Large-scale streaks in wall-bounded turbulent flows: amplication, instability, self-sustaining process and control

• Main Author: Hwang, Yongyun
• Language: ENG
• Published: Ecole Polytechnique X 2010
• Subjects: [SPI:MECA:MEFL] Engineering Sciences/Mechanics/Fluids mechanics; [PHYS:MECA:MEFL] Physics/Mechanics/Mechanics of the fluids; Large-scale streaks; wall turbulence; non-modal growth; instability; self-sustaining process; drag reduction
• Online Access: http://pastel.archives-ouvertes.fr/pastel-00564901; http://pastel.archives-ouvertes.fr/docs/00/56/49/01/PDF/Thesis_final.pdf

Wall-bounded turbulent flows such as plane Couette flow, channel, pipe flows and boundary layer flows are fundamental problem of interest that we often meet in many scientific and engineering situations. The goal of the present thesis is to investigate the origin of large-scale streaky motions observed in the wall-bounded turbulent flows. Under a hypothesis that the large-scale streaky motions sustain with a process similar to the well-known near-wall self-sustaining cycle, the present thesis have pursued on four separate subjects: (i) non-modal amplification of streaks, (ii) the secondary instability of the finite amplitude streaks, (iii) existence of a self-sustaining process at large scale and (iv) turbulent skin friction reduction by forcing streaks. First, using a linear model with turbulent mean flow and the related eddy viscosity, it is shown that the streaks are largely amplified by harmonic and stochastic forcing. The largely amplified streaks undergo the secondary instability and it has been associated with the formation of the large-scale motions (bulge). The existence of a self-sustaining process involving the amplification and instability of streaks at large scale is proved by quenching the smaller-scale energy carrying eddies in the near-wall and logarithmic regions. Finally, it is shown that artificially forcing of large-scale streaks reduce the turbulent skin friction up to 10\% by attenuating the near-wall streamwise vortices.