Chemistry Reference
In-Depth Information
Wave tank study of phase velocities and damping of
gravity-capillary wind waves in the presence of surface
films
Stanislav A. Ermakov, Irina A. Sergievskaya, Emma M. Zuikova,
Vladimir Yu. Goldblat, and Yury B. Shchegolkov
Institute of Applied Physics, Nizhny Novgorod, Russia
Abstract.
The wave number-frequency spectra of gravity-capillary waves
were measured using two optical spectrum analysers and an artificial gra-
dient illuminator. It was found that phase velocities of centimetre-milli-
metre (cm-mm) scale waves on clean water do not obey the dispersion re-
lation of free surface waves, but they increase with fetch, while frequen-
cies of dominant decimetre (dm)-scale wind waves decrease with fetch.
This observation implies that the wind wave spectrum contains nonlinear
cm-mm-scale harmonics bound to the dominant waves and propagating
with the phase velocities of the dominant waves. The relation between
bound and free waves can be estimated from measurements of phase velo-
city. Wind ripple damping was found to be maximum at wavelengths
around 5-7 mm. The latter effect agrees with results of our field experi-
ments using artificial slicks and can be explained qualitatively by a non-
linear “cascade” damping mechanism, when the damping of dm-scale
dominant waves leads to strong damping of their cm-mm-scale nonlinear
harmonics including damping of “parasitic” capillary ripples.
1 Introduction
It is well known that marine slicks detected by radars and optical systems
may be used to characterize areas of biological productivity or pollution.
In order to retrieve parameters of marine films from radar/optical observa-
tions of slicks one needs to know the physical mechanisms responsible for
the damping of short wind waves. Classical linear wave damping theory
(Lucassen-Reynders and Lucassen 1969 and references therein) predicts a
maximum of the relative damping coefficient in the centimetre wavelength
range. Our recent field experiments with artificial slicks (Ermakov et al.
1998), however, showed that the relative damping (contrast) of short wind
