Rotating fluid dynamics

So we’ve seen from our turntable demonstration that the Coriolis force results in an apparent deflection of anything that’s moving over the surface of a rotating body. Now we’re going to do a slightly more realistic experiment with a tank full of water. And this is going to be a bit like the atmosphere. And we’re going to imagine that we’re kind of looking down from the North Pole, looking at the weather systems developing. So you have a tank full of water here that’s rotating in the same way that the atmosphere is rotating with the Earth as it spins around. What I’m going to do is that I’m going to add some colour dye to this tank.

I’m going to stir the tank a little bit to induce just a little bit of motion, add some coloured dye. And then we’re going to look at the motions that develop. And the patterns are going to spin up fairly quickly. They’re going to start looking like the kind of weather systems that we see from satellite images. So we’ll see some swirling patterns start to develop, the patterns that look like the motion of the clouds around swirling round, say, a centre of a depression. So I’m going to take my mixing rod here.

And what I’ll do is I’ll just give the tank a little bit of a stir, as I say, just to induce a little bit of motion within the tank. So I’ll give it a couple of whizzes around like that. We’ve got a little bit of motion in the tank there now. And I’ll take some blue dye to start with, I think.

I’m going to drop a few little blobs of blue dye into the tank in various places and one more there.

I’ll perhaps put some red in as well, just for contrast.

OK. and I think we can start to see already, particularly with the blue patterns, that we’re starting to have some swirling motion starting to develop. And we can see the red patterns are wrapping in with that. And we’re starting to get swirling patterns that look very much like the weather systems that we see from satellite imagery. We’re also filming this from a camera that’s mounted above the tank so that we can see the motion with the motion of the tank itself subtracted out. So you can see on the screens we’re looking at over here, we can see that we’ve got these swirling patterns of ink motion that look just like the weather systems, say, the clouds circling around a depression.

Some very beautiful patterns there, looking just like weather systems viewed from a satellite in space. So if you like, this is a model of the atmosphere, and the swirling patterns that are developing within this tank are due to the fact that the tank is rotating, and the fluid that’s within it, just like the air in the atmosphere is rotating with the earth and the effect of that rotation is resulting in these swirling motions, the fact that weather systems are rotating is all down to the fact that the earth itself is rotating. So we’ve got a little model here of the Earth’s atmosphere, in very simple terms, developing little individual weather systems due to the rotation.