Internal Waves 2- mode formation


To show the unusual, anisotropic phase and group velocity properties of internal waves in a density-stratified fluid.

What Happens:

A tank about 30 cm deep is filled with a salt stratification of buoyancy period, , about 6 seconds. A solid cylinder of a few cm diameter runs across the tank at mid depth, in the right of the field of view. This cylinder is oscillated horizontally at frequency less than N, generating internal waves. The flow is visualized with a schlieren system that shows regions of positive isopycnal slope in red, and negative isopycnal slope in green. Slopes close to zero show as yellow. The movie is in time lapse, so that the waves appear to have higher than real frequency. The wave field fills outwards from the paddle, reflects from the walls, top, and bottom, and begin to form modes.

Physics of the phenomenon:

(For discussion of wave crests, phase, and group velocity, see Internal waves 1- low frequency.)

The wave "crests", or lines of constant phase, are the red and green bands. The energy moves outwards from the paddle with the group velocity, and reflects into waves of the same frequency at walls, top, and bottom. A standing wave would be formed if the frequency were chosen such that the reflected wave returns to the paddle with just the right phase so as to reinforce the paddle-generated wave.

This set-up would be even more interesting if the bottom were sloped, due to the strange reflection properties of internal waves at sloping top/bottom. Read about wave reflection from a sloping bottom in Phillips (1966), then have a look at the interesting consequences at Leo Maas' web-site:


Phillips, O.M. 1966. The dynamics of the upper ocean. Cambridge University Press.


Movie and text - Barry Ruddick
Digitization of movie - Dave Hebert

Load and run mode formation movie