This blog post was written for Elin Darelius & team’s blog (link) which you should totally follow if you aren’t already!
We have started rotating and filling water into our 13-meter-diameter rotating tank! So exciting! Pictures of that to come very soon.
But first things first: Why do we go to the trouble of rotating the swimming pool?
The Earth’s rotation is the reason why movement that should just go straight forward (as we learned in physics class) sometimes seems to be deflected to the side. For example, trade winds should be going directly towards the equator from both north and south, since they are driven by hot air rising at the equator, which they are replacing. Yet we see that they blow towards the west in addition to equatorward. And that is because the Earth is rotating: So even though the air itself is only moving towards the equator, when observed from the Earth, the winds seem to be deflected by what is called the Coriolis force.
The influence of the Coriolis force becomes visible when you look at weather systems, which also swirl, rather than air flowing straight to the center where it then raises. Or when you look at tidal waves that propagate along a coastline rather than just spreading out in all directions. Or when you look at ocean currents. But all of these effects are fairly large-scale and not so easy to observe directly by just looking up in the sky or out on the ocean for a short while.
There are however easy ways to experience the Coriolis force when you play on a merry-go-round or with a record player or with anything rotating, really. Those are obviously spinning much faster than the Earth, and that’s exactly the point: The faster rotation makes it easy for us to see that something is going on. And obviously, Nadine and I had to test just that on the best merry-go-round that I have ever seen:
And that is what we’ll use in our experiments, too: Since our topography is a lot smaller than the real world it is representing, we also have to turn the tank faster than the real world is turning in order to get comparable flow fields. How to exactly calculate how fast we need to turn we’ll talk about soon. Stay tuned! :-)
Nadine demonstrating the — southern hemisphere! — Coriolis defliection
I just realized that I never explicitly showed the difference between rotation and no rotation, even though I do have the footage to do so: Two experiments set up to create a monopole, which both turned dipole.
In the non-rotating experiment (which was, by the way, set up carefully in preparation for a rotating experiment, but then the v-belt on the rotation table failed [but luckily this was on the last night of the JuniorAkademie, so we had otherwise run everything we had been planning to run], so we ended up with a non-rotating experiment), the dipole shown below develops within seconds of the central dense column being released.
A dipole created by releasing a column of dense water in the middle of a non-rotating tank.
In the rotating experiment, however, this is what the dipole looks like after a similar amount of time:
And we see that in the non-rotating case, the eddies are spreading to fill the whole width of the tank within seconds, whereas in the rotating case the eddies mainly stay confined into their respective columns. This is the often quoted phenomenon of conservation of vorticity in a rotating system, where movements happen mostly in the horizontal plane, whereas in non-rotating system, vertical movements happen easily, too (i.e. the dense water from the upper part of the initial dense column can sink to the bottom of the tank in this case, which it could not do in the rotating case), and turbulence can hence develop in 3D and not only 2D.
Because sometimes it’s easier to control a computer than rotation, salinity, water and dye.
After looking at a non-rotating cylinder collapse the other day, it is time to look at proper hetonic explosions (you know? The experiment on the rotating tank where a denser column of water at the center of the tank is released when the whole tank has reached solid body rotation). In Bergen, we used to show this experiment as a “collapsing column” experiment, the tilting of a frontal surface under rotation. For those cases, all the parameters of the experiment, e.g. the rotation rate, the density contrast, the water height, the width of the cylinder, were set up such as to ensure that one single column would persist in the middle of the tank. At JuniorAkademie, we’ve also run it in other setups, to form dipoles or quadrupoles. For a real hetonic explosion, we would typically go for even more eddies than that.
Students watching the experiment shown below. We put paper on the outside of the tank because all the feet swiping past are kind of distracting on the movie later, but that is obviously really annoying for live observers. But in our defense – we only did this once for one experiment late one evening, and didn’t expect so many people to be interested in the experiment! Plus they got to watch on the tablet which showed the top-camera’s view via WiFi… ;-)
To show us what to expect, Rolf did some model simulations for us. This is what a monopole looks like:
Shown is an isoline in density, separating the dense water below from the lighter water above. Superimposed are the horizontal velocities, so you get a sense of the rotation.
For more advanced experimentalists to recreate, here a dipole:
As for the monopole, you see chimneys that are open on top. That is because the density is higher than the one of the isoline inside the eddy, so you get the impression that you can look inside.
But the picture is different for quadrupoles, here the four eddies (that form when the central column breaks up) do not reach the water surface any more, hence they appear closed in the visualization below.
Btw, the time is of course not measured in weekdays, that’s just a glitch in the visualization that we didn’t fix.
Seeing the simulated situations for the three cases above was quite comforting after having run this experiment a couple of times. When you run the experiment in a tank, there is always a lot of turbulence that you wish wasn’t there. But it really helps to keep your expectations in check when you see that in the simulation there are always little vortices, trying to break away from the main ones, too, and that that is how it is supposed to be.
So now for an attempted experimental monopole, which turned out as a dipole due to turbulence introduced when removing the cylinder, similarly to what happened to us in the no-rotation collapsing column experiment.
When you watch the side views closely, you can see that the tank appears to be wobbling (which we usually can’t see, because this is the only time we taped a camera to the side of a tank – usually when filming from the side, I film from outside the rotating system, holding the camera in my hand). You see it most clearly when the yellow dye crystals are added – the water is sloshing back and forth, and that is most definitely not how it is supposed to be. Oh, the joys of experimentation! But what is pretty awesome to see there is how the vertical dye streaks get pulled apart into sheets as they get sucked into the vortices. Reminds me of Northern Lights! :-)
Water running uphill during spin-down – how much more awesome can it get?
After hours, when all but the most curious students had left, Rolf and I ran another collapsing cylinder experiment, this time on Rolf’s old disk player turned rotating table.
Rolf setting up the experiment
Rolf has a cone-inset for the round tank, and we set a cylinder on top of the cone and filled it with dyed salt water. The rest of the tank was filled with fresh water and the whole system spun up into solid body rotation. Then the cylinder was pulled out and here is what happened:
Column sitting on top of the cone!
The column sat right on top of the cone! And stayed there, and stayed there, and stayed there. Slowly a bottom boundary layer started creeping down the slope, so we decided to add more color.
Still only one column on top of the cone
Nice to see that, for a change, we calculated all the parameters correctly! But then The Boss himself had done the calculations this time round…
The column creeping back up the slope during spin-down
But the most fascinating thing happened during spin-down when we had stopped the tank: The column slowly withdrew up the slope again! Our two fascinated students were absolutely wowed (and that’s saying something – they were really impressed with the salt fingers earlier already).
Watch the movie below for some impressions of the experiment.
In order to not be in the eddying regime, this time we are rotating our tank as slowly as possible.
Since we ran the Hadley cell experiment the other day, I’ve been obsessed with running it again, this time with the slowest rotation possible in order to visualize a different flow regime – one were the heat transport happens through an overturning circulation rather than through eddies.
Unfortunately the camera we had mounted above the tank only started up halfway through the experiment (no idea how that happened!), so today you’ll only get snippets of this experiment. But all the more reason for us to run it again soon!
Or, an experiment on this blog often known as “slumping column”. (deutscher Text unten)
If you don’t scale your tilting of frontal surfaces under rotation experiment correctly, you get a phenomenon called “hetonic explosion”: the formation of a cloud of baroclinic point vortices. From the densities, the rotation rate, the dimensions etc you can calculate the Rossby radius and determine how many eddies you will generate. In our case, though, the calculation went wrong by a factor 10 (9.81, to be precise) and what we ended up getting is shown below.
Watch the movie below for the whole experiment (though most of it in time lapse).
Heute haben wir ein sehr spannendes Experiment gemacht. In einem Drehtank hatten wir in der Mitte einen Zylinder mit gefärbten Salzwasser und außen herum klarer Süßwasser ins Gleichgewicht gedreht. Dann wurde der Zylinder entfernt und die Säule blauen Wassers musste ein neues Gleichgewicht finden.
Im Film oben zeigen wir das Experiment – zum Teil allerdings im Zeitraffer. Viel Spaß!
This is an experiment that Pierre and I ran two years ago in Bergen but that – as I just realized – has not been featured on this blog before. Which is a pity, because it is a pretty cool experiment.
Under rotation, vertical fronts with different densities on either side can persist for a long time without leading to the density-driven adjustment shown in the non-rotating Marsigli experiment. This is what we demonstrate with this experiment.
In a not-yet-rotating tank, dyed salt water is filled into a centered cylinder while, at the same time, fresh water is filled in the tank outside of the cylinder.
This setup is then spun up for approximately half an hour. Then, the cylinder can be carefully removed and the column of dense water can adjust to the new conditions.
The rotating tank just as the cylinder is being removed
When the cylinder is being removed, disturbances are being introduced. Hence, several columns with sloping fronts develop in the rotating system.
Dense columns developing towards an equilibrium state in the rotating system.
This is what the rotating tank looks like from the side several minutes after the cylinder has been removed.
Side view of the sloping front around the dense column
Here are a couple of movies of this experiment. First a top view (note how you can see the deformation of the surface when you focus on the reflection of the ceiling lights on the water’s surface!):
Then a side view:
And finally (just because it’s fun) this is what it looks like when you switch off the rotation of the tank when you are done with the experiment: