Numerical study of the start-up process in an optimized rocket nozzle

This work is to analyze the start-up process of the subscale optimized J2-S rocket nozzle by means of axisymmetric time-accurate Reynolds Averaged Navier–Stokes simulations. Specifically, the phenomena of the FSS → RSS transition (from fully separated to reattached flow), onset of backflow on the nozzle axis and opening of the separation bubble are investigated. Numerical predictions show a remarkable good agreement with experimental data. In particular, the FSS → RSS transition is correctly captured by axisymmetric modelling. The analysis of the flowfield across the shock pattern leads to a simple criterion for the FSS → RSS transition to occur. Furthermore, a mechanism and criterion for the appearance of backflow on the axis is suggested