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SUMMARY:Three-dimensional instability dynamics in jets - Lutz Lesshafft\, 
 Laboratoire d’Hydrodynamique\, École polytechnique
DTSTART:20191115T160000Z
DTEND:20191115T170000Z
UID:TALK132346@talks.cam.ac.uk
CONTACT:73696
DESCRIPTION:The shear instability in jets at high Reynolds number\, which 
 leads to the roll-up of the shear layer into vortices\, has been investiga
 ted 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 tu
 rbulent jets in the form of dominant coherent structures (1).\n\nBut the n
 onlinear 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 bifu
 rcation\, 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 ver
 y recently been identi€ed as strong co-herent structures in turbulent jet
 s\, and we are currently investigating their effect on the shear instabili
 ty that drives the turbulent wavepackets responsible for jet noise.\n\n---
 -----------------------\n\n1. see my seminar at DAMTP\, “Harmonic and St
 ochastic Wavepackets in Jets”\, May 3\, 2016\n\n2. Shaabani-Ardali\, Sip
 p & 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.
LOCATION:MR2\, Centre for Mathematical Sciences\, Wilberforce Road\, Cambr
 idge
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