Modal and non-modal linear wavepacket dynamics in turbulent jets

We perform data-driven analyses in order to explore some well-documented discrepancies between linear models and observed wavepacket dynamics in turbulent jets. The paper asks if these discrepancies may be due to non-modal effects that are not usually incorporated in linear models. A locally parallel, spatial transient-growth analysis is first performed, the objective being to address differences between linear model and data in the region downstream of the potential core. The results show how in this region, following stabilisation of the KelviHelmholtz mode, non-modal effects become important in terms of both the streamwise and radial wavepacket organisation. LES data is then used in conjunction with the linearised Euler equations, where linearisation is about the global, non-parallel, mean flow. The objective is to explore two further questions. (i) Are the mechanisms identified in the optimal, locally parallel, transient-growth study present in the real flow, which is non-parallel and does not necessarily contain optimally excited structures? (ii) Are non-modal mechanisms important for two-point coherence decay? The answers to both questions are affirmative: non-modal phenomena are shown to be a key feature of turbulent-jet wavepacket dynamics. The study further-more suggests that these mechanisms are activated by non-linear interactions distributed throughout the flow and that might be modelled as a volume forcing of the linear operator. © 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.