Ks. Breuer et al., THE LATE STAGES OF TRANSITION INDUCED BY A LOW-AMPLITUDE WAVEPACKET IN A LAMINAR BOUNDARY-LAYER, Journal of Fluid Mechanics, 340, 1997, pp. 395-411
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.