An experimental rocket model and numerical plume boundary studies for under-expanded flow

Abstract

Experiments were conducted to validate the numerical results generated for an argon/helium gas co-flow in a rarefied environment and to present images of a rocket nozzle plume boundary in background pressures ranging from a correctly-matched nozzle static pressure condition to a moderate vacuum. The experiments provided visualisation of ideal and under-expanded plume shapes for the various background pressures. Plume flow experiments were conducted at the Australian Defence Force Academy where a shock tube was modified to a Ludwieg tube and coupled with a test section, pump and dump tank. An argon gas delivery system was devised to achieve the flow requirements of the experiment. Visualisation of the jet was conducted with a schlieren system and pitot pressure probe results for the argon jet and helium plume were obtained. A numerical plume prediction methodology was employed to obtain approximate plume boundaries for the quiescent and co-flow tests. The results were overlaid on the experimental results. Good agreement was obtained for the ideal and slightly under-expanded conditions. Numerical studies were conducted to evaluate the correct boundary conditions for plume flow regions of interest in both the co-flow and stationary atmosphere conditions. Regions of noncontinuum flow were identified with Birds breakdown parameter and a numerical method for low density flow was employed. Correlation of the results for the experimental and numerical plume boundaries was achieved for the ideal and the underexpanded cases. Separation of the external boundary layer was identified as altering the pressure in the aft region of the rocket. The numerical methodology employed to correlate plume boundary visualisation for an experimental model of a rocket plume inserted in a quiescent and co-flow environment was utilised for flow not significantly removed from equilibrium. Regions of nonequilibrium flow were identified and designated as requiring a suitable numerical method for rarefied flows. The plume boundary data obtained from the experimental and computational results was used to model elevated Knudsen number wedge flow with the Modified Moment Method of Myong. The method was found superior for modelling nonequilibrium regions of flow near the leading edge of a wedge when compared to the Navier-Stokes/Fourier equations.