Kelvin Helmholtz
This experiment, to look at the Kelvin-Helmholtz instability of a shear layer between two layers
of fluid, begins with a clear layer of fresh water on top of a denser red dyed layer of
salty water in a long horizontal tank.
To set up this experiment, the empty tank is tilted and then half-filled with fresh
water.
Very slowly, so as not to mix the fluids, dyed salty water is introduced from the bottom
corner of the tilted tank.
This has been speeded up in the movie.
Still working very slowly, the tank is tilted back towards the horizontal, and frequent
checks are made that the fluids are not sloshing around.
When all is ready, the tank is tilted quickly to an angle of about 8 degrees.
The dense, salty water flows downhill, while the lighter, fresh water flows uphill, and
thus shear is generated between the two layers.
Suddenly, the interface between them becomes unstable.
Let's watch that again closer up.
Remember, the lower dyed layer is flowing to the left, while the clear layer flows to
the right.
The instability initially forms a distinctive pattern, but quickly generates turbulence
and mixing of the fluids.
Keep your eyes open, and you'll see interesting flows like this all around you.