THE FEEDBACK LOOP IN IMPINGING 2-DIMENSIONAL HIGH-SUBSONIC AND SUPERSONIC JETS

The instabilities in a supersonic impinging jet are investigated by so lving the two-dimensional Euler equations using the piecewise paraboli c method (PPM) and Roe's linearized Riemann solver. The predicted shoc k cell spacing agrees well with the observed and theoretical values. T he frequency and nature of the dominant instabilities are found to be a function of the impingement distance. Two instability modes are poss ible: a symmetric (or varicose) mode and an asymmetric (or sinuous) mo de. For two given jet exit Mach numbers (M = 0.98 and 1.29), the energ y in, and frequency of, these modes are functions of impingement dista nce, leading to an integral staging due to an acoustic feedback loop. The predicted frequencies of the fundamental symmetric and asymmetric instabilities agree with the theoretically allowed values. The staging of predicted frequencies that occurs in experimental work is also pre dicted.