THE LATE STAGES OF TRANSITION INDUCED BY A LOW-AMPLITUDE WAVEPACKET IN A LAMINAR BOUNDARY-LAYER

The evolution of a wavepacket in a laminar boundary layer is studied e xperimentally, paying particular attention to the stage just prior to the formation of a turbulent spot. The initial stages of development a re found to be in very good agreement with previous results and indica te a stage in which the disturbance grows according to linear theory f ollowed by a weakly nonlinear stage in which the subharmonic grows, ap parently through a parametric resonance mechanism. In a third stage, s trong nonlinear interactions are observed in which the disturbance dev elops a streaky structure and the corresponding wavenumber-frequency s pectra exhibit an organized cascade mechanism in which spectral peaks appear with increasing spanwise wavenumber and with frequencies which alternate between zero and the subharmonic frequency. Higher harmonics are also observed, although with lower amplitude than the low-frequen cy peaks. The final (breakdown) stage is characterized by the appearan ce of high-frequency oscillations with random phase, located at low-sp eed 'spike' regions of the primary disturbance. Wavelet transforms are used to analyse the structure of both coherent and random small-scale structure of the disturbance, In particular, the breakdown oscillatio ns are also observed to have a wavepacket character riding on the larg e-amplitude primary disturbance.