Chemistry Reference
In-Depth Information
2 Experimental
Experiments were carried out in the oval wind wave tank of the Institute of
Applied Physics. A sketch of the experimental approach is shown in Fig-
ure 1.
MOSA
OSA
wire
gauge
wind
30 cm
film
clean water
illumination system
I(x,s)
30 cm
10 cm
1.2 m
Fig. 1.
Schematic diagram of the present wave tank experiment; OSA { second
Optical Spectrum Analyser; MOSA { Multichannel ("Stroboscopic") Optical
Spectrum Analyser
The phase velocities of surface waves were measured using an optical
method based on the refraction of incident light by the rough water sur-
face. Surface waves can be imaged by refracted light, as a result of both
the dependencies of the refraction coefficient on the local surface slope
and the nonuniformity of the light source. The latter factor is usually more
important when observing the water surface at low incidence angles (near
the nadir direction). If the intensity of the image of the water surface at
each point is proportional to the local wave slope, the wave number spec-
trum of the image is linearly related to the wave number spectrum of sur-
face waves. In order to satisfy this condition, a gradient illumination sys-
tem has been designed, analogous to that reported in Keller and Gotwols
(1983) and later in Jähne and Riemer (1990).
The illumination system contained a transparent plastic box, installed at
the bottom of the wave tank and filled with an aqueous suspension of La-
tex particles with a diameter of about 0.3 P - 0.4 P. A nearly parallel light
beam from a line-source lamp is directed into the box. Measurements with
a photo receiver have shown that the resulting light intensity, in accor-
