Finite-Time Lyapunov Exponent Analysis of Intermittent Acoustic Events in a Round Jet

摘要

Acoustic motions in the simulated flow field of unforced and forced subsonic, unheated jets are explored from a Lagrangian point of view. The Finite-Time Lyapunov Exponent (FTLE) technique developed for the identification of dominant, flow-organizing Lagrangian coherent structures is used. Intermittent events are first identified based on validated near field pressure probe data subjected to Empirical Mode Decomposition (EMD). The FTLE field is then obtained from a series of snapshots encompassing the intermittent event. Lagrangian Coherent Structures (LCS) from the backward FTLE field show that each intermittent event is the result of a localized entrainment event. This connection is highlighted with a simple logarithmic transformation of the FTLE field, in conjunction with an integration time targeting small-scale motions. This basic dynamic holds for both unforced and forced jets, though excitation regularizes events in the latter. The method thus facilitates identification of instantaneous causal events in the vicinity of the jet shear layer that have been shown to correlate to far-field noise. The Eulerian connection between coherent structures and the near acoustic field is also summarized.