Sinuous breakdown in a flat plate boundary layer exposed to free-stream turbulence

In a flat plate boundary layer, perturbed with streaks, breakdown occurs due to a secondary instability acting on the streaks. An experimental study using a water channel with static turbulence grid revealed the presence of a sinuous secondary instability mode in the bypass transition process. Five sinuous instabilities are investigated in detail in the horizontal plane. The streamwise length scale of the sinuous instability is around 40 delta(*)(300) and the spanwise scale equals around delta(*)(300). Four main features are found in the underlying streak configuration and developing streak-streak interactions. Firstly, all instabilities arise in a streak configuration where two low-speed streaks are located at a small spanwise distance from each other. Patches of low-speed fluid (forming a discontinuity in the streak pattern) are present in the high-speed streaks surrounding the unstable low-speed streak. As a consequence of the streak-streak interactions at the discontinuities, vortices arise in a staggered configuration. Finally, the vortices develop into three-dimensional structures after which the flow falls apart into smaller three-dimensional flow regions. (C) 2007 American Institute of Physics.