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The Internal Wave Generator

A new
principle for internal wave generation has to be proposed,
if a monochromatic plane wave is necessary for
laboratory experiments. The key idea is to create
physical boundary conditions that effectively propagate
within the fluid interior, instead of a stationary

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The generator consists of a pile of 24 expanded PVC
sheets (2x36x150 cm) enclosed in a parallelepiped
half-opened box and free to slip one over the other.
The plates are weighted with thin lead sheets so that
they are buoyantly neutral in water; it minimizes friction
forces between plates. They are thus separated by
2.2 mm one from the other. Two rectangular holes in
each plates allow two identical camshafts
to go through the pile, imposing the relative position of
the plates.

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At rest, the plates are sinusoidally shifted,
due to the helicoidal repartition of the cams. The rotation of the camshafts applies a periodic
motion to the plates which propagates upward
(resp. downward) for a clockwise (resp. anti-clockwise)
rotation. The eccentricity of the camshafts defines the
amplitude of oscillation of the plates, namely 6 cm
peak to peak.

To optimize the emission process, the generator is
slightly tilted in direction of the emitted beam, i.e. the
direction of shear propagation. In the following, we
generate a downward propagating beam (defined by
the direction of the group velocity) corresponding to
an upward propagating shear (direction of the phase
velocity). As anticipated, the device can easily generate
upward propagating beams.

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