Millisecond-scale shock-train evolution in high-pressure ratio nozzles: Schlieren imaging and qualitative analysis of shock–boundary layer interaction

Abstract

The classical picture of shock evolution in nozzles holds that under over-expanded flow conditions, a single, nominally normal shock exists within the nozzle. Focusing on the highly dynamic flow produced during blow-down of an experimental, high-nozzle pressure ratio, planar nozzle, this article presents visual evidence that shock-trains – here, a pair of parallel, nominally normal shocks – dominate the rapidly evolving flow field. Three principal results are presented in this study. First, high-speed schlieren images of the evolving nozzle flow are reported. Second, a simple qualitative model of shock–boundary layer/recirculation zone interaction is proposed and used to explain observed millisecond-scale shock-train structure. Third, limited wall pressure measurements and schlieren images are combined to propose a second qualitative model of shock-train–boundary layer/recirculation zone evolution on the longer blow-down process time-scale. The results provide insight into millisecond-scale compressible flow dynamics within high-nozzle pressure ratios .