Over-expanded separation transitions of single expansion ramp nozzle in the accelerating and decelerating processes

Abstract

Flow separation is a basic fluid-dynamics phenomenon that occurs in supersonic nozzles at low pressure ratios. In the over-expanded flowfield of nozzles, the subsonic area is formed, such as separation bubble, recirculation zone, and flow downstream Mach reflection. In static ambient, the recirculation zone is opened with the environment. When the external flow is not still, it will interact with the recirculation zone. As a result, the external flow will make significant influences on the over-expanded separation flowfield. This paper aims to present a detailed description of separation patterns transitions caused by external Mach number variation, analyze the mechanism of separation transitions. Accelerating and decelerating processes of a long flap single expansion ramp nozzle (SERN) is investigated by numerical simulation. Separation transitions and the performance of SERN influenced by the separation transitions have been discussed. In the accelerating process, free shock separation (FSS) is a stable separation pattern and lasts for a long time. The decelerating process is not just the inverse process because a new over-expanded flowfield pattern appears in the decelerating process. Meanwhile, there is an apparent hysteresis between the accelerating and decelerating processes.