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CATEGORIES:Isaac Newton Institute Seminar Series
SUMMARY:Transition in wall-bounded flows - Lee\, C (Peking
 )
DTSTART;TZID=Europe/London:20080911T121000
DTEND;TZID=Europe/London:20080911T123000
UID:TALK13376AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/13376
DESCRIPTION:In this article we present direct comparisons of e
 xperimental results on transition in wall-bounded 
 flows obtained by flow visualizations\, hot-film m
 easurement\, and particle-image velocimetry (PIV)\
 , along with a brief mention of relevant theoretic
 al progresses\, based on a critical review of abou
 t 120 selected publications. Despite somewhat diff
 erent initial disturbance conditions used in exper
 iments\, the flow structures were found to be prac
 tically the same. The following observed flow stru
 ctures are considered to be of fundamental importa
 nce in understanding transitional wall-bounded flo
 ws: The three-dimensional nonlinear wave packets c
 alled solitons-like coherent structures (SCS) in b
 oundary layer and pipe flows\; the -vortex\; the s
 econdary vortex loops\; and the chain of ring vort
 ices. The dynamic processes of the formation of th
 ese structures and transition as newly discovered 
 by recent experiments include\, among others: (1) 
 The sequential interaction processes between the -
 vortex and the secondary vortex loops\, which cont
 rols the manner by which the chain of ring vortice
 s is periodically introduced from the wall region 
 into the outer region of the boundary layerG (2) T
 he generation of high-frequency vortices\, which i
 s one of the key issues for understanding both tra
 nsitional and developed turbulent boundary layers 
 (as well as other flows)\, of which several explan
 ations have been proposed but a particularly clear
  interpretation can be provided by the experimenta
 l discovery of secondary vortex loops. The ignoran
 ce of secondary vortex loops would make the dynami
 c processes and flow structures in a transitional 
 boundary layer inconsistent with previous discover
 ies\; and (3) The dominant role of SCS in all turb
 ulent bursting\, which is considered as the key me
 chanism of turbulent production in a low Reynolds-
 number turbulent boundary layer. Of direct relevan
 ce to bursting is the low-speed streaks\, whose fo
 rmation mechanism and link to the flow structures 
 in wall-bounded flows can be answered more clearly
  than before in terms of the SCS dynamics. We comb
 ine these newly observed structures and processes 
 to those well-known ones to form a more integrated
  physical picture of the transitional dynamics. Th
 is not only enables revisiting the classic story o
 f wall-bounded flow transition\, but also opens a 
 new avenue to reconstruct the possible universal s
 cenario for wall bounded flow transition. \n
LOCATION:Seminar Room 1\, Newton Institute
CONTACT:Mustapha Amrani
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