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4 Conclusions
Optical measurements of the wave number-frequency spectrum of wind
waves in the wave tank have shown that the phase velocities of cm-mm-
scale waves do not obey the linear dispersion relationship of gravity-
capillary waves (GCW), both for clean water and in the presence of surfac-
tant films, since the spectrum in this wavelength range is essentially de-
termined by bound waves, which are nonlinear high-order harmonics of
longer, dm-cm-scale waves. Nonlinear damping of “parasitic” capillary
ripples on the profile of the dominant wind waves results in a contrast
maximum in the mm-scale wavelength range of the spectrum. Relative in-
tensity of bound waves in the spectrum of cm-mm-scale wind waves has
been estimated from the measured phase velocities. It is obtained that the
contribution of bound waves is large and can either increase or decrease in
the presence of surfactant films, depending on a wind wave regime.
Note that the effect of damping of dm-waves and their nonlinear har-
monics as well as free waves depend on the film elasticity. Therefore, one
can expect that the resulting phase velocity of the system “free
waves/bound waves” should depend on the characteristics of films. This
effect will be studied in further experiments.
Acknowledgements.
The authors are grateful to Dr. J. Scott for his help-
ful discussion and interest to this work. The work was supported by the
UK Ministry of Defence through the DERA, Winfrith, UK, via the Interna-
tional Science and Technology Center (Project 1774p), by INTAS (Project
03-51-4987 “SIMP”), and RFBR (projects 05-05-64137, 04-05-64763).
5 References
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