Mechanism and prediction of flow oscillation based on counter-flow jet for drag reduction in hypersonic flow

초록

The counter-flow jet, which is utilized for drag reduction in high-speed aircraft, creates an extremely unstable flow mode known as the long penetration mode (LPM) when the injection mass flow rate is low. Although the LPM provides a high drag reduction efficiency, it can cause an unstable condition that disrupts aircraft control owing to the oscillation characteristics. In this study, the oscillation mechanism of the LPM was investigated through experiments and computational simulations. It was confirmed that fluid impulse is the main factor affecting the LPM flow behavior, and that the recirculation region pressure determines the fluid impulse variation. To validate the effect of the fluid impulse, the frequency of the LPM oscillation was predicted based on the analyzed mechanism using a mass-spring model. The results showed that the applied modeling predicts the oscillation frequency satisfactorily. Therefore, the results confirmed that fluid impulse is a dominant factor that affects the LPM flow oscillation. In addition, the condition under which the flow structure undergoes a transition was investigated via fluid impulse. Based on some physical assumptions, the transition condition was analyzed. It was confirmed that the fluid impulse and static pressure in the recirculation region impose a dominant effect on the flow oscillation and the transition from LPM to SPM.

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