Screech: Edge‐tone‐like behavior induced by a small plate

Abstract

The dominant screech frequency of round choked jets falls steadily with increasing pressure ratio, interrupted by jumps as the jet instability mode—A1 and A2 (toroidal), B (sinuous), C (helical), and D (sinuous again≈B)—changes. A small normal plate (diam/nozzle diam = 0.35) on the axis radically changes the behavior. Preliminary results indicate that oscillations in the screech frequency range take place only in four “preferred” frequency bands, three being narrow of constant frequency, separated by prominent “dead” bands of no activity. With variable nozzle‐plate distance, the composite plot for all pressure ratios shows each composite band to be reminiscent of classical sawtooth edge tones, with the number of cycles N in the apparent feedback loops from 2 to at least 8, with continuity of each N from band to band. But each band may contain more than one screech‐like mode: Just toroidal for the band of highest frequency, then sinuous and helical, then sinuous, helical and sinuous again and finally just sinuous for the lowest frequency band. [Partial support from the Texas Advanced Research Program.]