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SUMMARY:Optimal transient growth and very large scale structures in turbul
 ent boundary layers - Cossu\, C (LadHyx)
DTSTART:20080909T143000Z
DTEND:20080909T145000Z
UID:TALK13349@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:The optimal energy growth of perturbations sustained by a zero
  pressure gradient turbulent boundary is computed using the eddy viscosity
  associated with the turbulent mean flow. It is found that even if all the
  considered turbulent mean profiles are linearly stable\, they support tra
 nsient energy growths. The most amplified perturbations are streamwise uni
 form and correspond to streamwise streaks originated by streamwise vortice
 s. For sufficiently large Reynolds numbers two distinct peaks of the optim
 al growth exist respectively scaling in inner and outer units. The optimal
  structures associated with the peak scaling in inner units correspond wel
 l to the most probable streaks and vortices observed in the buffer layer a
 nd their moderate energy growth is independent of the Reynolds number. The
  energy growth associated with the peak scaling in outer units is larger t
 han that of the inner peak and scales linearly with an effective turbulent
  Reynolds number formed with the maximum eddy viscosity and a modified Rot
 ta-Clauser length based on the momentum thickness. The corresponding optim
 al perturbations consist in very large scale structures with a spanwise wa
 velength of the order of 8 $delta$. The associated optimal streaks scale i
 n outer variables in the outer region and in wall units in the inner regio
 n of the boundary layer\, there being proportional to the mean flow veloci
 ty. These outer streaks protrude far into the near wall region\, having st
 ill 50% of their maximum amplitude at $y^+=20$. The amplification of very 
 large scale structures appears to be a robust feature of the turbulent bou
 ndary layer: Optimal perturbations with spanwise wavelengths ranging from 
 4 to 15 $delta$ can all reach 80% of the overall optimal peak growth.
LOCATION:Seminar Room 1\, Newton Institute
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