Laminar–turbulent transition in oscillating boundary layer: experimental and numerical analysis using continuous wavelet transform

The laminar–turbulent transition of a forced oscillating boundary layer with a varying pressure gradient is experimentally and numerically investigated for two Strouhal numbers. Time-dependent characterization of the natural instability modes is carried out using continuous wavelet analysis of velocity signals. The periodic evolution of the total growth rates of the most unstable disturbances are measured and compared to the results of the linear stability theory. The “Tollmien–Schlichting” (TS) and “convective” transition modes are identified. It is shown that they correspond to the extrema of opposite signs of the skewness factor of unstable wavelet transform of the most unstable frequencies.