Resonant Amplification of Instability Waves in Quasi-subharmonic Triplets with Frequency and Wavenumber Detunings

Probably the first experimental evidence of an important role of the subharmonic resonance in the process of the laminar-turbulent transition in adverse pressure gradient (APG) boundary layers was obtained in [1]. Until recently, the only available direct experimental study of the subharmonic resonant interactions in the APG boundary layer was performed in [2] for rather small values of the Hartree parameter (βH = –0.06 and –0.09). In agreement with the theoretical prediction, a rapid growth and subsequent saturation of subharmonic amplitudes was found under controlled disturbance conditions. However, the double-exponential growth predicted by theory has not been found, and resonant interactions with frequency and wavenumber detunings have not been examined. Some new results were obtained in [3] were all main properties of tuned resonances where studied for a moderate Hartree parameter βH = –0.115. Optimal parameters of resonant triplets were found, a strong dependence of the interaction on phase relationships was examined, and the double-exponential growth was discovered. However, many questions are still open. The goal of the present experiments was to narrow the gaps existing in this area. In particular, we aimed to study the resonant interactions in the presence of frequency and spanwise-wavenumber detunings for quasi-subharmonic modes.