Principal Parameters in Flow Separation Patterns of Over-Expanded Single Expansion RAMP Nozzle

Abstract

The paper investigates the loss of performance due to flow separation in an over-expanded single expansion ramp nozzle (SERN) during the startup and shutdown process. Characterization of the performance of SERN in this flight regime is complicated as the high degree of asymmetry in the geometry of SERN results in asymmetric separation patterns. In this investigation, the various separation patterns of SERN have been studied by way of numerical simulations. The characteristics of the over-expanded separation patterns have been detailed, and the effects of the critical parameters, such as the length of the flap, the expansion ramp angle and the Mach number, have been analyzed. The outcome of the simulations indicates that restricted shock separation (RSS) accounts for the largest range of over-expanded pressure ratios. The length of the flap can affect the separation patterns, and RSS on the flap does not occur for a short flap SERN. Moreover, with the ramp angle larger than 27 degrees, RSS on the ramp can be avoided, while an angle smaller than 20 degree can circumvent free shock separation (FSS) as well as RSS on the flap. When the vehicle accelerates from very low speeds, the separation pattern is changed in a complex manner, transitioning from FSS to RSS and then back to FSS.