Observations of the oscillation modes of choked circular jets

Under-expanded jets exhausting from convergent circular nozzles are known to emit a powerful acoustic tone called screech. The steady decrease of the screech frequency with increasing pressure ratio is interrupted by four frequency jumps as the mode of jet instability changes successively from an axially symmetric (varicose, torroidal) one labeled A1 (varicose, torroidal), to another but similar one labeled A2, a sinuous (lateral, flapping) one B, a helical one C, and finally one that is identified here as a sinuous one, D. Digital FFT analyses disclose the simultaneous presence of associated secondary tones about 25 dB weaker than the dominant ones, the frequency of these secondary tones forming smooth continuations of the dominant tones. As a secondary tone connects the dominant B and D modes, it is inferred that these are actually the same mode interrupted by the helical mode C. However, their detailed behavior is significantly different: the plane of oscillation of B appears to have a preferred orientation but is accompanied by rather random oscillations in other planes, occasionally having a helical appearance, while D appears be in a single plane that rotates relatively slowly and unsteadily about the jet axis. The helical mode C appears very stable: its frequency is significantly higher (≊14%–23%) than that of the adjacent sinuous mode(s). The convection velocity of the unstable waves in the jet varies from about 0.6 of the ambient sound speed for the modes A to the maximum of about 0.8 for mode B, these corresponding to about 0.6 of the fully expanded jet exit velocity.