Oscillations in Rectangular Supersonic Inlets with Large Internal Contraction Ratio

Abstract

Increasing the internal contraction ratio (defined as the area ratio of the duct entrance and throat, denoted as ICR) can improve the operating performance of a variable geometry supersonic inlet, however, excessive ICR typically will bring the inlet into an unstarted state with oscillation. To uncover the underlying mechanism of a large ICR-induced oscillatory-unstarted phenomenon, we employed a high-speed schlieren and dynamic pressure acquisition system, and experimentally examined the flowfields of a rectangular supersonic inlet with different ICRs of 1.5, 2.0, 2.5, and 3.0 in the wind tunnel at a freestream Mach number of 2.9. The inlet stayed unstarted while the ICR ranged from 2.0 to 3.0, and the flowfield had three features. First, the inlet flowfields showed high- and broadband-frequency oscillations, whose dominant frequency and oscillatory amplitude increased with ICR. Second, the inlet flowfields revealed similar oscillatory flow structures, which contained the periodic variation of the ramp-side boundary layer separation, cowl-side flow spillage, and shock train in the internal contraction part. Third, the unstarted flow structure and oscillatory phenomenon mainly occurred upstream of the inlet geometric throat, downstream of which the main flowfield was supersonic. To characterize these three features, we proposed a self-excited oscillatory model consisting of a disturbing source, a disturbing feedback terminal, and a signal feedback loop. Practically, the disturbing source is a scale variation of the entrance separation, the disturbing feedback terminal is an aerodynamic throat near the geometric throat, and the signal feedback loop is composed of communication pathways of shock train motion, acoustic waves, and convection waves. With this model, we explained the underlying mechanism of the high- and broadband-frequency characteristics of the oscillation caused by large ICR, and the ICR’s influence on the flow oscillation.