Experimental investigation of unstart dynamics driven by subsonic spillage in a hypersonic scramjet intake at Mach 6

Abstract

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.

Length scale for the estimation of buzz frequency in the limit of high mechanical blockage in mixed-compression intakes

“…However, the flow features corresponding to this higher- case, which were already reported in Devaraj et al. (2020), are similar to those of . At , the separation bubble present on the ramp side deflects SJ away from the ramp wall and allows it to enter the isolator.…”

“…(2020) and the authors (Devaraj et al. 2020) have reported intake unstart at hypersonic Mach numbers, and have observed that the frequencies of oscillation are in reasonable agreement with the predictions based on the quarter-wave resonator model. The geometric differences between supersonic and hypersonic intakes – i.e.…”

“…Tan, Shu & Zhi-Long (2009) have used the model given by Newsome (1984) and have reported discrepancies with the experimentally observed frequencies in a hypersonic intake. On the other hand, recently, Berto et al (2020) and the authors (Devaraj et al 2020) have reported intake unstart at hypersonic Mach numbers, and have observed that the frequencies of oscillation are in reasonable agreement with the predictions based on the quarter-wave resonator model. The geometric differences between supersonic and hypersonic intakes -i.e.…”

“…In our previous work (Devaraj et al 2020), we have shown that the flow features in the intake differ significantly between static and dynamic flap experiments in lower-TR cases. For example, the intake remains in the started condition in a dynamic flap experiment, (Ferri criterion) and subsonic spillage, and (iii) ramp-side separation and supersonic spillage (Tan et al 2009;Li et al 2013).…”

“…Further details of the facility and instrumentation are given in Devaraj et al. (2020). The dynamic content associated with the flow is analysed with the help of fast Fourier transform (FFT) of the unsteady pressure signals as well as dynamic mode decomposition (DMD) of the schlieren image sequence.…”