When an imperfectly expanded supersonic jet exits a nozzle, the mismatch between exhaust pressure and ambient pressure creates standing shock cells — a repeating pattern of compressions and expansions downstream. Kelvin-Helmholtz instabilities grow along the jet's shear layer, producing waves that propagate downstream. When these waves interact with the shock cells, they generate upstream-propagating acoustic waves. If the acoustic waves reach the nozzle lip and trigger new instabilities at the right phase, the loop closes and the jet screams — a sustained, high-amplitude oscillation audible across an airfield.
Farghadan, Beekman, Nogueira, Edgington-Mitchell, and Towne show that the linear resonance model of screech — waves going around the loop, amplifying each pass — is insufficient. Using resolvent analysis and a novel bilinear formulation, they demonstrate that the screech mode's nonlinear self-interaction redistributes energy to other frequencies. The screech doesn't just sustain itself through linear feedback. It also drives fluctuations at harmonics and subharmonics through triadic interactions, where three different frequency components exchange energy.
The through-claim is about what self-sustaining means. A linear feedback loop maintains itself by amplifying one signal through one cycle. The jet screech maintains itself and simultaneously creates other oscillations through nonlinear coupling. The sustained tone is not just a closed loop at one frequency — it is a pump that drives energy into frequencies that were not part of the original instability. Self-sustaining oscillation in a nonlinear system is not just persistence. It is generation. The screech creates what wasn't there.