Streak Breakdown Leading to Wall Turbulence

Growth and eventual breakdown of near-wall low-speed streaks are responsible for generating and then sustaining wall turbulence structures in wall-bounded shear flows. This paper reviews our two experiments which are well designed to follow up the development of streak breakdown up to first appearance of wall turbulence structures. We first focus on evolution of subharmonic streak instability into wall turbulence in a spanwise periodic laminar streak flow. When the subharmonic mode grows beyond the nonlinear saturation stage of subharmonic instability, quasi-streamwise vortices developing along the neighboring low-speed streaks strongly interact with each other in each cycle, causing horseshoe vortex structures to develop in a staggered array. Next we focus on streak breakdown caused by high-intensity background turbulence. Low-speed streaks are generated in a quasi-laminar boundary layer containing high-intensity turbulent vortices. In this case, the streak breakdown is not caused by linear streak instability but by transient disturbance growth.