SCREECH TONES FROM FREE AND DUCTED SUPERSONIC JETS

It is well known that screech tones from supersonic jets are generated by a feedback loop. The loop consists of three main components. They are the downstream propagating instability wave, the shock cell struct ure in the jet plume, and the feedback acoustic waves immediately outs ide the jet. Evidence will be presented to show that the screech frequ ency is largely controlled by the characteristics of the feedback acou stic waves. The feedback loop is driven by the instability wave of the jet. Thus the tone intensity and its occurrence are dictated by the c haracteristics of the instability wave. In this paper the dependence o f the instability wave spectrum on the azimuthal mode number (axisymme tric or helical/flapping mode, etc.), the jet-to-ambient gas temperatu re ratio, and the jet Mach number are studied. The results of this stu dy provide an explanation for the observed screech tone mode switch ph enomenon (changing from axisymmetric to helical mode as Mach number in creases) and the often-cited experimental observation that tone intens ity reduces with increase in jet temperature. For ducted supersonic je ts screech tones can also be generated by feedback loops formed by the coupling of normal duct modes to instability waves of the jet. The sc reech frequencies are dictated by the frequencies of the duct modes. S uper resonance, resonance involving very large pressure oscillations, can occur when the feedback loop is powered by the most amplified inst ability wave. It is proposed that the observed large amplitude pressur e fluctuations and tone in the test cells of Arnold Engineering Develo pment Center were generated by super resonance. Estimated super-resona nce frequency for a Mach 1.3 axisymmetric jet tested in the facility a grees well with measurement.