The current work is devoted to the study of weakly non-linear interactions of Tollmien-Schlichting waves in an incompressible, 2D airfoil boundary layer. Selected resonant regimes are investigated with emphasis to the regimes where more than one fundamental T-S wave are present in the flow. The results show that amplification rates of the effective sub-harmonic modes are not affected by modulations in time of the fundamental wave. It is also shown that modulations of the 2D fundamental waves tend to generate additional modes at modulation frequency. Furthermore, these modes can resonate with the fundamental ones as detuned subharmonics. This mechanism seems to be responsible for the initial seeds of subharmonics in cases of 'natural' transition.
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