Category Archives: hands-on activity (easy)

Ekman layers in my kitchen

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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

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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? :-)

Brine rejection and overturning, but not the way you think! Guest post by Robert Dellinger

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It’s #KitchenOceanography season! For example in Prof. Tessa M Hill‘s class at UC Davis. Last week, her student Robert Dellinger posted a video of an overturning circulation on Twitter that got me super excited (not only because as of now, April 15th, it has 70 retweets and 309 likes. That’s orders of magnitude more successful than any kitchen oceanography stuff I have ever posted! Congratulations!).

Robert is using red, warm water on one side and melt water of blue ice cubes on the other side to provide heating and cooling to his tank to create the overturning. Why did I get so excited? Because of this: the head of the meltwater plume was very clearly not blue (see above)! Rob kindly agreed to write a guest post about these observations:

“I first and foremost want to start off by thanking Dr. Mirjam Glessmer for doing a phenomenal job at SciCom through Kitchen Oceanography. I was able to replicate her physical oceanography experiment regarding oceanic overturning circulation for my oceanography class with Dr. Tessa Hill.

As mentioned in her previous post, oceanographic currents are often simplified to give an easier understanding of how oceanic overturning circulation operates. The top 10% of our oceans are controlled by wind-driven currents and tidal fluctuations while the bottom 90% of our ocean currents are controlled by density-dependent movements.Originally this process was defined as thermal circulation but was later expanded to thermohaline circulation. Thermohaline circulation is dependent upon both temperature (thermal) and salinity (haline.) These density-dependent reactions occur when either freshwater fluxes meet saltwater and from thermal differences in water masses. Due to differential heating in our planet, colder formations of dense water masses are formed at the poles, which in turn causes the convective mixing and sinking of water masses driving oceanic circulation.

(Video by Robert Dellinger; thanks for letting me use it!)

In this experiment, we primarily focus on the thermally dependent reactions between two water masses. As seen in the video, the warmer water mass is dyed red, while the cold water mass formed by ice melting, is blue. As expected the more dense water mass (cold) is pushed under the warmer mass once they meet. One feature I would like to point out is the clear plume head feature in my experiment (see picture on top of this post). My theory is that part of the ice cube that was not dyed melted first and was pushed under the warm water mass. This feature is most likely due to the ice cube experiencing unequal cooling, which in turn led to an uneven dye distribution as seen in the previous post “Sea ice formation, brine release, or: What ice cubes can tell you about your freezer.”

As seen in the experiment, thermohaline circulation is thermally driven therefore, the role of salinity causes the system to be non-linear. Salinity serves as a positive feedback mechanism by increasing the salinity of deeper water and strengthens the circulation. Furthermore, current studies are focusing on how atmospheric warming is altering thermohaline circulation attributed by increases in ocean heat absorption and freshwater fluxes (primarily from melting ice caps.)”

On melting ice cubes and molecular diffusion of heat

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First of all, let me say how much I love having chats like the one Elin and I had over the weekend (which you only see the very beginning of above). I had gotten into a bit of a rut kitchen oceanography-wise, which, I am happy to report, is over now! Thanks, Elin! :-)

One of my favourite experiments of all times are the ice cubes melting in fresh water and salt water. I’ve written about this experiment many times before, so if you aren’t familiar with it, check out my introductory post or all the posts related to the original experiment, because now we are going to take it one step further.

Ready?

Last year, Elin told me about a conversation she had had with Prof. Emeritus Arne Foldvik (our hero when it comes to tank experiments!) about someone considering towing icebergs from Antarctica to some tropical place to use as freshwater supply. Arne mentioned that when the water in which the iceberg swims gets warmer than 27°C, the situation changes, as in that the iceberg’s melt water now is denser than the warm saltwater it is swimming in. So the assumption from that would be that the melt water would sink, rather than form a layer floating around the iceberg.

And that’s the experiment I had been wanting to try and only got around to doing when Elin reminded me on Saturday. Results were … a bit disappointing. At least at first:

So what is happening is that even though the melt water is initially denser than the salt water, it doesn’t stay that way for very long, because diffusion of temperature is very fast and the fresh meltwater plumes don’t have to warm up by very much before they are less dense than the warm saltwater, so that happens very quickly.

In the movie below we see evidence of this: Around minute 1 (marked in the video) we can spot plumes of dense water sinking down, but at the same time we very clearly see a (green) freshwater layer forming on top of the salt water.

(The “Happy Birthday Arne” in this movie refers to Arne Foldvik’s 90th Birthday which was yesterday!)

For the experiment, I kinda eyeballed the salinity and also my thermometers might not be the most suitable choice for measuring water temperatures, at least in that temperature range. But as Elin and I discussed as the chat above went on: I think it won’t make a big difference to fiddle with temperatures and salinities, in the end the dominant process will be the molecular diffusion of heat that will always quickly warm the meltwater, making it buoyant. And I actually think that this makes this experiment even more interesting — to show how different processes are acting at the same time, and it’s not always obvious right away which one will be the most important one. Kinda similar to what I showed in yesterday’s post: molecular diffusion of heat will sneak up on you faster than you think :-)

Would the same thing also happen if we didn’t just have small ice cubes with low meltwater “production”, but icebergs, where the meltwater plumes would have a larger volume and so wouldn’t be warmed up as easily? Who knows… But my kitchen is too small to try and I’m too lazy to do the maths, so now it’s your turn! :-)

Temperature dependency of molecular diffusion, and convection taking over

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I saw the idea for this experiment on Instagram (Max is presenting it for @glaeserneslabor) and had to try it, too!

The idea is to put drops of dye into hot and cold water and observe how in hot water the dye is mixed a lot faster than in cold water — after all, molecules in hot water should move a lot more due to more energy and thus more Brownian motion. And we see that nicely in the upper panel of the picture: In hot water, structures look blurred, whereas in the cold water, we nicely see the vortex rings of dye falling into the water.

But what I found super interesting: Molecular diffusion of dye is only the dominant process in the very beginning of the experiment! Very quickly, molecular diffusion of heat is taking over. By warming the dye, we now get a convective flow that moves dye upward in the warm water (see lower panel).

For someone who worked on double-diffusive mixing (i.e. me) this is very exciting: It’s so nice to observe the effects of both diffusion of dye and diffusion of heat in one experiment! And to be able to show how different processes are important at different times.

What’s next? I think next time I’ll use dye at the different temperatures of the two glasses, that should get rid of the convection. Very curious to see what will happen then! :-)

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?