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CATEGORIES:Isaac Newton Institute Seminar Series
SUMMARY:Optimal transient growth and very large scale stru
ctures in turbulent boundary layers - Cossu\, C (L
adHyx)
DTSTART;TZID=Europe/London:20080909T153000
DTEND;TZID=Europe/London:20080909T155000
UID:TALK13349AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/13349
DESCRIPTION:The optimal energy growth of perturbations sustain
ed by a zero pressure gradient turbulent boundary
is computed using the eddy viscosity associated wi
th the turbulent mean flow. It is found that even
if all the considered turbulent mean profiles are
linearly stable\, they support transient energy gr
owths. The most amplified perturbations are stream
wise uniform and correspond to streamwise streaks
originated by streamwise vortices. For sufficientl
y large Reynolds numbers two distinct peaks of the
optimal growth exist respectively scaling in inne
r and outer units. The optimal structures associat
ed with the peak scaling in inner units correspond
well to the most probable streaks and vortices ob
served in the buffer layer and their moderate ener
gy growth is independent of the Reynolds number. T
he energy growth associated with the peak scaling
in outer units is larger than that of the inner pe
ak and scales linearly with an effective turbulent
Reynolds number formed with the maximum eddy visc
osity and a modified Rotta-Clauser length based on
the momentum thickness. The corresponding optimal
perturbations consist in very large scale structu
res with a spanwise wavelength of the order of 8 $
delta$. The associated optimal streaks scale in ou
ter variables in the outer region and in wall unit
s in the inner region of the boundary layer\, ther
e being proportional to the mean flow velocity. Th
ese outer streaks protrude far into the near wall
region\, having still 50% of their maximum amplitu
de at $y^+=20$. The amplification of very large sc
ale structures appears to be a robust feature of t
he turbulent boundary layer: Optimal perturbations
with spanwise wavelengths ranging from 4 to 15 $d
elta$ can all reach 80% of the overall optimal pea
k growth.
LOCATION:Seminar Room 1\, Newton Institute
CONTACT:Mustapha Amrani
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