Tag Archives: DIYnamics

Tilting frontal surface under rotation / cylinder collapse

Torge and I are planning to run the “tilting of a frontal surface under rotation / cylinder collapse” experiment as “remote kitchen oceanography” in his class on Thursday, so I’ve been practicing it today. It didn’t work out quite as well as it did when Pierre and I were running it in Bergen years ago, so if you are looking for my best movie of that experiment, you should go read the old blog post.

The idea is that a density front is set up by spinning up a tank in which a bottom-less cylinder contains a denser fluid, set up into a less dense fluid. Once the tank is spun up, the cylinder is removed, releasing the denser fluid into the less dense one. In contrast to the non-rotating case, where the dense water would sink to the bottom of the tank and form a layer underneath the less dense water, here the cylinder changes its shape to form a cone that retains its shape. The slope of the front is determined by both the rotation rate and the density contrast.

What I can show you today is what it looks like on my DIYnamics rotating table in my kitchen (and it’s pretty cool that all these different experiments can be run on such a simple setup, isn’t it?!). This is from two weeks ago:

And a second attempt done today (I’m not showing you all the failed ones in between, and since I’m a little sick, I’m also not showing you what I look like, and spare you the sound of my incoherend explanations ;-)). But: Now everything is set up so I can use my right hand to pull out the cylinder to introduce fewer disturbances (spoiler alert: didn’t work out — see all the waves on the tank after I remove the cylinder?)

Check out the flower “floats” — the ones on the remains of the cylinder are rotating in the same direction as the tank, only faster! That’s something we didn’t show in Bergen and that I think is really neat.

What I learned about how to set up the experiment: I filled the cylinder with ice cubes and then filled water into the donut outside of the cylinder. That way, water pressure would push water through the petroleum jelly seal at the bottom of the cylinder inside, but the dye of the melting ice cubes would not seep out (very much). Also, the cold melt water would make the water inside the cylinder denser (make sure to stir!). The whole fancy “get water out and refill using a syringe” stuff sounds nice but just isn’t feasible in my setup…

In this case, having a larger tank would be really helpful, because the disturbances introduced in either case are probably more or less the same, but the smaller the tank, the larger the relative effect of a disturbance… Also, my tripod was making it really difficult for me to reach into the tank without hitting it, both for filling the tank and for removing the cylinder. I guess if we didn’t need a top view, things would be a lot easier… ;-)

Rotating vs non-rotating turbulence

Last Thursday, Torge & I invited his “atmosphere & ocean dynamics class” to a virtual excursion into my kitchen — to do some cool experiments. As you know, I have the DIYnamics rotating table setup at home, so this is what it looked like:

We did two experiments, the very boring (but very important) solid body rotation, and then the much more exciting (and quite pretty, see pic at the very top or movie below!) comparison of turbulence in a non-rotating and a rotating system.

We didn’t manage to record the class as we had planned, so I redid & recorded the experiments. Here are 8 minutes of me talking you through it. Enjoy!

A common misconception in rotating tank experiments, and one way of maybe not reinforcing it

A very common misconception when looking at atmosphere & ocean dynamics in a rotating tank is that the center of the tank represents one of the poles and the edge of the tank the equator. And there is one experiment that — I fear — might reinforce that misconception, and that is the one we love to show for rotation vs thermal forcing, baroclinic instabilities (fast
rotation), Hadley cell circulation (slow rotation).

When we do this experiment, the tank looks like a polar stereographic view of the Earth, with the pole (represented by the blue ice in the picture below) in the center and the equator at the edge of the tank. And when we then talk about the eddies we see as representing weather pattern, it’s all too easy to assume that the Coriolis parameter also varies throughout the tank similarly as it would on Earth, only projected down into the tank. Which is not the case!

But the good news is that it’s super easy to drive this experiment by heating rather than cooling in the center of the tank. The physics are exactly the same, only the heat transport is now happening radially outward rather than radially inward. And that it’s now not the easiest assumption any more that we are looking down at the pole.

Also: Heating in the middle is a lot easier to do spontaneously than cooling using ice — no overnight stay in the fridge required, just a kettle! :-)

What are other misconceptions related to rotating tanks that you commonly come across? And do you have any advice on how to prevent these misconceptions or elicit, confront, resolve them?

Solid body rotation

Several of my friends were planning on teaching with DIYnamics rotating tables right now. Unfortunately, that’s currently impossible. Fortunately, though, I have one at home and enjoy playing with it enough that I’m

  1. Playing with it
  2. Making videos of me playing with it
  3. Putting the videos on the internet
  4. Going to do video calls with my friends’ classes, so that the students can at least “remote control” the hands-on experiments they were supposed to be doing themselves.

Here is me introducing the setup:

Today, I want to share a video I filmed the spinup of a tank until it reaches solid body rotation. To be clear: This is not a polished, stand-alone teaching video. It’s me rambling while playing. It’s supposed to give students an initial idea of an experiment we’ll be doing together during a video call, and that they’ll be discussing in much more depth in class. Watching a tank until it reaches olid body rotation is probably the most boding tank experiment ever done, but understanding the concept of solid body rotation and why we need it in tank experiments is the foundation of everything we do on a rotating tank. So here we go!

Thermal forcing vs rotation tank experiments in more detail than you ever wanted to know

This is the long version of the two full “low latitude, laminar, tropical Hadley circulation” and “baroclinic instability, eddying, extra-tropical circulation” experiments. A much shorter version (that also includes the end cases “no rotation” and “no thermal forcing”) can be found here.

Several of my friends were planning on teaching with DIYnamics rotating tables right now. Unfortunately, that’s currently impossible. Fortunately, though, I have one at home and enjoy playing with it enough that I’m

  1. Playing with it
  2. Making videos of me playing with it
  3. Putting the videos on the internet
  4. Going to do video calls with my friends’ classes, so that the students can at least “remote control” the hands-on experiments they were supposed to be doing themselves.

Here is me introducing the setup:

Today, I want to share a video I filmed on thermal forcing vs rotation. To be clear: This is not a polished, stand-alone teaching video. It’s me rambling while playing. It’s supposed to give students an initial idea of an experiment we’ll be doing together during a video call, and that they’ll be discussing in much more depth in class. It’s also meant to prepare them for more “polished” videos, which are sometimes so polished that it’s hard to actually see what’s going on. If everything looks too perfect it almost looks unreal, know what I mean? Anyway, this is as authentic as it gets, me playing in my kitchen. Welcome! :-)

In the video, I am showing the two full experiments: For small rotations we get a low latitude, laminar, tropical Hadley circulation case. Spinning faster, we get a baroclinic instability, eddying, extra-tropical case. And as you’ll see, I didn’t know which circulation I was going to get beforehand, because I didn’t do the maths before running it. I like surprises, and luckily it worked out well!

Thermal forcing vs rotation

The first experiment we ever ran with our DIYnamics rotating tank was using a cold beer bottle in the center of a rotating tank full or lukewarm water. This experiment is really interesting because, depending on the rotation of the tank, it will display different regimes. For small rotations we get a low latitude, laminar, tropical Hadley circulation case. Spinning faster, we get a baroclinic instability, eddying, extra-tropical case. Both are really interesting, and in the movie below I am showing four experimentsm ranging from “thermal forcing, no rotation”, over two experiments which include both thermal forcing and rotation at different rates to show both the “Hadley cell” and “baroclinic instability” case, to “no thermal forcing, just rotation”. Enjoy!

Ekman layers in my kitchen

Several of my friends were planning on teaching with DIYnamics rotating tables right now. Unfortunately, that’s currently impossible. Fortunately, though, I have one at home and enjoy playing with it enough that I’m

  1. Playing with it
  2. Making videos of me playing with it
  3. Putting the videos on the internet
  4. Going to do video calls with my friends’ classes, so that the students can at least “remote control” the hands-on experiments they were supposed to be doing themselves.

Here is me introducing the setup:

Today, I want to share a video I filmed on Ekman layers. To be clear: This is not a polished, stand-alone teaching video. It’s me rambling while playing. It’s supposed to give students an initial idea of an experiment we’ll be doing together during a video call, and that they’ll be discussing in much more depth in class. It’s also meant to prepare them for more “polished” videos, which are sometimes so polished that it’s hard to actually see what’s going on. If everything looks too perfect it almost looks unreal, know what I mean? Anyway, this is as authentic as it gets, me playing in my kitchen. Welcome! :-)

In the video, I am stopping a tank that was spun up into solid body rotation, to watch a bottom Ekman layer develop. Follow along for the whole journey:

Now. What are you curious about? What would you like to try? What would you do differently? Any questions for me? :-)

Rossby-#WaveWatchingWednesday

Several of my friends were planning on teaching with DIYnamics rotating tables right now. Unfortunately, that’s currently impossible. Fortunately, though, I have one at home and enjoy playing with it enough that I’m

  1. Playing with it
  2. Making videos of me playing with it
  3. Putting the videos on the internet
  4. Going to do video calls with my friends’ classes, so that the students can at least “remote control” the hands-on experiments they were supposed to be doing themselves.

Here is me introducing the setup:

Today, I want to share a video I filmed on planetary Rossby waves. To be clear: This is not a polished, stand-alone teaching video. It’s me rambling while playing. It’s supposed to give students an initial idea of an experiment we’ll be doing together during a video call, and that they’ll be discussing in much more depth in class. It’s also meant to prepare them for more “polished” videos, which are sometimes so polished that it’s hard to actually see what’s going on. If everything looks too perfect it almost looks artificial, know what I mean? Anyway, this is as authentic as it gets, me playing in my kitchen. Welcome! :-)

In the video, I am using an ice cube, melting on a sloping bottom in a rotating tank, to create planetary Rossby waves. Follow along with the whole process:

Also check out the video below that shows both a top- and side view of a planetary Rossby wave, both filmed with co-rotating cameras.

Previous blog posts with more movies for example here.

Now. What are you curious about? What would you like to try? What would you do differently? Any questions for me? :-)

Reposting my “field report” for the DIYnamics blog

Reposting a guest post I wrote for the @DIYnamicsTeam‘s blog:

When we came across the DIYnamics article right after its publication, Torge and I (Mirjam) were very excited about the endless possibilities we saw opening up with an affordable tool like the DIYnamics rotating table. We applied for, and were granted, money for an “innovative teaching” project by Kiel University’s PerLe (1) and built four DIYnamics rotating tables (five if you count the one I built for personal use ;-)), which we’ve been working with for about a year now. Since we are using them in a slightly different context than we’ve seen described before, and also have modified and added some of the experiments, I thought I’d report on it here. If you have any comments or suggestions for us, please do get in touch!

DIYnamics tables in undergraduate education

In contrast to using the DIYnamics rotating tables mainly for outreach purposes which is the most common application I am aware of, we are using them as part of a regular Bachelor-level class on “ocean and atmosphere dynamics” at Geomar, Germany. I have gained a lot of experience using a rotating table in undergraduate education at GFI, Norway, but there we only had one – much bigger – table available. Now we have four that can be used simultaneously! This is great for so many reasons:

Time efficiency for the instructor

With only one rotating table available, the typical setup I have used was to have small groups of students come in at different times over the course of a week or so, to do that week’s experiment with me. But that meant that I would spend a lot of time in the lab, and a large part of that time would be spent on waiting for the water on the rotating table to have spun up into solid body rotation, as well as prep time or cleaning, drying, putting away time.

Of course, wait times can easily be used for discussions of the upcoming experiment, of the concept of a “spun up” body of water, of how to judge whether or not a body of water is spun up or not, and many other things. But those are things that don’t necessarily have to be discussed in a setting of one instructor per each small student group, they could just as well be discussed in student groups and then in a larger plenum.

Exchange between student groups

In the setup with one table and student groups coming in one after the other, students would then write lab reports, submit them, and come back for the next experiment. While I am sure there was some exchange between student groups happening (which we could sometimes see from eerily similar ways of expressing things between groups, or errors propagating through several groups’ reports), that wasn’t an instructional design choice.

Now, however, when we have student groups working on the same experiment at the same time in the same room, it is very easy to have discussions both within individual groups and then across groups. There are many instructional methods to choose from that facilitate that kind of exchange, for example “think, pair, share”, where students first think about a question individually, then pair up with a partner (or the group at their tank) to discuss, and then results are shared with and discussed in the whole class.

Seeing many implementations of the same experiment

One reason why these discussions are especially fruitful if done in connection with simultaneously-run tank experiments is that, even if all student groups get the exact same instructions, two implementations of the same experiment do not ever look the same. There is always something that’s different. Maybe one tank is spun up less than the others, or a bit wobbly, because someone bumped into the table mid-spinup. Or the dye that is used as flow tracer has a different density for one group (less diluted? Different temperature?) and thus behaves differently. Or dye is put in a different spot in the tank and thus shows different features of the flow field. There are so many tiny things that can and will be different from experiment to experiment, and it’s a great learning experience to see an ensemble of different implementations and discuss what change in boundary conditions made results look different rather than just seeing one implementation and wrongly assuming that this is the one and only way this experiment always turns out.

Seeing many different experiments at the same time

And then, there is of course the opportunity to have different student groups work on different experiments simultaneously, which cuts down on total prep and spin-up time and enables students to see a larger variety of experiments. It also opens up the possibility that students pick what experiment they want to work on – either new ones or repeating older ones that they would like to take better pictures of or modify something in the boundary conditions. This seems to be very motivating!

Our group of students coming together at one tank to discuss observations. Note that in the background a square tank is sitting on a different rotating table, being spun up for the Rossby wave experiment described below

Why we are using higher-walled tanks

In contrast to the tanks we’ve seen used on the DIYnamics rotating tables before, we chose to invest in some high-walled tanks.

An interesting feature of rotating fluid dynamics is that the flow becomes 2D, so technically having a shallow layer of water is completely sufficient to give a good representation of those flows. And if we look at the aspect ratios of the oceans, they are in fact extremely shallow compared to their horizontal extent. But it’s easy to see a 2D flow in a shallow tank and assume that it’s 2D because of the shape of the tank, not because of the flow itself. So having an exaggerated third (depth) dimension that can be easily observed by looking into the tank from the side actually helps drive home the point that there is “nothing to see” on that dimension because the flow really is as 2D as theory told us it would be.

But then there are of course the cases where the flow isn’t 2D, and then a higher-walled tank is really convenient. For some experiments where there are exciting things to discover by looking into the tank from the side, check out this blog post over on my blog.

Students working with an experiment on the Ekman bottom boundary layer – one of the experiments that really benefit from a larger water depth because this allows for observations of the development of the boundary layer over time.

Experiment on planetary Rossby waves

One of the first experiments we tried on our DIYnamics rotating table was a “planetary Rossby wave” experiment. We hadn’t bought our cylindrical tanks yet (read more about those below), so using a clear plastic storage box was very convenient. Btw, this is one of the experiments where looking into the tank from the side gives a lot of interesting insights!

For the planetary Rossby wave experiment, we need a sloping bottom (easiest done in a rectangular tank, but we’ve also run the same experiment on a cone-shaped insert in a cylindrical tank) and a dyed ice cube. When the tank is in solid body rotation, a dyed ice cube is inserted in the shallow “eastern” corner of the tank (make sure it is sitting far enough from the edge of the tank so it can turn around its own axis unobstructed). For more details see this blog post, but in a nutshell: The cold melt water sinks, setting up a column of spinning water that sheds spinning eddies at regular intervals. Eddies and ice cube propagate westwards. This is a really easy and fool-proof experiment, and it looks beautiful!

Planetary Rossby waves in a square tank with a sloping bottom.

Presenting the DIYnamics tanks at an institute colloquium

In January, Torge and I gave a seminar presentation titled “you should really play more often! Using tank experiments in teaching” at our institute’s colloquium. In the abstract, we announced that we would give a brief overview over our project and then give people the opportunity to play – which we did. We had been expecting that maybe a handful of our loyal friends would stick around after the presentation to look at the tanks, but we were very surprised and excited to see that pretty much everybody in the audience wanted to stay. We had set up the four rotating tables with four different experiments and a student presenting at each in the back of the room, and we ended up running all the experiments repeatedly, until we were kicked out of the room because the next lecture was about to start. It was really motivating to see how everyone – students, PhD students, postdocs, staff, professors – got really excited and wanted to learn more about the tank experiments, discuss observations, modify and experiment. This goes to show that an affordable rotating table is an amazing tool at every level of education: There is always more to discover!

A snapshot of the audience of our seminar presentation interacting with the tanks and each other. We were running four experiments simultaneously.

Follow our experiences

If you want to follow our experiences with the DIYnamics rotating tables, there are several options for you:

Torge and I have recently launched the “Teaching Ocean Science” blog at https://www.oceanblogs.org/teachingoceanscience/, which is a joint effort of ourselves and other instructors, describing fun hands-on stuff they do in their teaching, and students, who write course assignments on what they are currently learning for the blog.

Additionally, all the outgoing links in my guest post above are to posts on my own blog, “Adventures in Teaching and Oceanography”, at https://mirjamglessmer.com/blog. I use that blog as my own archive and document every new thing I try on there, so you might want to sign up for email alerts or follow via my Twitter @meermini.

In addition to writing about the DIYnamics rotating tables and what we are up to with those (and you can be sure that if I played with a tank, you will read it on that blog right away ;-)), I write a lot about#KitchenOceanography (which are very easy hands-on experiments on ocean and climate topics that you can do with household items) and #WaveWatching (observing ocean physics everywhere, all the time). I also write about science communication issues.

Please feel free to check out those blogs and as I wrote in the beginning: Please get in touch if you have any comments or suggestions for us. I am always happy to chat!

(1) This project has been funded by the PerLe fund for innovative teaching through resources from the Federal Ministry of Education and Research, grant number: 01LP17068. The responsibility for the content of this publication lies with the authors.

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! :-)