Tag Archives: rotating tank

Working on our own affordable rotating table for oceanographic experiments!

Inspired by the article “Affordable Rotating Fluid Demonstrations for Geoscience Education: the DIYnamics Project” by the Hill et al. (2018), Joke, Torge and I have been wanting to build an affordable rotating table for teaching for a while now. On Saturday, we met up again to work on the project.

This post is mainly to document for ourselves where we are at and what else needs to happen to get the experiments working.

New this time: New rotating tables, aka Lazy Susans. After the one I’ve had in my kitchen was slightly too off-center to run smoothly, we bought the ones recommended by the DIYnamics project. And they work a lot better! To center our tank on the rotating table and keep it safely in place, we used these nifty LEGO and LEGO Duplo contraptions, which worked perfectly.

We also used a LEGO contraption to get the wheel close enough to drive the rotating table. The yellow line below shows where the rim of the rotating table’s foot needs to sit.

And this is how the engine has to be placed to drive the rotating table.

First attempt: Yes! Very nice parabolic surface! Very cool to see time and time again!

Now first attempt at a Hadley cell experiment: A jar with blue ice is placed at the center of the tank. Difficulties here: Cooling sets in right away, before the rotating tank has reached solid body rotation. That might potentially mess up everything (we don’t know).

So. Next attempt: Use a jar (weighted down with stones so it doesn’t float up) until the tank has reached solid body rotation, then add blue ice water

Working better, even though the green dye is completely invisible…

We didn’t measure rotation, nor did we calculate what kind of regime we were expecting, so the best result we got was “The Heart” (see below) — possibly eddying regime with wavenumber 3?

Here is what we learned for next time:

use better dye tracers and make sure their density isn’t too far off the water in the tank
use white LEGO bricks to hold the tank in place (so they don’t make you dizzy watching the tank)
measure the rotation rate and calculate what kind of regime we expect to see — overturning or eddying, and at which wave number (or, even better, the other way round: decide what we want to see and calculate how to set the parameters in order to see it)
use white cylinder in the middle so as to not distract from the circulation we want to see; weigh the cylinder down empty and fill it with ice water when the tank has reached solid body rotation
give the circulation a little more time to develop between adding the cold water at the center and putting in dyes (at least 10 minutes)
it might actually be worth reading the DIYnamics team’s instruction again, and to buy exactly what they recommend. That might save us a lot of time ;-)

But: As always this was fun! :-)

P.S.: Even though this is happening in a kitchen, I don’t think this deserves the hashtag #kitchenoceanography — the equipment we are using here is already too specialized to be available in “most” kitchens. Or what would you say?

My first attempt at building a rotating table for kitchen oceanography using LEGO

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Inspired by the article “Affordable Rotating Fluid Demonstrations for Geoscience Education: the DIYnamics Project” by the Hill et al. (2018), I spent a fun Sunday afternoon with my friends Joke and Torge in their kitchen, playing with Legos, water and food dye.

Turns out building a rotating table isn’t as easy as we had hoped, because my Lazy Susan’s axle is unfortunately really off centre (how did I never notice before?), which makes it pretty difficult to drive it with a grinding wheel, and the LEGO motor we were using only has one speed (which would have to be regulated by changing the diameter of the gears). That makes it really difficult to spin up a tank at rest if you go at it zero to full force…

But we got it to spin! Look at the cool paraboloid surface!

Next issue, though: my awesome glass vase which looks like it should work well as a tank has a really irregular bottom, which makes it very difficult to have anything stand in the centre without too much of a wobble. Also, for the Hadley Circulation experiment we were trying to set up here, when do you add in the cooling in the center? Would be best to do it after the tank is spun up, but that is such a pain to do! And I messed up the dye here, too.

But at least you can see a little bit of what it will be like when we are done, right?

Next time:

better Lazy Susan
better lighting
think about how to film it, therefore either have a co-rotating camera or a white background

And then it will be almost ready to be used in teaching. Well, almost…

Funny how tank experiments that you think should be quick and easy to set up & run always take sooo much longer than expected. But it’s so much fun that I really don’t mind! :-)

Bottom Ekman layer without a rotating table

Move the “bottom”, not the water

My initial idea was to use a Lazy Susan (you know, the kind of tray on a swivel base that you can use for your jam and honey etc on your breakfast table, but which you shouldn’t turn too rapidly (ask me how I know)) and to have a cylindrical vase sit on it, which will then be put in rotation and will rotate around and under the (initially still stagnant) water. The friction with the moving vase will then lead to a bottom-intensified circulation.

Problem here: While I have a Lazy Susan at home as well as a vase that would work as “tank”, I am currently in Bergen where I don’t have access to my own equipment. Instead, though, I have access to a rotating table in GFI’s basement which I used to simulate my Lazy Susan idea (Cool, eh? Simulating a non-rotating-table situation on a rotating table ;-)).

That worked quite well, didn’t it?

This, btw, is what the setup looked like:

So how would that work as kitchen oceanography without an actual rotating table?

The physics themselves obviously work in this setup. However, they will be really difficult to observe for several reasons:

Scales. A small dish (like the one I used; for comparison see the usual tank in the background in the picture above) makes it a lot more difficult to see what’s going on, and in my case the circulation is quickly influenced by the sides of the dish (which is obviously not what we wanted).
Rotation. It’s not difficult to set a Lazy Susan into rotation, but I imagine it will be quite difficult to keep it at a constant rotation for any length of time. But you will only see the nice spiral for as long as you keep the rotation constant. As soon as it changes, so will your currents and that will be clearly visible in the dye (which is why you put it in in the first place).
Documentation. If you want to document your experiment, if want to have your cameras co-rotating with the Lazy Susan, it’s going to be quite difficult to install them (but maybe you would just want one that sits stationary above the center of rotation? That would obviously be easy to do with a tripod)

So all in all: it was a nice idea, but either I haven’t thought it through well enough, or it is a whole lot easier to do with a rotating table. I would imagine that it’s quite hard to observe when you don’t know very well what you are looking for, so it is unfortunately not useful as a demonstration to introduce people to the topic. What do you think? Any suggestions on how to improve this and make it work at home?

Of a pool that sits on a merry-go-round and how we use it to investigate ocean circulation in Antarctica

Adventures in Oceanography and Teaching

Dr. Mirjam S. Glessmer