Investigation of the bypass transition in flat plate boundary layer

Abstract

In a transitional boundary layer subjected to free-stream turbulence streaks develop downstream and eventually breaks down to turbulence after wavy motions. Purpose of the present study is to reveal the streak generation mechanism, so called lift-up, that is the transport of low velocity fluid near the wall to higher regions by a wall-normal velocity disturbance. For direct observation of the lift-up, a stereo Particle Tracking Velocimetry using hydrogen bubbles has been developed and validated against Laser Doppler Velocimetry. Measurements of instantaneous spanwise distributions of the streamwise and wall-normal velocities show strong negative correlation between the wall-normal and streamwise velocities in the streaks.