On the instability and reproduction mechanism of a laminar streak

Abstract

The instability and spanwise reproduction process of one single low-speed streak evolved in a laminar flat-plate boundary layer is experimentally investigated. The primary goal of this experiment is to bridge the gap between the understandings on streak characteristics and the dynamic mechanisms of free stream turbulence-induced bypass transition. The wake of an upstream wall-normal positioned interference wire is used to generate a low-speed streak through the leading edge receptivity process. Moreover, the wire wake introduces irregular perturbations with quasi-antisymmetry into the boundary layer, thus triggers a sinuous-typed secondary instability of the streak. The downstream evolution of the streak presents a re-growth trend in streak strength statistics. The first growth stage is dominated by the streak instability, while the secondary growth stage might be associated with the unsteady breakdown of the streak. Streamwise vortices generated through breakdown process are frequently observed on either flank of the streak in the secondary growth stage. They always organize into streamwise-elongated train with alternating vorticity sign, thus introducing antisymmetric perturbations onto the streak with significant strength to trigger additional breakdown. In the far downstream, two new-born low-speed streaks appear on either side of the original one, indicating the completion of the spanwise reproduction of the streak. On considering the consistent lift-up inducement from the streamwise vortex trains onto the underlying inner fluid, the streak breakdown process provides a dynamical precondition for the streak reproduction process.