BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Lagrangian measurements of inertial particle accelerations in shea
 r\, and in grid generated turbulence - Warhaft\, Z (Cornell)
DTSTART:20080910T092000Z
DTEND:20080910T094000Z
UID:TALK13359@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:We describe two dimensional Lagrangian acceleration statistics
  of inertial particles in a turbulent boundary layer and contrast the resu
 lts with those observed in decaying grid generated turbulence (Ayyalasomay
 ajula PRL .Vol. 97\,144507 (2006)) . The statistics were determined by mea
 ns of particle tracking techniques using a high speed camera moving along 
 the side of a wind tunnel at the mean flow speed . Water droplets were fed
  into the flow using two different methods: sprays placed down-stream from
  an active grid\, and from tubes fed into the boundary layer from humidifi
 ers. The boundary layer\, which had considerable free stream turbulence\, 
 was formed above a flat plate placed horizontally in the tunnel. \n\nThe f
 lows are described in terms of the Stokes\, Froude and Reynolds numbers. F
 or the flow conditions studied\, the sprays produced Stokes numbers varyin
 g from 0.47 to 1.2\, and the humidifiers produced Stokes numbers varying f
 rom 0.035 to 0.25\, where the low and high values refer to the outer bound
 ary layer edge and the near-wall region\, respectively. The Froude number 
 was approximately 1.0 for the sprays and 0.25 for the humidifiers. The bou
 ndary layer momentum thickness Reynolds number was approximately 800. The 
 free stream turbulence was varied by operating the grid in the active mode
  as well as a passive mode (the latter behaves as a conventional grid). Th
 e effects of the free stream turbulence on the acceleration statistics wer
 e systematically studied. \n\nAt the outer edge of the boundary layer\, wh
 ere the shear was weak\, the acceleration probability density functions we
 re similar to those previously observed in isotropic turbulence for inerti
 al particles. As the boundary layer plate was approached\, the tails of th
 e probability density functions narrowed\, became negatively skewed\, and 
 their peak occurred at negative accelerations (decelerations in the stream
 -wise direction). The mean deceleration and its rms increased to large val
 ues close to the plate. These effects were more pronounced at higher Stoke
 s number. Although there were free stream turbulence effects\, and the com
 plex boundary layer structure played an important role\, a simple model su
 ggests that the acceleration behavior is dominated by shear and inertia. T
 he results are contrasted with inertial particles in isotropic turbulence 
 and with fluid particle acceleration statistics in a boundary layer. \n\nT
 he work was supported by the US National Science Foundation. \n
LOCATION:Seminar Room 1\, Newton Institute
END:VEVENT
END:VCALENDAR