Numerical investigation of annular expansion-deflection nozzle flow under varying backpressure changing rate

The characteristics of flow and thrust evolution of an annular Expansion-Deflection (ED) nozzle are numerically investigated under varying backpressure changing rates during ascending and descending trajectories. The objective is to test the sensitivity of unsteady behaviors of shock waves in the ED nozzle to backpressure changing rate, and to further elucidate the thrust evolution mechanism and mode transition hysteresis. The movement of shock reflection points on the nozzle wall follows two flow mechanisms, namely, shock self-excited oscillations and rapid backpressure changes. A low backpressure changing rate enables shock self-excited oscillations, leading to a reciprocating motion of the shock waves accompanied by thrust oscillations, while a high backpressure changing rate suppresses the shock self-excited oscillations, leading to a unidirectional motion of the wave system on the nozzle shroud wall. A criterion for distinguishing ED nozzle operation modes is proposed, which relies on the loading inflection points of the nozzle pintle base and exhibits a fast and user-friendly feature. A dual-wake mode hysteresis region is defined to quantify the hysteresis in nozzle mode transition, with the span of the region decreasing as the backpressure changing rate slows down. The present work helps in understanding the unsteady flow mechanism and thrust evolution in ED nozzles.