Effect of the jet pressure ratio in supersonic axisymmetric jets of a polyatomic gas SF$_6$ on their gas-dynamic structure

The influence of the jet pressure ratio ($n$ =1.18 $\div$ 3.35) in a nonisobaric supersonic jet of a vibrationally excited gas SF$_6$ exhausting from a convergent axisymmetric nozzle 0.25 mm in diameter is studied numerically and experimentally. The experiments aimed at studying the gas-dynamic structure of the jets are performed in a specially designed jet setup of the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences. The numerical simulations are performed by solving two-dimensional Navier–Stokes equations within the framework of the ANSYS Fluent software and the thermally perfect gas model. The influence of excitation of vibrational degrees of freedom of the SF$_6$ gas is studied in both an equilibrium gas and vibrationally nonequilibrium gas. The nonequilibrium state of vibrational degrees of freedom is simulated with the use of a two-temperature model of relaxation flows. It is shown that the jet pressure ratio of the SF$_6$ gas affects the length of the wave structure cells, which is responsible for the change in the vibrational relaxation rate. The coefficient of density amplitude reduction in gas-dynamic cells is derived as a function of the jet pressure ratio.