Numerical analysis of flow separation in rocket nozzles

Abstract

The focus of present study is to have a comprehensible understanding of flow physics involved in separation in rocket nozzle. Numerical analysis of flow separation in rocket nozzles is challenging and involved task because of the complex turbulent flow behaviour that develops different shock-patterns. The study will have to consider shock-boundary layer interaction with shear layers and formation of vortices that includes the generation of flow features like the Mach disk, separation shock, Mach stem, vortex core, contact surface, slip stream and shock front. When the back pressure is high enough, the boundary layer separates and moves freely away from the nozzle wall. A recirculation zone of ambient air is created at the nozzle exit which will not allow the flow reattachment. This is a basic separation pattern, commonly known as Free Shock Separation (FSS) that can be observed in all kinds of nozzles; especially, conical, contoured or Truncated Ideal Contoured (TIC) nozzles. The flow separation pattern is considerably different in strongly overexpanded nozzles with an internal shock, like Thrust Optimized Contoured nozzles (TOC) or Thrust Optimized Parabolic bell nozzles (TOP). The flow is getting reattached to the nozzle wall creating a restricted circulation region forming a closed separation bubble and is named as Restricted Shock Separation (RSS). The paper presents numerical simulations carried out with different nozzle geometries and chamber-to-ambient pressure ratios with comprehensive assessment of the flow features and flow separation structures. Separation of supersonic flow in convergent–divergent nozzles is investigated by solving the Reynolds-averaged Navier–Stokes equations with a two-equation k-ω turbulence model.