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SUMMARY:Cause-and-effect of linear mechanisms sustaining in wall turbulenc
 e - Adrian Lozano-Duran\, MIT\, USA
DTSTART:20210521T150000Z
DTEND:20210521T160000Z
UID:TALK160477@talks.cam.ac.uk
CONTACT:virginia mullins
DESCRIPTION:Despite the nonlinear nature of turbulence\, there is evidence
  that part of the energy-transfer mechanisms sustaining wall turbulence ca
 n be ascribed to linear processes. The different scenarios stem from linea
 r stability theory and comprise exponential instabilities\, neutral modes\
 , transient growth from non-normal operators\, and parametric instabilitie
 s from temporal mean-flow variations\, among others. These mechanisms\, ea
 ch potentially capable of leading to the observed turbulence structure\, a
 re rooted in simplified physical models. Whether the flow follows any or a
  combination of them remains elusive. Here\, we evaluate the linear mechan
 isms responsible for the energy transfer from the streamwise-averaged mean
 -flow U to the fluctuating velocities u’. To that end\, we use cause-and
 -effect analysis based on interventions: manipulation of the causing varia
 ble leads to changes in the effect. This is achieved by direct numerical s
 imulation of turbulent channel flows at low Reynolds number\, in which the
  energy transfer from U to u’ is constrained to preclude a targeted line
 ar mechanism. We show that transient growth is sufficient for sustaining r
 ealistic wall turbulence. Self-sustaining turbulence persists when exponen
 tial instabilities\, neutral modes\, and parametric instabilities of the m
 ean flow are suppressed. We further show that a key component of transient
  growth is the Orr/push-over mechanism induced by spanwise variations of t
 he base flow. 
LOCATION:GKB 100 Fluid Mechanics Webinar Series
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