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SUMMARY:Optimal streaks in 2D wakes and their stabilizing effect on local 
 and global instabilities
DTSTART:20140526T150000
DTEND:20140526T170000
DTSTAMP:20260510T000032Z
UID:0276554471ee1d374d7f39ef6e44333312028c113c3c8f83595b9ed5
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Carlo Cossu\, Directeur de recherche CNRS\, Institut de 
 mécanique des fluides (IMFT)\, Toulouse\nThis talk will summarize recent 
 results obtained on the forcing of optimal perturbations in wakes and on t
 heir stabilizing action on local and global wake instabilities.\nIn the fi
 rst part of the talk\, parallel wakes are considered. Optimal temporal tra
 nsient energy growths of  streamwise uniform perturbations are computed. 
 Optimal initial conditions consist in streamwise vortices and the optimall
 y amplified perturbations are streamwise streaks. It is shown that\, when 
 forced at finite amplitude in absolutely unstable wakes\, optimal streaks 
 can completely suppress the absolute instability converting it into a conv
 ective instability.\nNext\, a synthetic non-parallel and the circular cyli
 nder wakes are considered.\nOptimal spatial energy amplifications of stead
 y disturbances are computed\, finding again that streamwise streaks are th
 e most amplified structures. It is shown that global instabilities can be 
 stabilized by optimal streaks of sufficiently large amplitude.\nWe also sh
 ow that the amplitude of optimal spanwise periodic (3D) perturbations of t
 he basic flow required to stabilize the global instability is smaller than
  the one required by spanwise uniform (2D) perturbations despite the fact 
 that the first order sensitivity of the global eigenvalue to basic flow mo
 difications is zero for 3D spanwise periodic modifications and non-zero fo
 r 2D modifications.\nWe therefore conclude that first-order sensitivity an
 alyses can be misleading if used far from the instability threshold\, wher
 e higher order terms are the most relevant.\nBio: I'm interested in fluid 
 mechanics and applied mathematics and\, more specifically\, in the underst
 anding of transition to turbulence and the design of innovative flow contr
 ol strategies. These topics are relevant to a number of applications rangi
 ng from the `vaccination' of boundary layers to delay transition and reduc
 e drag to the understanding of the dynamics of accretion disks. You can le
 arn more browsing the pages listed on the on the left. If you are a highly
  motivated student and you feel interested in these topics do not hesitate
  to contact me.
LOCATION:BM 1130 https://plan.epfl.ch/?room==BM%201130
STATUS:CONFIRMED
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