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CATEGORIES:Isaac Newton Institute Seminar Series
SUMMARY:When turbulence first reaches the wall - Tamer Zak
 i (Johns Hopkins University)
DTSTART;TZID=Europe/London:20220328T143000
DTEND;TZID=Europe/London:20220328T150000
UID:TALK171128AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/171128
DESCRIPTION:Laminar-to-turbulence transition in boundary layer
 s is accompanied by concentration of&nbsp\;the mea
 n spanwise vorticity near and at the boundary.&nbs
 p\; The increase of wall vorticity\, or equivalent
 ly the wall stress\, is even more pronounced in th
 e instantaneous fields when a nascent patch of tur
 bulence first reaches the wall.&nbsp\; The fluid d
 ynamical mechanism for this increase is examined r
 igorously using the stochastic Lagrangian formulat
 ion of the Navier-Stokes equations\, which quantif
 ies all&nbsp\;possible contributors.&nbsp\; We exp
 ress the instantaneous wall vorticity as the expec
 tation of a stochastic Cauchy invariant in backwar
 d time\, with terms due the&nbsp\;(a) wall-vortici
 ty flux (Lighthill source)&nbsp\;and (b) interior 
 vorticity evolved by&nbsp\;nonlinear advection\, v
 iscous diffusion\, vortex&nbsp\;stretching&nbsp\;a
 nd tilting.&nbsp\; We evaluate these terms for an 
 ensemble of transition events from a simulation of
  bypass transition that is publicly available thro
 ugh the&nbsp\;JHU database. &nbsp\;The results yie
 ld a&nbsp\;clear&nbsp\;picture of the origin of th
 e&nbsp\;enhanced wall vorticity in this flow\, whi
 ch is not the Lighthill source but instead a diffe
 rent mechanism suggested&nbsp\;by Lighthill (1963)
 .\n&nbsp\;
LOCATION:Seminar Room 1\, Newton Institute
CONTACT:
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