pressure signal and Dynamic Mode Decomposition (DMD) of the schlieren images and density contours. In this work, two different modes of shock oscillation during unstart are observed when the flap is moved while the cowl is held stationary. At ICR = 1.19, the intake shows started behavior for throttling ratio 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 and 1100 Hz, respectively. A staged evolution (5 stages) to a subsonic spillage oscillatory unstart is detailed using corroborative evidence from both time-resolved schlieren and pressure measurements. The ramp side separation bubble drives the high amplitude oscillatory unstart. At ICR = 1.37, the shear layer emanating from the triple point of shock interaction drives the low amplitude oscillatory unstart with a dominant frequency of about 3.7 kHz for a throttling ratio of 0.69. A criterion for demarcating the modes of unstart is evolved using current and previous data. The actual shock on lip condition during started operation demarcates the two modes of oscillatory unstart. Unsteady numerical computations are performed to study the effect of enthalpy on the unstart frequency. The frequency of unstart varies linearly with stagnation acoustic speed and is an appropriate velocity scale. During unstart, the extent of the subsonic region is the appropriate length scale to be used in the quarter-wave resonance model to estimate unstart frequency pertaining to high mechanical blockage