Mode switch in tonal under-expanded impinging jets

Abstract

The dominant resonance mode switch pattern concerning nozzle pressure ratio and impinging distance for under-expanded jets issuing from a convergent nozzle and impacting a flat surface is demonstrated for the first time. Schlieren visualization and acoustic measurements have been carried out to simultaneously capture the dynamic and acoustic properties of impinging jets with a nozzle pressure ratio ranging from 2.00 to 4.30 (from moderately under-expanded to highly under-expanded) and an impingement distance extent 3.0D to 11.0D, where D is the nozzle exit diameter. Five bands within which symmetric mode predominates have been discovered. Jets within these bands also exhibit high amplitudes in overall sound pressure level. Besides, dominant frequencies within any band remain relatively stable when impinging distance is fixed, while a negative correlation between dominant frequencies and impingement distance has been found. Dynamic mode decomposition has been applied to schlieren snapshot sequences to achieve dimension reduction, by which the sound source location of tones can be determined. It is found that symmetric mode finds its genesis near the impingement surface, whereas the source related to asymmetric mode lies in the shear layer near the third shock cell or the standoff shock, depending on the number of shock cells between the nozzle exit and the impinging surface. This difference in acoustic feedback paths serves as the cause of the mode switch. Further, the symmetric mode's source location's stability results in the stability of dominant frequencies within a band when impinging distance is fixed.