Hot-wire anemometry was employed to examine the laminar-to-turbulent transition of low speed, two-dimensional boundary layer. The modification of the mean and fluctuation characteristics of flat-plate boundary layer subjected to free stream turbulence(T<0.3%) was studied experimentally. The study was focused on the region of upstream of transition onset, where the fluctuations inside of the boundary layer were dominated by elongated flow structures which grew downstream both in amplitude and length.
Data obtained for the pre-onset stage indicated that the streamwise-component fluctuation- amplitude distributions, frequency distributions were similar to those reported in the literature for low-freestream-turbulence transition. It is found that the T-S waves with very small amplitude were involved in nonlinear interactions which led to a generation T-S waves in the whole unstable frequency band.