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SUMMARY:Transition in wall-bounded flows - Lee\, C (Peking)
DTSTART:20080911T111000Z
DTEND:20080911T113000Z
UID:TALK13376@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:In this article we present direct comparisons of experimental 
 results on transition in wall-bounded flows obtained by flow visualization
 s\, hot-film measurement\, and particle-image velocimetry (PIV)\, along wi
 th a brief mention of relevant theoretical progresses\, based on a critica
 l review of about 120 selected publications. Despite somewhat different in
 itial disturbance conditions used in experiments\, the flow structures wer
 e found to be practically the same. The following observed flow structures
  are considered to be of fundamental importance in understanding transitio
 nal wall-bounded flows: The three-dimensional nonlinear wave packets calle
 d solitons-like coherent structures (SCS) in boundary layer and pipe flows
 \; the -vortex\; the secondary vortex loops\; and the chain of ring vortic
 es. The dynamic processes of the formation of these structures and transit
 ion as newly discovered by recent experiments include\, among others: (1) 
 The sequential interaction processes between the -vortex and the secondary
  vortex loops\, which controls the manner by which the chain of ring vorti
 ces is periodically introduced from the wall region into the outer region 
 of the boundary layerG (2) The generation of high-frequency vortices\, whi
 ch is one of the key issues for understanding both transitional and develo
 ped turbulent boundary layers (as well as other flows)\, of which several 
 explanations have been proposed but a particularly clear interpretation ca
 n be provided by the experimental discovery of secondary vortex loops. The
  ignorance of secondary vortex loops would make the dynamic processes and 
 flow structures in a transitional boundary layer inconsistent with previou
 s discoveries\; and (3) The dominant role of SCS in all turbulent bursting
 \, which is considered as the key mechanism of turbulent production in a l
 ow Reynolds-number turbulent boundary layer. Of direct relevance to bursti
 ng is the low-speed streaks\, whose formation mechanism and link to the fl
 ow structures in wall-bounded flows can be answered more clearly than befo
 re in terms of the SCS dynamics. We combine these newly observed structure
 s and processes to those well-known ones to form a more integrated physica
 l picture of the transitional dynamics. This not only enables revisiting t
 he classic story of wall-bounded flow transition\, but also opens a new av
 enue to reconstruct the possible universal scenario for wall bounded flow 
 transition. \n
LOCATION:Seminar Room 1\, Newton Institute
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