Flow field and noise characteristics of a supersonic impinging jet

This paper describes the results of a study examining the flow and acoustic characteristics of an axisymmetric supersonic jet issuing from a sonic and a Mach 1.5 converging-diverging (C-D) nozzle and impinging on a ground pla ne. Emphasis is placed on the Mach 1.5 nozzle with the sonic nozzle used ma inly for comparison. A large-diameter circular plate was attached at the no zzle exit to measure the forces generated on the plate owing to jet impinge ment. The experimental results described in this paper include lift loss, p article image velocimetry (PIV) and acoustic measurements. Suckdown forces as high as 60% of the primary jet thrust were measured when the ground plan e was very close to the jet exit. The PIV measurements were used to explain the increase in suckdown forces due to high entrainment velocities. The se lf-sustained oscillatory frequencies of the impinging jet were predicted us ing a feedback loop that uses the measured convection velocities of the lar ge-scale coherent vortical structures in the jet shear layer. Nearfield aco ustic measurements indicate that the presence of the ground plane increases the overall sound pressure levels (OASPL) by approximately 8 dB relative t o a corresponding free jet. For moderately underexpanded jets, the influenc e of the shock cells on the important flow features was found to be negligi ble except for close proximity of the ground plane.