BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Stabilization of absolute instability in spanwise wavy wake/Stabil
 ity of a downflowing gyrotatic microorganism suspension in a two-dimension
 al vertical channel - Dr Yongyun Hwang\, DAMTP
DTSTART:20131101T160000Z
DTEND:20131101T170000Z
UID:TALK48051@talks.cam.ac.uk
CONTACT:Dr C. P.  Caulfield
DESCRIPTION:1: Controlling vortex shedding using spanwise-varying passive 
 or active actuation (namely three-dimensional control) has recently been r
 eported as a very efficient method for regulating two-dimensional bluff-bo
 dy wakes. However\, the mechanism of how the designed controller regulates
  vortex shedding is not clearly understood. To understand this mechanism\,
  we study a linear stability analysis of two-dimensional wakes\, the base 
 flow of which is modified with a given spanwise waviness. Floquet theory f
 or determining absolute instability in spatially periodic flows is first i
 ntroduced\, and we show that the spanwise wavy base-flow modification stab
 ilizes the absolute\ninstability of two-dimensional parallel wakes. The ph
 ysical mechanism of the stabilization is explained in terms of vortex dyna
 mics. Finally\, the sensitivity of absolute instability to spanwise wavy b
 ase-flow modification is investigated\, showing that absolute instability 
 of two-dimensional wakes is much less sensitive to spanwise wavy base-flow
  modification than to two-dimensional modification.\n\n2: Hydrodynamic foc
 using of cells is a robust feature in downflowing\nsuspensions of swimming
  gyrotatic microorganisms. In the early experiments with a downward pipe f
 low\, Kessler (1986\, J. Fluid Mech\, 173:191-205) observed that the focus
 ed beam-like structure of cells in the region of most rapid downflow exhib
 its regular-spaced axisymmetric blips\, but the mechanism by which the bli
 ps are formed has not been well understood yet. For this purpose\, we perf
 orm a linear stability analysis of a downflowing suspension of randomly sw
 imming gyrotactic cells in a two-dimensional vertical channel. For relativ
 ely small flow rates\, the focused beam in the channel exhibits a\nvaricos
 e instability strikingly similar to the blips in the pipe flow\, and\nthis
  becomes gradually damped out as the flow rate increases. It is found that
  the varicose instability essentially originates from the interaction of c
 ell-concentration fluctuations with the horizontal gradient of the cell-or
 ientation vector field\, which does not appear in uniform suspensions. A c
 omparison is finally made with recent experimental results by Croze & Bees
  (2013\, In preparation)\, showing qualitatively good agreement. 
LOCATION:MR2\, Centre for Mathematical Sciences\, Wilberforce Road\, Cambr
 idge
END:VEVENT
END:VCALENDAR