The nonlinear evolution of wavepackets in a laminar boundary layer has been
studied experimentally. The packets were generated by acoustic excitations injected
into the boundary layer through a small hole in the plate. Various packets with
different phases relative to the envelope were studied. It was found that for all the
packets the nonlinearity involved the appearance of oblique modes of frequency
close to the subharmonic of the dominant two-dimensional wave. Moreover, the
results confirmed that the phase had a strong influence on the strength of the
nonlinear interaction. The experimental observations also indicated that although a
subharmonic resonance appeared to be present in the process, it alone could not
explain the nonlinear behaviour. The experiment demonstrated that the process must
also involve a mechanism that generates oblique waves of frequency lower than the
Tollmien–Schlichting band.
Nonlinear evolution of a second-mode wave in supersonic boundary layers
