Chemistry Reference
In-Depth Information
spectrum water waves, but for shorter wavelengths, from about 5 cm to 2-3
mm. The two analysers were positioned over the wave tank, with co-
located look areas. Co-located wave height measurements were also per-
formed, using a wave gauge. Wind wave measurements were carried out
for clean water and for water surfaces covered with surfactant films at a
mean wind velocity of 2.5 m s
-1
at a height of 15 cm and at two different
fetches: 12 m and 8 m. Wind drift velocities were measured between 4 cm
s
-1
for clean water and about 3 cm s
-1
for contaminated water using small
floats.
The films were produced by spreading of 10%-solution of oleic acid in
ethanol, the solvent was used to ensure better spreading of surfactants and
formation of monomolecular films. Two different amounts of oleic acid
were spread on the water surface for each fetch - about 1 mg and 2 mg
(“film 1” and “film 2”). In the stationary wind wave regimes the films oc-
cupied a part of the tank from about 2.5 m to 3 m length, depending on the
fetch and the amount of the surfactant. The surfactant concentrations
ranged above those necessary for monomolecular coverage of oleic acid
monolayers (which is about 1.1 mg m
-2
), the elasticity of saturated oleic
acid films can be considered as constant (Ermakov et al. 2006). Therefore,
differences between wave propagation conditions in the experiments with
different films could be characterized mainly by a distance from the up-
wind edge of the film to the look area. The largest distance from the edge
of the film and, correspondingly, the most strong wave damping was for
the case of “small fetch/film 2”, while the case “large fetch/film 1” was
characterized by the shortest distance and the weakest damping. Results
below are presented for these two limiting cases.
3 Results
3.1 Clean water surface
3.1.1 Regular waves
Firstly, measurements of the wave number-frequency spectrum were con-
ducted for steep mechanically generated waves of frequency 4.67 Hz. In
order to detect the waves, the parameter
L
of the MOSA was chosen to be
equal to 7.5 cm, i.e., equal to the basic wavelength of the steep waves. The
wave amplitudes were 0.6 mm and 3 mm. The phase velocities and ampli-
tudes of the first four harmonics in the wave number spectrum are plotted
in Figure 2.
