A signal processing perspective of acoustically-excited jet shear layers

Abstract

An alternative perspective of the behaviour of jet shear layers in response to acoustic excitation is developed using concepts in signal processing. A central idea is that the vortices that roll-up in the shear layer are similar to the discrete samples of a digital control system. This analogy has the interesting corollary that the Nyquist-Shannon sampling theorem should apply. It has also been understood since the 1990s that shear layers act as demodulators when subjected to amplitude-modulated perturbations – some distance downstream of the source of excitation, only perturbations at the modulating frequency persist (at baseband). It is possible to explain this behaviour through the lens of signal processing by describing the relationship between the shear layer vorticity signal and the excitation waveform as a rectifier, along with the sampling action already described. The rectifier + sampler model offers insights into shear layer behaviour that are not perceptible using the tools of fluid mechanics alone. The model is verified through a series of experiments on both free and attached jets. The implications of the analogy are explored, one of which is the advent of an excitation technique where the amplitude modulation signal driving the acoustic source is overmodulated. This overmodulation technique has application in flow control, and results in a doubling of the response frequency and hence the input bandwidth. This talk is based on two recently published journal papers.