TWO-LAYER BOX:   THIS SIMPLE SELF-CONTAINED DEMONSTRATION PERMITS ILLUSTRATION OF BUOYANCY, GRAVITY CURRENTS, RAYLEIGH-TAYLOR AND KELVIN-HELMHOLTZ INSTABILITY.

This closed plexiglass box, about 125cm long, 8cm high and 1.5cm thick, contains two immiscible fluids.  The denser "lower" fluid is dyed blue.  The top is clear.  It may be necessary to remove a little air from the top and replace with 1.5cs silicone oil.

The procedure is simply to shift the box orientation around to illustrate many different states and tranisitons.

Sitting on end leads (after a while) to a stable, stably stratified state with the dense fluid on the bottom.

Going from the vertical stable state to the horizontal position generates a gravity current with KH waves superposed on the interface.  Option:  Work out the gravity current speed  [root (g * del rho/rho * H) ] and compare.

Quickly flipping the vertical orientation of the tank produces Rayleigh-Taylor (convection) two-fluid instability.  Show simply that heavy fluid over light is an unstable configuration.  This tank is too thick to illustrate stabilization of the R-T modes by capillarity.  For a fluids class using this, along with a small tube closed-at-one-end tube, filled in a dish and then held up vertically with the open end down, one could show both stable and unstable R-T states.