Abstract

The shear instability in jets at high Reynolds number, which leads to the roll-up of the shear layer into vortices, has been investigated over many decades by means of local and global linear analysis. It has even been shown in recent years that the linear perturbation wavepackets that are identi€ed by such approaches are present in the statistics of turbulent jets in the form of dominant coherent structures (1).

But the nonlinear and three-dimensional instability dynamics in jets do not thrive on Kelvin–Helmholtz-type shear instability alone. In this seminar, two other scenarios will be presented: €first, the secondary instability of developed vortex streets, which gives rise to vortex pairing and jet bifurcation, is characterised by way of Floquet analysis and optimal forcing in time-periodic base flows (2). Second, the formation of streaks in jets is explored, underpinned by the lift-up mechanism. Streaks have only very recently been identi€ed as strong co-herent structures in turbulent jets, and we are currently investigating their effect on the shear instability that drives the turbulent wavepackets responsible for jet noise.

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1. see my seminar at DAMTP , “Harmonic and Stochastic Wavepackets in Jets”, May 3, 2016

2. Shaabani-Ardali, Sipp & Lessha (2019) “Optimal triggering of jet bifurcation, an example of optimal forcing applied to a time-periodic base ƒow”, under review for J. Fluid Mech.