Stability and Transition to Turbulence in 2-D Boundary Layer Under the Influence of Adverse Pressure Gradients

A laminar-turbulent transition of the natural laminar flow airfoils is simulated at high Reynolds numbers and with the acoustic excitation of unstable Tollmien-Schlichting waves. The 2D boundary layer over a smooth flat plate is investigated with and without longitudinal adverse pressure gradients. Initial amplitudes of TS waves, induced by free stream sound due to receptivity, are directly measured. The RMS amplitude had extremely small value of O(10-6) in comparison with experiments in which a vibrating ribbon had been used. The linear and nonlinear regions of TS wave evolution and N-factors corresponding to them are detected. The e N - method for predicting the transitional Reynolds number is discussed. For the first time it is found an appearance of unsteady microseparation lenses during strong nonlinear development of TS waves. Sizes of the lenses are determined. The discovery of the microseparations, induced by unsteady pressure gradients of the TS wave, supports both current points of view (Tollmien-Schlichting and Taylor) on the physical mechanisms of transition to turbulence in boundary layers.