BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Boundary layer transition by discrete and continuous modes - Durbi
 n\, R (Iowa State)
DTSTART:20080910T103000Z
DTEND:20080910T105000Z
UID:TALK13357@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:The natural and bypass routes to boundary layer turbulence hav
 e traditionally been studied independently. One can characterize our study
  as an exploration of the situation in which both occur. Experiments indic
 ate that this may be pertinent in certain flow regimes\, particularly in a
 dverse pressure gradients. We study this case by direct numerical simulati
 on (DNS). \n\nThe inflow condition is a superposition of a 3-D continuous 
 mode and a 2-D T-S wave onto a Blasius mean flow. The T-S and continuous m
 odes are obtained by solving the Orr-Sommerfeld and Squire equations by we
 ll established numerical methods. The DNS is accomplished with a finite vo
 lume\, staggered mesh\, fractional step algorithm for incompressible Navie
 r-Stokes equations. \n\nEither mode\, of itself\, is unable to provoke tra
 nsition. With both modes present\, transition usually occurred within the 
 computational domain. Transition was preceded by the appearance of Lambda-
 shaped velocity contours. Although this is reminiscent of secondary instab
 ility of T-S waves\, the lateral spacing between Lambda's was very much na
 rrower and seemed to be controlled by spanwise wavelength in the continuou
 s mode. However\, the spacing and wavelength were not necessarily equal. T
 wo broad classes of behavior were seen\, as epitomized by modes 2 and 5. \
 n\nIn mode 2 the Lambda's were grouped in staggered rows. The elements of 
 a row are pairs Lambda's. The pairs are aligned in $z$ within the row\, wh
 ich is followed by another row of pairs\, shifted horizontally half way be
 tween the previous row. Flow visualization will be presented. The lateral 
 spacing between Lambda's within a row is equal to that of the continuous m
 ode --- actually of the perturbation jets spawned by the continuous mode. 
 \n\nMode 5 produced a more irregular pattern\, but still arranged in pairs
  Lambda's. Unlike mode 2\, their spanwise spacing differs from that of the
  continuous mode\; it appears to be about three times as wide. Another cur
 ious aspect of mode 5 is that a larger streak amplitude can delay transiti
 on. Mode 2 shows transition to move upstream as the Klebanoff streaks get 
 stronger. Mode 5 initially promotes transition\, then\, as its amplitude i
 ncreases further\, it delays transition. \n
LOCATION:Seminar Room 1\, Newton Institute
END:VEVENT
END:VCALENDAR