Numerical Investigation of Over-Expanded Nozzle Flows: Influence of Internal Shock Waves

Introduction

During ascent in low altitudes, space launchers experience high nozzle exit pressure leading to flow separation in the divergent. Two flow separation regimes have been identified: free shock separation (FFS) and restricted shock separation (RSS) regimes [1]. The transition between these two regimes with potential asymmetry of the flow could lead to high side loads which are of great practical importance. Meanwhile, modern rocket nozzles are optimized for maximum thrust during the entire ascent trajectory. In compliance with the launcher overall dimensions, the divergent part of these nozzles is truncated and the nozzle area-ratio are readjusted to satisfy the exit Mach number. Thus, the resulting nozzle contour (TOC contour) is characterized by a high angle of divergence at the throat compared to an ideal nozzle contour. As a consequence, an internal shock emanating from the nozzle throat region can interact with the shock waves system produced by flow separation at the end of the nozzle. Recently for the first time, the influence of the internal shock wave on the transition between free separation and restricted separation regimes has been questioned [2]. An experimental work has been carried out in the framework of a PhD thesis at ONERA, Meudon, France. This work has had both a theoretical part by analyzing shock waves interferences inside the divergent and an extensive experimental program during which measurements of both stationary and non stationary pressures were obtained.