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October, 1953
HOULT I WATER EFFECTS
97
oscillation is at an infinite depth, since the wave motion decreases exponentially with depth (in Fig. 3, a' = a.e~^). This is the theoretically perfect form of a wave generator, but a close approximation can be obtained by a flat plate hinged about an axis at a depth of approximately A, and oscillated about a vertical mean as in Fig. 22. Such a plate can be oscillated by an arm driven by a crank, which may be provided with a variable throw and a variable speed, to provide waves of variable height and length.
Such a wave generator will produce a train of waves on either side, the two trains being out of phase, and if it is situated close to the wall of a tank, the train of waves propagated towards the wall will be reflected and the reflected train will interfere with the other train. The unwanted train can however, be
STILL WATER LEVEL
1
Fig. 23. Cross-section of an artificial beach.
destroyed by breaking them on an artificial beach situated at the rear of the wave-generator. An artificial beach consists of a sloping ramp terminating in a series of transverse channels which receive the water thrown up the beach and return it to the tank, as shown in section in Fig. 23.
Summary of Practical Conclusions
i. Depth
the length of the longest wave to be produced artificially.
IV.
V.
VI.
Vll.
Vlll.
IX.
XI.
Xll.
Waves must be produced with a minimum velocity of 045 knots. Allowance should be made for " heave " set up by prolonged wave-making. Wind used to maintain waves must have an adequate velocity to maintain the longest wave required. It must exceed 1 10 cm. /sec.
" White Horses " can be produced by a strong enough wind and will form on small waves before large ones. Short-crested waves can be produced by converging different systems of longcrested waves.
A sea-surface should consist of systems of wind-maintained waves on a resultant swell which may be produced by wave-generators.
A model may be propelled at any speed in deep water above 696 cm. /sec. (1*35 knots).
Depth of tank should be greater than one-eighth of the square of the maximum velocity at which a model will be propelled.
The speed of a model should be the speed of the full size vessel, reduced by the square root of the scale. The camera speed should then be increased from normal by the square root of the scale. A mechanical wave generator should be of the form described and hinged at as great a depth as possible. An artificial beach should likewise be of the form described.
of tank should be at least i
Acknowlegement
I wish to acknowledge the J. Arthur Rank Organisation for permission to reproduce these notes, which first appeared as a Studio Technical Advice Note in November, 1947.
BIBLIOGRAPHY
Lamb. H. " Hydrodynamics."
Havelock, T. H. " The propagation of groups of waves in dispersive media with application to waves of water produced by a travelling disturbance," Proc. Roy. Soc, (A), 81, 398, 1908.
Cornish, V. " Ocean Waves and kindred geophysical phenomena." Cambridge Press, 1934.
Fleming. " Waves and ripples in water, air and aether."