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SUMMARY:Minimal seeds for turbulent bands in channel flow - Stefania Cheru
 bini (Politecnico di Bari)
DTSTART:20220328T110000Z
DTEND:20220328T113000Z
UID:TALK171119@talks.cam.ac.uk
DESCRIPTION:Recently\, many authors have investigated the origin and growt
 h of turbulent bands in shear flows\, highlighting the role of streaks and
  their inflectional instability in the process of band generation and sust
 ainment. Recalling that streaks are created by an optimal transient growth
  mechanism\, and motivated by the observation of a strong increase of the 
 disturbance kinetic energy corresponding to the creation of turbulent band
 s\, we use linear and nonlinear energy optimizations in a tilted domain to
  unveil the main mechanisms allowing the creation of a turbulent band in c
 hannel flow. While linear optimal perturbations induce turbulence in the w
 hole domain\, spatially-localized perturbations obtained by nonlinear opti
 mization\, or by artificially confining the linear optimal to a localized 
 region in the transverse direction\, lead to the generation of a localised
  turbulent band through the formation of large-scale vortices. These resul
 ts suggest that two main elements are needed for inducing turbulent bands 
 in a tilted domain: i) a linear energy growth mechanism\, such as the lift
 -up\, for generating large-amplitude flow structures\, which produce infle
 ction points\; ii) spatial localisation\, linked to the presence or genera
 tion of large-scale vortices. We show that these elements alone can genera
 te isolated turbulent bands also in large non-tilted domains. In such fram
 ework\, using nonlinear optimization together with energy bisection\, we f
 ind a scaling law for the minimal energy threshold needed to trigger incli
 ned bands. A similar scaling is found in a much smaller domain\, where lam
 inar-turbulent stripes are not observed\, but the values of the minimal en
 ergy and the shape of the minimal seed strongly differ. Finally\, based on
  the evolution of such minimal seeds\, two different mechanisms of formati
 on of turbulent bands in channel flow are discussed\, depending on the Rey
 nolds number and initial energy of the perturbation. The selection of one 
 of these two mechanisms appears to be dependent on the probability of deca
 y of the newly-created stripe\, which increases with time\, but decreases 
 with the Reynolds number. 
LOCATION:Seminar Room 1\, Newton Institute
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