Numerical investigation of the instability of a supersonic jet impacting a substrate.

A numerical model is used to explain the high-frequency surface pressure fluctuations experimentally observed during the transient impact of a supersonic jet onto a substrate. The observations are made during experiments associated with the Shockwave-Induced Spray Process. Various approximations of the experimental arrangement and operating conditions are made to simplify the model and maintain a reasonable computational cost. The model geometry is axially symmetric, and a single mean jet Mach number value is used. The selected Mach number corresponds to the transient value that occurs during the high-frequency oscillation. The steady Reynolds-Averaged Navier-Stokes equations are first solved to establish the mean velocity field, followed by an unsteady Reynolds-Averaged Navier-Stokes solution to resolve the fluctuations. Due to the presence of acoustic wave propagation, non-reflecting boundary conditions are employed on the atmospheric boundaries of the solution domain. The numerical results indicate that the substrate pressure fluctuations are due to an instability of the bow shockwave that forms ahead of the substrate. The shock experiences an axially pulsing oscillation which is consistent with that previously described in the literature. The predicted frequency of oscillation however does not agree with the experiment. This is explained by the differences in the current model geometry compared to that of the actual experimental equipment. [ABSTRACT FROM AUTHOR]