Discussion
The energy of internal waves is
generally lower than that of surface
waves. This is because the gravitational
restoring force is smaller for
internal waves, due to the relatively
slight difference in density between
water layers (compared to that
between water and air for surface
waves). This lower energy means that,
for a tank (or water basin) of a given
size, the natural frequency of the
internal waves will also be lower than
for surface waves.
In additional to surface waves, stratified
fluids support internal waves; in
two-layer fluids, these waves ride on
top of the interface between the two
fluids. Their periods are significantly
longer than those of surface waves
and their amplitudes can be significantly
higher. When we perturb the
two-layer system, many waves are
initially excited, but only those that
fit (resonate) with the geometry of the
basin remain. Inserting the piece of
plastic at one end of the tank, simulating
an increasingly shallow seabed,
can cause internal waves to break,
similar to surface waves breaking
on a beach, but occurring below the
surface.