Bypass transition in a boundary layer subject to sheet-type freestream disturbance

In this study, the receptivity of a flat-plate boundary layer was studied by introducing a thin sheet-type disturbance. An airfoil-shaped device was used to generate a thin disturbance without velocity deficit in which a steady jet was ejected from its trailing edge to the downstream. Despite the absence of strong disturbances in the freestream outside the boundary layer, streaky structures similar to an ordinary bypass transition were generated. They meandered slowly in the spanwise direction where their downstream parts were oscillating in a delayed fashion. Turbulent spots were formed in the further downstream region. Consequently, the energy growth of the low frequency band in the velocity fluctuation spectrum was found to originate from this meandering motion of the streaks, whereas the growth of the middle- and high-frequency bands was attributed to the appearances of the turbulent spots.