Category Archives: tank experiment

Playing in a 13-m-diameter pool on a merry-go-round results in Nature article

A long, long time ago (ok, in fall of 2017) I got the chance to join Elin Darelius and Anna Wåhlin’s team for a measuring campaign at the Coriolis platform in Grenoble for several weeks. I was there officially in an outreach officer-like role: To write and tweet about the experiments, conduct “ask me anything” events, write guest posts newsletters and websites, etc.. A lot of my work from that time is documented on Elin’s blog, that I blogged on almost daily during those periods. And we had so many amazing pictures to share (mostly green, that’s because of the lasers we used).

Turbulence in a rotating system is 2D, therefore the whole water column is rotating in this eddy that we accidentally made when moving parts of the structure in the tank

But I was extremely lucky: Neither Elin nor Anna nor anyone else on the team saw me as “just the outreach person”, which is a role that outreach people are sadly sometimes pushed in. Instead, they knew me as an oceanographer and that’s how I was integrated in the team: We discussed experiments all the way from the setup in an empty tank (below you see Elin with her “Antarctica”)

No matter how carefully you planned your experiments, once you start actually conducting them, there is always something that doesn’t work quite the way you imagined. But since time in facilities like the Coriolis platform is limited, it is hugely important to think on your feet, come up with ideas quickly, and fix things. Which is the part of science that I enjoy the most: Being confronted with a problem “in the field” and having to fix it right then and there, using whatever limited equipment and information you have available.

Speaking of “limited information”: Sometimes you have to make educated guesses about what’s in the data you are currently collecting in order to make decisions on how to proceed, without being able to know for sure what’s in the data. We took tons of pictures and videos and obviously also observed by eye what was happening in the tank, but in the end, the “real” data collection was happening with images that we couldn’t analyse on the spot (and that’s what the research part is about that took place in between fall of 2017 and now: many many hours of computing and analysing and discussing and rinse and repeat).

Grenoble was also an amazing experience just because of the sheer size of the Coriolis platform. Below you see the operations room, an office that is built above the tank and rotates with it. And let me tell you, being on a merry-go-round all day long isn’t for everybody!

I really also enjoy the hands-on work. Below is me in waders in the 13-m-diameter rotating pool (while it’s rotating, of course), using a broom to sweep up “neutrally buoyant” particles that we use to track the flow that over night settled on the topography (so much for “neutrally buoyant”, but close enough). Sometimes it comes in handy to be an early bird and doing this work before everybody else gets up, so the tank has the chance to settle into solid body rotation again before experiments start for the day.

Here you see the layer of particles in different stages of disturbance, and me having fun with it (it might not be obvious from the picture, but I’m standing in waist-deep water there)

But then we weren’t playing all day long for weeks. There were times of intense discussions of preliminary results. Exciting times! And of course, those discussions only intensified when all the data was in and could be analysed in more depth.

I loved being part of the whole process and contributing to this exciting publication now!

Giving it a side eye: Why we use high-walled tanks on our rotating table #FlumeFriday

Inspired by a recent twitter comment on how our tanks are higher-walled than those usually used on the DIYnamics rotating tables, today I’ll talk about why we went for those.

Full disclosure: Mainly for practical reasons (see below). BUT: having high-walled tanks is really helpful for many experiments because they make it a lot easier to observe the vertical dimension. Even though oceanic flows are largely 2D and thus a shallow tank should be enough (and it is for many purposes!), if you look at representations of sections of oceanic properties, the vertical dimension is always stretched to make the important 3D features visible. That’s basically what we are doing here, too: In order to make the point that rotating flows are largely 3D, we blow up the vertical dimension so people can actually observe that claim. Plus then there are all those cases where rotating flow actually isn’t 2D!

For which experiments might a high-walled tank (or a higher water level) be helpful?

For example the Ekman layer experiment. If you want to see the bottom boundary layer thicken over time as friction propagates upwards through the water column, you need to look at it from the side and over a certain period of time, so the water needs to be deep enough to be able to see parts of the water column that are already affected by friction, and then the upper part that isn’t and that’s still in solid body rotation.

Or if you want to observe the difference between rotating and non-rotating fluids, the extra height helps to show that rotating fluids are 2D whereas non-rotating fluids are 3D. So just to make it easier to observe that structures are really 2D, it helps to stretch the vertical axis.

For example of thermal forcing in rotating and non-rotating cases (And yes, I see the irony that i am showing a top-view of the rotating case. But observing by eye and taking pictures in which you can actually see what you saw by eye are two very different things).

Or non-rotating vs rotating turbulence (check out the movie in the linked blog post; makes it much clearer than the picture below).

Or those cases of rotating fluid dynamics where we force the flow to become 3D by mean tricks like slow rotation on a sloping bottom

In reality, there were other reasons, too: Firstly, we couldn’t find cheap options that matched all our requirements (We wanted something that had a diameter close to the maximum that we could fit on our rotating tables, that was cylindrical, had a flat bottom, had clear walls and would be robust enough to use with students).

Secondly, I own a glass vase with a similarly high walls that we used as tank on our prototype of the rotating table. I still use it at home, but we didn’t want to go with glass for the tanks we use all the time with students & for outreach, for obvious reasons. But since we were happy with the dimensions of the vase, we just went with it. Never change a running system, right?

And a practical reason: Emptying a high-walled tank by carrying it to a sink and throwing out the water there is much less likely to make a mess than emptying a lower-walled tank with the same water height in it. Waves created by moving the tank is all I am saying…

And also I think observing vertical structures develop in fluids is always fun! :-)

Playing for #FlumeFriday

Yesterday, we’ve had four rotating tables operating simultaneously, for three different experiments. The one that everybody is gathering around in the picture above is our favourite experiment: a slowly rotating tank with cooling in the middle that shows a nice 2D circulation instead of an overturning as we would expect in a non-rotating system.

A second group was doing an Ekman spiral experiment similar to this one.

If you are interested in observing the bottom boundary layer of a tank, it might look a bit weird to people who don’t know what you are up to…

And the other two experiments were the planetary Rossby wave experiments that I’ve written about so much before that it doesn’t really matter that I didn’t take any pictures this time round.

24 Days of #KitchenOceanography — Crystal structure of ice

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.

24 Days of #KitchenOceanography — Fresh water and salt water ice crystal structure

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.

24 Days of #KitchenOceanography — Diffusive layering

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.

24 Days of #KitchenOceanography — Double-diffusive mixing with cream and tea

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.

24 Days of #KitchenOceanography — Double-diffusive mixing

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.

24 Days of #KitchenOceanography — Topographic mixing

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.

24 Days of #KitchenOceanography — Wind-driven mixing

Welcome to 24 days of #KitchenOceanography! Both English and German instructions below.

Herzlich Willkommen zu 24 Tagen Küchen-Ozeanographie! Deutsche und Englische Anleitungen weiter unten.