Topographic Rossby waves in a tank

This experiment just doesn’t want to be filmed by me. Even though I spent more time on preparation of this experiment than on almost any other experiment I have ever done! I have written up the theory behind this experiment, run it with a blob of dye to visualize the wave, then with a ring of dye. But for some reason, something goes wrong every time. Like people opening the door to the lab to come and visit me just the very second I am about to put dye into the tank, resulting in me jumping and a lot of dye ending up in the wrong spots… Or the tank itself getting the hickups. Or the cameras not playing nicely if for once the experiment itself goes well.

Anyway, it is still a very cool experiment! So here are some pictures.

In all those pictures, the tank is rotating a lot more slowly than recommended in the instructions. I thought that might make it all easier to run (5rpm; dial at approximately 7 for GFI big tank, similar to Taylor column). And it looks just fine, except that the restoring force back to the middle isn’t really there (as was to be expected, since the surface is almost flat and the parabolic shape is needed for a difference in water depth).

Third attempt

Below, you see the “ridge”, a piece of hose that connects a solid cylinder in the middle of the tank to the tank’s outer wall. The tank is turning counter-clockwise.

The flow looks substantially different upstream and downstream of the ridge: Upstream, it is laminar and close to the middle cylinder. Downstream, it’s meandering (the Rossby waves!) and diffusive.

Fifth attempt (same as above)

In this experiment, the difference between the flow up- and downstream of the ridge are even more obvious. Look at those eddies!

It’s quite amazing to see how a small disturbance can make the entire system unstable.


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