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CATEGORIES:Fluid Mechanics (DAMTP)
SUMMARY:Three-dimensional instability dynamics in jets - L
 utz Lesshafft\, Laboratoire d’Hydrodynamique\, Éco
 le polytechnique
DTSTART;TZID=Europe/London:20191115T160000
DTEND;TZID=Europe/London:20191115T170000
UID:TALK132346AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/132346
DESCRIPTION:The shear instability in jets at high Reynolds num
 ber\, which leads to the roll-up of the shear laye
 r into vortices\, has been investigated over many 
 decades by means of local and global linear analys
 is. It has even been shown in recent years that th
 e linear perturbation wavepackets that are identi€
 ed by such approaches are present in the statistic
 s of turbulent jets in the form of dominant cohere
 nt structures (1).\n\nBut the nonlinear and three-
 dimensional instability dynamics in jets do not th
 rive on Kelvin–Helmholtz-type shear instability al
 one. In this seminar\, two other scenarios will be
  presented: €first\, the secondary instability of 
 developed vortex streets\, which gives rise to vor
 tex pairing and jet bifurcation\, is characterised
  by way of Floquet analysis and optimal forcing in
  time-periodic base flows (2). Second\, the format
 ion of streaks in jets is explored\, underpinned b
 y the lift-up mechanism. Streaks have only very re
 cently been identi€ed as strong co-herent structur
 es in turbulent jets\, and we are currently invest
 igating their effect on the shear instability that
  drives the turbulent wavepackets responsible for 
 jet noise.\n\n--------------------------\n\n1. see
  my seminar at DAMTP\, “Harmonic and Stochastic Wa
 vepackets in Jets”\, May 3\, 2016\n\n2. Shaabani-A
 rdali\, Sipp & Lessha (2019) “Optimal triggering 
 of jet bifurcation\, an example of optimal forcing
  applied to a time-periodic base ƒow”\, under revi
 ew for J. Fluid Mech.
LOCATION:MR2\, Centre for Mathematical Sciences\, Wilberfor
 ce Road\, Cambridge
CONTACT:
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