In recent experiments on the growth of localized disturbances in a Blasius boundary
layer, Medeiros & Gaster (1999a, b) observed that the development of nonlinear
effects depends markedly on the initial phase of their imposed disturbance. Here,
a simple explanation of this phenomenon is proposed. Because the disturbance is
localized in space and time, it has a spread of wavenumbers and frequencies: among
these are components which can initiate a pair of resonant subharmonic waves with
well-determined phase, which are then amplified by the familiar three-wave resonance
mechanism. The amplitude attained after some time is strongly phase-dependent,
consistent with the experimental observations.
