Category Archives: demonstration (easy)

Reposting my “field report” for the DIYnamics blog

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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.

A recent seminar presentation on “one should really play more!” and our rotating tanks

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Using “One should really play more!” as title of a presentation in a serious scientific colloquium might seem like a bold move, but the gamble payed off: a large, interested audience including everyone from students to professors enthusiastically dropped ice cubes and food dye in our LEGO-driven rotating tanks and passionately discussed their observations when on Monday, Torge and I gave a presentation in the “Ocean Circulation and Climate Dynamics” colloquium at GEOMAR. After briefly presenting the context of our PerLe-funded “Dry Theory to Juicy Reality” project, we invited everybody to play, no wait … conduct experiments with four of our rotating tanks that we had set up. Nils, Ludwig, Jakob and Hendrik from our current atmosphere and ocean dynamics class were there to help out at each of the tanks to make sure that people actually dared to touch the equipment but also make sure that they would see something meaningful in each experiment, while David took amazing pictures (which you see over on our new teaching ocean sciences blog, these are all mine).

It was such a pleasure to see everybody — from students to retired professors — drop ice cubes and drip dye, falling to their knees to have a better angle to look at tanks, and enthusiastically discussing observations and theory. Even though I am convinced that everybody should really play more, it felt really good to see people enjoying it, and not only for the aspect of play, but also for the scientific discussions that are inevitably provoked when you look at tanks.

Also it was great to be back in that auditorium 10 years after having defended my PhD there. So many things have changed, yet so much remained the same!

Playing for #FlumeFriday

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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.

Thermal forcing in a non-rotating vs rotating case: Totally different results

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On Thursday, I wrote about the thermally driven overturning circulation experiment that Torge and I did as past of our “dry theory 2 juicy reality” experiments, and mentioned that it was a non-rotating experiment in a class about rotating fluid dynamics.

I showed you the rectangular tank, but we also used a cylindrical tank with cooling in the middle that is a rotational symmetric version of the “slice” in the rectangular tank. In both cases we see the same: Cold water sinks and spreads at the bottom and is then replaced by warmer water.

But when we start turning the cylindrical tank with the cooling in the middle, cool things start to happen. I’ve blogged about that experiment before, but here is a pic of the circulation that develops. Instead of an overturning, we now get heat transport via eddies!

This is actually a really nice way to show again how hugely important the influence of rotation is on the behaviour of the ocean and atmosphere!

Salt fingers in my overturning experiment

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You might have noticed them in yesterday’s thermally driven overturning video: salt fingers!

In the image below you see them developing in the far left: Little red dye plumes moving down into the clear water. But wait, where is the salt? In this case, the “double” in double diffusion comes from heat and dye which are diffusing at different rates. As temperature’s molecular diffusion is about 100x faster than that of salt (or other things that have to physically change their distribution, rather than just bump into each other to transfer energy), the red and clear water quickly have the same temperature, but then the red dye makes the red water more dense, hence it sinks.

Over time, those fingers become more and more clearly visible…

Until after a couple of minutes, we see that they are really contributing to mixing between the two layers.

Even though double diffusive mixing happens in the ocean, too, the scaling of these fingers is of course totally off if we think of this tank as for example the northern half of the Atlantic. But then so is the density stratification… But it’s always good to keep in mind that while this experiment is showing some things quite nicely, there are also things that are artefacts of the way the experiment is set up and that aren’t analogous to how things work in the ocean.

A really nice and very new-to-me way of observing them is from above:

This is a picture that was taken fairly early in the experiment, when the layers hadn’t propagated far yet and the salt fingers weren’t being pulled back by the shear between the layers. But it’s nice to see how the dye is concentrated in those downward moving fingers, isn’t it?

Thermally driven overturning circulation

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Today was the second day of tank experiments in Torge’s and my “dry theory 2 juicy reality” teaching innovation project. While that project is mainly about bringing rotating tanks into the theoretical teaching of ocean and atmosphere dynamics, today we started with the non-rotating case of a thermally driven overturning circulation.

Very easy setup: A rectangular glass vase filled with luke-warm water. A frozen cool pack for sports injuries draped over one end (which we’ll think of as the northern end) provides the cooling that we need for deep water formation. The deep water is conveniently dyed blue with food dye. Red food dye is warmed up and added to the “southern end” of the tank, and voilà! An overturning circulation is set up.

Watch the sped-up movie to see what happens:

As you will notice, this circulation won’t last for a very long time. Since we are adding neither warming nor mixing, the cold water will eventually fill up the tank. But it’s still quite a nice experiment!

(And should you have noticed the “salt fingers” forming towards the end of the movie, I’ll write about those tomorrow)

And here is the nice group of students that humoured me and posed for this picture. It’s fun with such a motivated group that comes up with new things to try all the time! :-)

If one rotating table is awesome, four rotating tables are…?

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I’m actually at a loss for words. Amazing? Spectacular? So much fun? All of that!

Today was the first time Torge and I tried our four DIYnamics-inspired rotating tables in teaching. (Remember? We want to use 4 rotating tables simultaneously so students can work in small groups rather than watching us present experiments, and also so we could quickly see how slightly different conditions might lead to different results. Having 4 tanks running at the same time cuts down on a lot of spin-up wait time! And we wanted affordable rotating tables so a) we could afford them and b) students would really just be able to play without them, or us, being afraid that they might break something). And it went even better than we had hoped, and we were already pretty convinced that it would be awesome!

It all started out, even before class started, with one of the students asking if it was me who had done the recent takeover of Kiel University’s Instagram account with the awesome tank experiments in Bergen. Yep, that was me, and it was great that she remembered she had seen the experiments and even recognized me! Made me very happy. If I had needed convincing that social media is awesome, here it was!

But then the students started playing, and they got really into it. We started out with just tanks filled with water on the Lazy Susans, and the students moved them by hand to get a feel for how water behaves under rotation. We looked at deformation of surfaces, how confetti as tracers behaved on the surface and on the bottom, all the good stuff. Already with such simple experiments there is so much physics to discuss!

And then we moved on to turbulence in a non-rotating and rotating system. Look at the cool vortex rings you can make with food coloring :-)

And then we moved on to turbulence in the rotating system. Our final tanks haven’t arrived yet, so we made do with whatever we had at hand (see the green bowl as tank below…). Students also started improvising to include a topography and other modifications that we hadn’t planned for. This is so great if students are so keen to figure things out that they take the initiative to make it happen themselves!

Judging from what I could observe, students were really enjoying themselves and got into deep discussions, trying to connect their observations to the theory they had learned. Additionally, there were lots of “oh wow!”s and “coooool”s everywhere. And I overheard this one exchange between two students: “careful, don’t drop the phone into the tank!” “oh, it’s ok, it’s waterproof” “I don’t care about the phone, I don’t want you to mess up the experiment!” :-D

Btw, note below the small Lego motor that drives the Lazy Susan. That’s really the whole setup. Speaking of affordable and easy. And portable. And all-around awesome!

And it was great fun for Torge and me, too, to observe what the students were up to, and to discuss with them. There were already several curious questions as to what experiments we are planning to do throughout the course. The next sessions, Torge will connect the experiments we did today to theory, and start on the theory we need for the next set of experiments we are planning to run, but I can’t wait to continue working with the tank experiments with such a motivated group of students! :-)

Phase and group velocities in deep and shallow water

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When Tor came to visit me in GFI’s basement lab a couple of days ago, he told me about an experiment he had seen in Gothenburg in the seventies. So Elin and I obviously had to recreate it on the spot. Therefore today, we are comparing phase- and group velocities in deep and shallow water!

Waves are excited by means of an oscillating, hand-helt beer can, curtesy of the beer brewing club at GFI. The experiments are filmed and wave lengths and phase velocities are determined from the videos, which is a lot easier than measuring them directly while the experiment is being run.

Shallow water waves

For shallow water, we are using a water depth of 10 cm. Waves are very easy to see and phase velocities are equally easy to measure.

There is another experiment on (standing) shallow water waves being run at GFI the year before students attend GEOF213, which I described back in 2013.

Deep water waves

For deep water waves, we use a water depth of 42.5 cm (the exact number only matters when the tank filling is also used to fiddle with the dead water experiment, as I had been when the idea for this experiment came up).

Typical wave lengths that are easy to do are between 10 and 25 cm (wave lengths obviously have to be short enough that the water is still “deep”, i.e. H>>wave length) — Elin’s instruction to me for the kind of waves she wanted was “Allegro!” :-D Elin, you are really the coolest and most fun person to play with tanks with!

In deep water, we now have the added difficulty that the phase speed is twice as fast as the group speed. This makes observing the whole thing a lot more difficult. Also amplitudes are a lot smaller now, since the tank was so full and we wanted to keep the water inside…

Here is t0 — Elin has just dipped the beer can into the water for the first time

t1 — can you see the wave signal has propagated up to where the red arrow is pointing to?

t2 — the signal has reached my thumb at the left edge of the picture.

From timing this, we can calculate the group speed. We can also measure the wave length on the video and then calculate a theoretical phase speed from that. For the experiments Elin and I did, the results were pretty good, as in phase speed was usually about twice as fast as group speed. And I am curious to hear how well this works out when the students run the experiment!

Planetary Rossby waves on Beta-plane. A super easy tank experiment!

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This is seriously one of the easiest tank experiments I have ever run! And I have been completely overthinking it for the last couple of weeks.

Quick reminder: This is what we think hope will happen: On a slope, melt water from a dyed ice cube will sink, creating a Taylor column that will be driven down the slope by gravity and back up the slope by vorticity conservation, leading to a “westward” movement in a stretched, cyclonic trajectory.

We are using the DIYnamics setup: A LEGO-driven Lazy Susan. And as a tank, we are using a transparent plastic storage box that I have had for many years, and the sloping bottom is made out of two breakfast boards that happened to be a good size.

Water is filled to “just below the edge of the white clips when they are in the lower position” (forgot to take measurements, this is seriously what I wrote down in my notes. We didn’t really think this experiment would work…)

The tank is then rotated at the LEGO motor’s speed (one rotation approximately every 3 seconds) and spun into solid body rotation. We waited for approximately 10 minutes, although I think we had reached solid body rotation a lot faster. But we had a lot of surface waves that were induced by some rotation that we couldn’t track down and fix. But in the end they turned out to not matter.

To start the experiment, Torge released a blue ice cube in the eastern corner of the shallow end. As the ice cube started melting, the cold melt water sank down towards the ground, where it started flowing towards the bottom of the tank. That increased the water column’s positive relative vorticity, which drove it back up the slope.

This was super cool to watch, especially since the ice cube started spinning cyclonically itself, too, so was moving in the same direction and faster than the rotating tank.

You see this rotation quite well in the movie below (if you manage to watch without getting seasick. We have a co-rotating setup coming up, it’s just not ready yet…)

Very soon, these amazing meandering structures appear: Rossby waves! :-)

And over time it becomes clear that the eddies that are being shed from the column rotating with the ice cubes are constant throughout the whole water depth.

It is a little difficult to observe that the structure is really the same throughout the whole water column since the color in the eddies that were shed is very faint, especially compared to the ice cube and the melt water, but below you might be able to spot it for the big eddy on the left.

Or maybe here? (And note the surface waves that become visible in the reflection of the joint between the two breakfast boards that make up the sloping bottom. Why is there so much vibration in the system???)

You can definitely see the surface-to-bottom structures in the following movie if you don’t let yourself be distracted by a little #HamburgLove on the back of the breakfast boards. Watching this makes you feel really dizzy, and we’ve been starting at this for more than the 8 seconds of the clip below ;-)

After a while, the Taylor column with the ice cube floating on top starts visibly moving towards the west, too. See how it has almost reached the edge of the first breakfast board already?

And because this was so cool, we obviously had to repeat the experiment. New water, new ice cube.

But: This time with an audience of excited oceanographers :-)

This time round, we also added a second ice cube after the first one had moved almost all the way towards the west (btw, do you see how that one has this really cool eddy around it, whereas the one in the east is only just starting to rotate and create its own Taylor column?)

And last not least: Happy selfie because I realized that there are way too few pictures like this on my blog, where you see what things look like (in this case in the GEOMAR seminar room) and who I am playing with (left to right: Torge, Franzi, Joke, Jan) :-)

Tidal mixing on a (fjord’s) sill

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A pink swirl going across a styrofoam block underneath a layer of yellow water? What’s going on here?

The picture was taken in a water tank, simulating the circulation of water masses in a fjord. A fjord is a long and narrow bay, usually with a sill that is separating the bay from the open ocean. And those sills play an important role in on the one hand preventing water exchange between the fjord and the open ocean (because everything below sill depth has a really hard time getting across the sill) and on the other hand mixing water masses inside and outside of the fjord (which we see visualized with the pink dye).

And here is why the sill is so important: Every time the tide goes in or out of the fjord (so pretty much all the time), the sill acts as an obstacle to the water that wants to go in or out. And flow across a ridge tends to create mixing downstream of the ridge.

In the picture below, we see a sketch of the situation in an outgoing tide, which is what we also see represented in the photo above: Water wants to push out of the fjord and has to accelerate to get through the much smaller cross section where the sill is located. This leads to strong currents and strong mixing “downstream” of the obstacle.

Except that “downstream” is on the other side of the sill only a couple of hours later, when the tide is pushing water into the fjord, but is again hindered by the sill.

So what is happening is this: The tidal current goes in and out, and mixing occurs on one or the other side of the sill. So the situation looks like this:

This is what that looks like in our tank (the “tidal waves” are generated by lifting the right end of the tank and then just slushing back and forth):

Of course, in reality we don’t see pink swirls, and the surface layer isn’t a different color from the deep layer, either. But that’s why tank experiments are so cool: They show us what’s going on deep below the waves, that we can otherwise only deduce from complicated measurements of temperatures, salinities or mixing rates, which require highly specialized equipment, a research ship, and lots of technical know how to process and analyse and display. Which, of course, is also being done, but this demonstration gives a quick and easy visual representation of the processes at play at sills all around the world.

P.S.: The photos in this blog post were taken when I ran the fjord circulation experiment with Steffi and Ailin at GFI earlier this year. I am posting about this again now because I wanted to use the picture for other purposes and realized that I never actually wrote about this feature in as much detail as it deserves!