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SUMMARY:Nonlinear development and acoustic radiation of instability waves/
 coherent structures in transitional/turbulent free shear flows - Professor
  Xuesong Wu\, Imperial College London
DTSTART:20150227T130000Z
DTEND:20150227T140000Z
UID:TALK55129@talks.cam.ac.uk
CONTACT:Aleksandra Przydrozna
DESCRIPTION:  Free shear flows\, such as mixing layers\, jets and wakes\, 
 are inviscidly unstable due to their inflectional velocity profiles. Insta
 bility modes\, which are usually excited by external perturbations\, ampli
 fy on the shear flow\, leading to vortex roll-up and randomization in the 
 nonlinear stage. Interestingly\, in turbulent state free shear flows exhib
 it a high degree of order\, characterised by the prevalent presence of so-
 called coherent structures\, the most striking of which are 'Brown-Roshko 
 rollers'.\n\nInstability waves and coherent structures are known to be dyn
 amically significant for entrainment and mixing\, noise generation as well
  as for turbulence modelling. In this talk\, I will present a nonlinear cr
 itical-layer theory to describe first the development of instability modes
  on laminar free shear layers. The theory predicts vortex roll-up and rand
 omisation through a generalized side-band instability mechanism. The theor
 y is then modified to describe formation and evolution of 'Brown-Roshko ro
 llers' on turbulent mixing layers. The mechanism of acoustic radiation by 
 coherent structures will be highlighted.
LOCATION:LR3B\, Inglis Building\, CUED 
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