Predicting supersonic jet mode-switching using non-negative matrix factorization

Abstract

Imperfectly expanded supersonic jets contain a shock-cell structure in the jet plume which plays a role in generating screech tones along with other jet noise sources. Depending on the Mach number of the jet, two types of jet motion or “jet modes”—a toroidal mode and flapping mode may occur, resulting in the generation of different screech tones that are not harmonics of each other. At a given jet condition, these jet modes (and the corresponding screech tones) can switch between each other based on environmental conditions in a seemingly non-deterministic manner. Understanding when mode-switching occurs at a fine time resolution can provide better understanding of the sensitivity of the jet structure dynamics. This work presents the preliminary results of a novel method that predicts when a particular screech tone is active using non-negative matrix factorization. Using this technique, spectrograms of jet noise are decomposed into a set of basis vectors corresponding to the screech tones present in the jet, and a set of activation functions that indicate when each screech basis is dominant. An activation function comparison technique is then used to identify activations corresponding to mode-switching.