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The subharmonic instability in two-dimensional boundary layer on a
flat plate is analyzed using the parametric instability model and the
resonant triad model. The problems arising from both models are solved
numerically using the shooting technique and results are presented. It
is found that in the presence of a strong interaction (e.g., large
amplitude of the two-dimensional wave), results from the resonant triad
model are inaccurate as compared with the experimental data and the
t results from the parametric instability model. This is mainly because
the resonant triad model is a weakly nonlinear model, and it does not
account for the modification of the eigenfunctions of the interacting
waves which really takes place as we find out from the experiments.
The parametric instability model is a powerful model, despite all
the assumptions included. The model, however, does not introduce a
clear understanding of how the subharmonic mode originates from the
three-dimensional Tollmien-Schlichting modes.
For a weak interaction results from the resonant triad model and
the parametric instability model get close to each other.