Experimental Study of Boundary Layer Transition Subjected to Weak Free Stream Turbulence

Abstract

An experimental investigation of laminar turbulent transition in a flat plate boundary layer subjected to weak free stream turbulence has been done with focusing on transition scenarios. Flow visualization clearly indicated that a packet of Tollmien-Schlichting(T-S) wave emerges in the boundary layer and immediately breaks down to turbulence with deformation to j-shape structures. In a much weaker turbulence intensity case, the transition assumes more complication that a short streak breakdown coexists with the T-S wave breakdown in addition to interaction between the wave packets and streaky structures considerably elongated in the streamwise direction. There exist, at least, three scenarios of boundary layer transition due to free stream turbulent: wavy motion of the elongated streaky structure, the j-shape deformation of the T-S wave packet and the short streak breakdown to turbulence. In order to accomplish a proper prediction method for the boundary layer transition, further classification of boundary layer transition scenarios is needed.