Transition of streamwise streaks in zero presure gradient boundary layers

Författare: Brandt, L.B., Henningson, D.S.H.
Dokumenttyp: Artikel
Tillstånd: Publicerad
Tidskrift: J. Fluid Mech.
Volym: 472   229-262
År: 2002


A transition scenario initiated by streamwise low- and high-speed streaks in a flat plate boundary layer is studied. In many shear flows, in fact the perturbations that show the highest potential for transient energy amplification consist of streamwise aligned vortices. Due to the lift-up mechanism these optimal disturbances lead to elongated streamwise streaks downstream, with significant spanwise modulation. In a previous investigation (Andersson, Brandt, Bottaro & Henningson 2001), the stability of these streaks in a zero pressure gradient boundary layer has been studied by means of Floquet theory and numerical simulations. The sinuous instability mode was found to be the most dangerous disturbance. For the first time, we present here the simulation of the breakdown to turbulence originating from the sinuous instability of streamwise streaks. The main structures observed during the transition process consist of elongated quasi-streamwise vortices located on the flanks of the low speed streak. Vortices of alternating sign are overlapping in the streamwise direction in a staggered pattern. The present scenario is compared with transition initiated by Tollmien-Schlichting waves and their secondary instability and by-pass transition initiated by a pair of oblique waves. The relevance of this scenario to transition induced by free-stream turbulence is also discussed.