Back-pressure effects on unsteadiness of separation shock in a rectangular duct at Mach 3

To evaluate back-pressure effects on unsteadiness of separation shock in a rectangular duct, some test cases were designed and conducted in a supersonic air-breathing wind tunnel. High-speed Schlieren technique and high-frequency pressure measurements were utilized for data acquisition. Different back-pressure levels were applied by changing downstream throttling ratio. Some parameters for estimating canonical separation flows, such as zero-crossing frequency and intermittency, have been introduced to analyze the duct separation flow. The experimental results show that the separation shock oscillation frequencies increase and the intermittent region lengths decrease with the increasing back-pressure level. A comparison between the unsteadiness of shock/boundary layer interactions for the canonical flow and the duct internal flow were made. It is found that the normalized Strouhal number for the duct internal separation flow ranges from 0.01 to 0.03, which share the similar range with canonical separation flows. That is to say, the physics of separation shock unsteadiness is similar over a wide range of interaction flows, which may share the inherent flow mechanism.