Weakly-nonlinear Interactions of Modulated T-S Waves in the Boundary Layer of an Airfoil

Weakly non-linear interactions of Tollmien-Schlichting (T-S) waves are investigated experimentally for an incompressible 2D boundary layer. The number of waves introduced in the boundary layer is varied, covering regimes composed of few waves or bands of T-S waves (frequency and wavenumber). The experiments are carried out on the pressure side of an airfoil designed to achieve a long region of laminar flow. The results show that modulations of fundamental wave tend to generate disturbances in a range of low frequencies, via nonlinear wave interaction, which can act as self-generated seeds for subharmonic resonance. This sequence of weakly non-linear events were recently described 2 for regimes composed of few waves. The results shown here, for wave bands, are in qualitative agreement with previous findings. The mechanism seems to be a possible route for transition in the case of Natural Laminar Flow airfoils under ‘natural’ environment disturbance conditions.