Abstract
The origin and propagation of low-frequency shock oscillation unsteadiness in the attached and separated flows are investigated. Wind tunnel experiments are performed in an isolator at Mach 1.85 and 2.7 with three types of upstream wedges, generating weak and strong background waves. High-speed schlieren imaging and high-resonance frequency pressure measurements are used to capture the flow features. In the attached flow with weak background waves, the impingement of the reflected shocks along the flow strengthens the original instability waves from the shock oscillation, resulting in the correlation drop and time-delay rise with the original instability waves. In the attached flow with strong background waves, two-point correlation analyses show that the shock oscillations propagate along the shock structure and convection of the boundary layer structures, which enhances the turbulence pulsation in the boundary layer. The correlation and coherence results for pressure indicate that the incident points of two independent background waves move in opposite directions, while the incident points of two merged background waves move in the same direction. Using downstream throttling, the shock train in the separated flow is introduced. Based on the phase analysis of schlieren images, the feedback mechanism of the shock train oscillation is described, which is related to the acoustic wave propagation and the duct volume effect. Power spectra of the pressure in the upstream attached and downstream separated flows of the shock train show that the perturbation pathways in the attached and separated flows do not affect each other.
Highlights
Over the past three decades, with the development of highspeed flight, ramjet engines, rockets, and space flight, significant progress has been made in the theory of compressible flow, which has become an important scientific field.1 Compressible flow can be categorized into subsonic flow (0.3 < M < 0.8),2 transonic flow (0.8 < M < 1.2),3 supersonic flow (1.2 < M < 5.0),4–7 and hypersonic flow (M > 5.0).8 In transonic and low-to-mid supersonic flows, the release of heat in the downstream combustor typically leads to the separation of the boundary layer, and a complex system of shock and compression waves, called shock trains, is housed in a short duct called the isolator
The results showed that the receptivity of fast sound waves under the conditions generated by an isothermal wall was less than that under adiabatic conditions
In the actual working conditions of a ramjet/scramjet, the attached flow develops from the freestream flow, the shock-wave/boundary-layer interaction (SWBLI) is caused by the impingement of the background waves generated from the geometry of the intake system on the boundary layer, and the shock train is caused by the high backpressure induced by the downstream combustion
Summary
Over the past three decades, with the development of highspeed flight, ramjet engines, rockets, and space flight, significant progress has been made in the theory of compressible flow, which has become an important scientific field. Compressible flow can be categorized into subsonic flow (0.3 < M < 0.8), transonic flow (0.8 < M < 1.2), supersonic flow (1.2 < M < 5.0), and hypersonic flow (M > 5.0). In transonic and low-to-mid supersonic flows, the release of heat in the downstream combustor typically leads to the separation of the boundary layer, and a complex system of shock and compression waves, called shock trains, is housed in a short duct called the isolator. Further coherence and phase analyses of high-speed schlieren images showed that the feedback mechanism of the shock train oscillation was related to the propagation of acoustic waves, the duct volume effect, and Kantrowitz limits.38 In summarizing these previous studies, it becomes apparent that investigations into the attached flow lack information concerning the evolution of the parameter spectra along the boundary layer, and few studies have been carried out on attached flow with incident shocks. In the actual working conditions of a ramjet/scramjet, the attached flow develops from the freestream flow, the SWBLI is caused by the impingement of the background waves generated from the geometry of the intake system on the boundary layer, and the shock train is caused by the high backpressure induced by the downstream combustion. III B 2, the perturbation pathway of the shock train oscillation in a separated flow with strong background waves and its relationship with the perturbation propagation in the attached flow are analyzed
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