In 1988, a group of Japanese physicists filmed the motion of Intertial Gravity waves in a tank of water. These waves, caused by a vibrating rod at the center of the tank, propagate out wards as disturbances in the density (and velocity, pressure) of the fluid. However, unlike the "usual" visualization of waves as spreading outwards isotropically, these waves spread in a "St Andrew's" cross - the properties of the fluid they are in limit the directions the waves can propagate.
Also interesting to note is the strong difference between the phase and group velocities of the waves. Although the waves are travelling outwards along the lines of the cross, (the group velocity) the lines of peaks and troughts can be clearly seen to be travelling perpendicular to this. The phase velocity is perpendicular to the group velocity!
See the films at dennou-k.gaia.h.kyoto-u...
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