How we can see vertical slices of the flow field in our tank

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Elin observing Antarctic currents close up and personal

Elin observing Antarctic currents close up and personal

We’ve talked before how we use the laser to light up neutrally-buoyant particles on horizontal slices of our tank, but we can actually also do this in the vertical.

Elin and Thomas observing Samuel working on the laser

Elin and Thomas observing Samuel working on the laser

This is sometimes very helpful to check whether the particle distribution is still good enough or whether someone needs to go in and stir up some particles before the next experiment.

Can you see the particles in the current?

Can you see the particles in the current?

Thumb wrestling, “Offshore”, and other simulation games

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“Ready? Set! Aaaaand go!” was the command given at the start of a thumb wrestling war. In every pair of workshop participants, thumbs were being twisted, squeezed, freed again. We were given only 30 seconds to win! And then the time was over. “Who got how many points? Anybody more than 10?”. Nope. Most people only had about three. Which, turns out, was because none of us had listened to the instructions given in the beginning: We had been told to play thumb wrestling, but with the instruction to make as many points as possible. NOT to win against the person we were playing with. So the best strategy would have been to just very quickly tap on the other person’s thumb, maybe taking turns, but definitely not to twist, wriggle, squeeze and waste time fighting! Ooooops.

This is how our very busy — but also very exciting — phase at the energie:labor continued. Yesterday, we hosted Klaus Masch, creator of the simulation game “Offshore” (all of the materials for this game are online here), who gave a workshop on the theory behind simulation games and how to implement them in teaching. And since we learn best by doing and then reflecting about it (the equation for this, we learned, is DExR=L, which I remember without looking at my notes, which are on my desk at work while I am home on my sofa. DE being Direct Experience, R reflection, and L learning. And since learning is the product of DE and R, both have to be bigger than zero for learning to occur…  (Or, technically, unequal to zero and of the same sign, but maybe the both-negative case doesn’t apply here ;-)) See, it left quite an impression!), we got to play the simulation game ourselves.

Now. Everybody who knows me in person knows I HATE playing any kind of games. Hate it with a passion. But since I had read so much about the benefits of simulation games in teaching, I really wanted to try it in order to get a better idea on whether I should get over myself and offer one as part of energie:labor. And I have to say, I am a convert.

In Offshore, students discuss the possible investment of their city into an offshore wind park as reaction to the political decision to quit nuclear power all throughout Germany. They research and adopt the roles of different stakeholders (the mayor, the city council, an investment banking firm, the people building the wind park, scientists, environmental groups) and debate risks and benefits. For us, that meant starting research before lunch, already in our roles, staying in the roles over our lunch breaks (which included lots of negotiations and bilateral conversations already!), doing some more research after, and then, finally, debating in the official debate. I got to play a double role: A student at the Institute of Applied Marine Science who is also active in the environmental group Save the Ocean. In a way, that role was probably the easiest of all: I had clear instructions that I was definitely against the project, so I could just be against everything but didn’t have to offer alternatives. All other roles had more balanced roles: Of course the investment people wanted to make as much money as possible from the project, but the role of my boss, for example, a professor in Applied Marine Sciences, was instructed to consider marine protection, but there was still room to interpret that in different ways.

Since Offshore is a frame game, it is quite easily adapted to different situations. One of our participants, for example, could only join after lunch. So a new role, the journalist, was invented for him and he was included in our simulation right away.

Participants in the workshop were all related to the energie:labor or KiFo in some way: Either having previously worked there, or currently working there with student projects or Master theses, or a visiting friend from the University of Applied Sciences who is involved in running their own energy lab that we cooperate with. That was great for me: Now future discussions on whether and how to use a simulation game for the energie:labor can refer to this common experience of playing together for a day, and getting so many useful tips and tricks along the way!

There were also so many useful micro teaching things going on that we can pick and choose from in other contexts, too. Below, for example, you see three participants holding hands: The one drawing on the flip chart has his eyes closed, while both others have their eyes open. And the left person has to guide the pen — through the other two! This, of course, causes a time lag between a signal going out and resulting in something on the paper, which sometimes makes it difficult to navigate the pen. Which is a great simulation of any complex system with a time lagged response: Any action you take doesn’t have visible effects right away, so it’s easy to over-correct and cause a mess just by being to fast in responding on the changes you (don’t) see that were caused by previous actions.

To get back to the thumb-wrestling in the beginning: That micro simulation game is one I, or any of the other participants, for that matter, will never forget. It was a very bold and impressive reminder that collaboration should be the first impulse rather than assuming that every situation, every game is about playing against someone. Maybe that’s what won me over to playing?

This was a really useful workshop, and I highly recommend both the instructor and the game! Thank you all for participating and thanks to our wonderful instructor! :-)

“Scientific evidence for warming of the climate system is unequivocal”: A #SciPoem

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“Scientific evidence for warming of the climate system is unequivocal”

This quote’s source: the IPCC
On what in total ninety-sev’n
Percent of scientists agree
The evidence of climate change

Risen by more than one degree
The av’rage surface temperature
Since anno nineteenhundr’d. See?
The evidence of climate change

The oceans have absorbed some heat
And have become a lot warmer
Espec’lly in the surface sheet
The evidence of climate change

Combined, four-hundred k-m-square
In Greenland and Antarctica
Of ice did melt, flowed off somewhere
The evidence of climate change

And everywhere around the sphere
Glaciers are retreating. Andes,
Himalayas and everywhere
The evidence of climate change

Satellite observations show
Northern Hemisphere snow cover
Was much more five decades ago
The evidence of climate change

Sea levels rising by the flow
Accumulated in the sea
Of all that melting ice and snow
The evidence of climate change

Not changing levels of the sea
But its own area and height
Arct’c sea ice declines rapidly
The evidence of climate change

Intense rainfalls destroy down streams
Temperatures are at record highs
The weather reaching new extremes
The evidence of climate change

Getting sourer through and through
Ocean waters acidify
When oceans absorb CO2
The evidence of climate change

We are constantly adding water to the tank — how is the water level kept stable?

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You’ve probably been wondering about this, too: We have a constant inflow from our “source” into the tank. How do we keep the water level stable?

Samuel adjusting the "skimmer" to regulate the outflow of the tank

Samuel adjusting the “skimmer” to regulate the outflow of the tank

Worry no more — here is the answer. In the picture above you see Samuel adjusting the skimmer — a sink inside the tank that height is adjusted such that its upper edge is exactly at where the water level should be. So any excess water is skimmed off and drained.

Sounds easy, but it’s actually not — we have a free surface in the tank and we are rotating quite fast, so there is a height difference of almost 10 cm between the center and the outer edge. So a little bit of fiddling around involved…

“Grubletegninger” — using sketches presenting alternative explanations of a situation to spark discussions

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In my personal #SciCommChallenge, one thing on my things-to-try list were “grubletegninger” — pondering sketches, or sketches to ponder. It’s a format developed in Norway and there is quite a collection available at naturfag.no. (And that my sketches below happen to be on a Norway-themed note pad is pure coincidence :-))

The idea is that the sketch of a situation is given, along with a couple of people who each give a statement explaining the situation. For example on the topic of whether a sundial can be used in both hemispheres, the characters state things like “yes, you just have to position it the other way round”, “yes, if you swap the numbers”, “no, because the sun moves in the opposite direction”, “it will work, but with a 12 hour offset”.

This can then be used to spark discussion in a student group. Since many possible misconceptions are made explicit on the sketch itself, it is easy for students to identify with one of the answers and explain why they think that it is the correct one. It is also useful to use answers to argue against, to use them as a starting point for experimentation, to bridge between school science and the real world. I really like this format and think it could be a very useful tool in science outreach, too!

And I think making many different possible answers explicit is actually the most important feature of this tool. My first Grubletegning-sketch (which I did based on a vage memory and before checking out the naturfag.no website) is not nearly as good for sparking thoughts and discussions as it could be if it was laying out different lines of argumentation!

Of course, in the end it is not very different from a multiple-choice exercise, with the different distractors giving the different answer options. But how much more appealing is it when combined with a nice sketch, and actual people (albeit sketched ones) giving the answers, rather than your teacher giving you one correct answer and a couple false ones, and not telling you which one is which? I think this might actually be an excellent tool in outreach to engage people in discussions.

And I am looking forward to coming up with situations that could be used for grubletegninger, and to actually sketching them in a slightly nicer way than I did above. I am trying out a sketch pad next weekend, there might be a huge increase in the quality of graphics on this blog in the near future ;-) Or not, we’ll see ;-)

energie:labor at European Researchers’ Night 2017

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Yesterday Alice and I spent the afternoon and evening in the cute coastal town of Eckernförde, enjoying the summer-y weather, the Baltic Sea, and — of course! — the science outreach. It was European Researchers’ Night!

We represented the energie:labor and our research group by entertaining many many people in our little blue tent:

The goal was to engage the public in thinking about physics, particularly about energy. What better tool to use than a thermal imaging camera?

I’ve talked about the many ways you can play with that sort of camera before (see here), but last night was special. To catch people’s eyes and engage children as well as grown-ups, we had prepared a couple of fun experiments, for example hitting gummi bears with a hammer and observing how that changes the temperature.

Despite the large media interest we didn’t make the local newspaper’s front page today ;-)

To get an impression of how much fun we had, watch the movie below. This was an hour before the official opening of the event, and the last seconds we had to actually do things ourselves before we got run over by curious crowds. Who knew that people are so keen on learning physics? ;-)

Thanks, Alice, we are a great team and I had so much fun! :-)

This is what experiments look like at our rotating tank

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experiment_14

Just so you don’t get bored over the weekend (and because they are so so beautiful to look at!) here are a couple more sneak peek gifs of our experiments.

experiment_15

Remember, though, that what we see are only particle distributions in one layer close to the surface, and also the very beginning of the experiments before the flow has reached a balance. So please don’t over-interpret :-)

experiment_16

About the influence of viscosity: The Reynolds number

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This blog post was written for Elin Darelius & team’s blog (link). Check it out if you aren’t already following it!

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I read a blog post by Clemens Spensberger over at  a scisnack.com couple of years ago, where he talks about how ice can flow like ketchup. The argument that he makes is that ketchup on your hotdog behaves in many ways similarly to glaciers on for example Greenland: If there is a layer of a certain thickness, it will start sliding off — both the ketchup off your hotdog and the glaciers off Greenland. After most of it has dripped to your shirt or in the ocean, a little bit still remains on the hotdog or the mountain. And so on.

What he is talking about, basically, are effects of viscosity. Water, for example, would behave very differently than ketchup or ice, if you imagine it poured on your hotdog or raining down on Greenland. But also ketchup would behave very differently from ice, if it was put on Greenland in the same quantities as the existing glaciers on Greenland, instead of on a hotdog as a model version of Greenland in a relatively small quantity. And if you used real ice to model the behaviour of Greenland glaciers on a desk, then you would quickly find out that the ice just slides off on a layer of melt water and behaves nothing like you imagined (ask me how I know…).

This shows that it is important to think about what role viscosity plays when you set up a model. And not only when you are thinking about ice — also effects of surface tension in water become very important if your model is small enough, whereas they are negligible for large scale flows in the ocean.

The effects of viscosity can be estimated using the Reynolds number Re. Re compares the effects of the velocity u of the flow, a length scale of an obstacle L, and the viscosity v: Re = uL/v.

Reynolds numbers can be used to separate different flow regimes: laminar flows for very low Reynolds numbers, nice vortex streets for Re > 90, and then flows with a stagnant backwater for high Reynold numbers.

Dependency of a flow field on the Reynolds number. Shown is the top view of a flow field. You see red obstacles, stream lines in blue (so any particle released at any point of a blue line would follow that line exactly, and in the direction shown by the arrow heads)

Dependency of a flow field on the Reynolds number. Shown is the top view of a flow field. You see red obstacles and blue stream lines (so any particle released at any point of a blue line would follow that line exactly, and in the direction shown by the arrow heads)

 

I have thought long and hard about what I could give as a good example for what I am talking about. And then I remembered that I did an experiment on vortex streets on a plate a while back.

Vortices created on a plate

Vortices created on a plate

If you start watching the movie below at min 1:28 (although watching before won’t hurt, either) you see me pulling a paint brush across the plate at different speeds. The slow ones don’t create vortex streets, instead they show a more laminar behaviour (as they should, according to theory).

https://vimeo.com/120239174

Vortex streets, like the one shown in the picture above, also exist in nature. However, scales are a lot larger there: See for example the picture below (Credit: Bob Cahalan, NASA GSFC, via Wikipedia)

Vortex street. Credit: Bob Cahalan, NASA GSFC, via Wikipedia

Vortex street. Credit: Bob Cahalan, NASA GSFC, via Wikipedia

While this is a very distinctive flow that exists at a specific range of Reynolds numbers, you see flows of all different kinds of Reynold numbers in the real world, too, and not only on my plate. Below, for example, the Reynolds number is higher and the flow downstream of the obstacle distinctly more turbulent than in a vortex street. It’s a little difficult to compare it to the drawing of streamlines above, though, because the standing waves disguise the flow.

2017-09-10-15-16-00

One way to manipulate the Reynolds number to achieve similarity between the real world and a model is to manipulate the viscosity. However that is not an easy task: if you wanted to scale down an ocean basin into a normal-sized tank, you would need fluids to replace the water that don’t even exist in nature in liquid form at reasonable temperatures.

All the more reason to use a large tank! :-)

Who is faster, the currents or the waves? The Froude number

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A very convenient way to describe a flow system is by looking at its Froude number. The Froude number gives the ratio between the speed a fluid is moving at, and the phase velocity of waves travelling on that fluid. And if we want to represent some real world situation at a smaller scale in a tank, we need to have the same Froude numbers in the same regions of the flow.

For a very strong example of where a Froude number helps you to describe a flow, look at the picture below: We use a hose to fill a tank. The water shoots away from the point of impact, flowing so much faster than waves can travel that the surface there is flat. This means that the Froude number, defined as flow velocity devided by phase velocity, is larger than 1 close to the point of impact.

img_84791

At some point away from the point of impact, you see the flow changing quite drastically: the water level is a lot higher all of a sudden, and you see waves and other disturbances on it. This is where the phase velocity of waves becomes faster than the flow velocity, so disturbances don’t just get flushed away with the flow, but can actually exist and propagate whichever way they want. That’s where the Froude number changes from larger than 1 to smaller than 1, in what is called a hydraulic jump. This line is marked in red below, where waves are trapped and you see a marked jump in surface height. Do you see how useful the Froude number is to describe the two regimes on either side of the hydraulic jump?

img_84791-copy

Obviously, this is a very extreme example. But you also see them out in nature everywhere. Can you spot some in the picture below?

hydraulic_jumps

But still, all those examples are a little more drastic than what we would imagine is happening in the ocean. But there is one little detail that we didn’t talk about yet: Until now we have looked at Froude numbers and waves at the surface of whatever water we looked at. But the same thing can also happen inside the water, if there is a density stratification and we look at waves on the interface between water of different densities. Waves running on a density interface, however, move much more slowly than those on a free surface. If you are interested, you can have a look at that phenomenon here. But with waves running a lot slower, it’s easy to imagine that there are places in the ocean where the currents are actually moving faster than the waves on a density interface, isn’t it?

For an example of the explanatory power of the Froude number, you see a tank experiment we did a couple of years ago with Rolf Käse and Martin Vogt (link). There is actually a little too much going on in that tank for our purposes right now, but the ridge on the right can be interpreted as, for example, the Greenland-Scotland-Ridge, making the blue reservoir the deep waters of the Nordic Seas, and the blue water spilling over the ridge into the clear water the Denmark Strait Overflow. And in the tank you see that there is a laminar flow directly on top of the ridge and a little way down. And then, all of a sudden, the overflow plume starts mixing with the surrounding water in a turbulent flow. And the point in between those is the hydraulic jump, where the Froude number changes from below 1 to above 1.

screen-shot-2017-09-08-at-14-37-48-copy-e1504936264627

Nifty thing, this Froude number, isn’t it? And I hope you’ll start spotting hydraulic jumps every time you do the dishes or wash your hands now! :-)

 

The ocean is very deep. It’s also very shallow. On the L/H aspect ratio and the size of the tank.

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When we come back from research cruises, one of the things that surprises people back home is how much time it takes to take measurements. And that’s for two reasons: Because the distances we have to travel to reach the area we are interested in are typically very large. And then because the ocean is also very deep.

People usually find it hard to imagine that it can easily take hours for an instrument, hanging on a wire from the ship, to go down all the way to the sea floor and then come back up to the ship again. A typical speed the winch is run at is 1 m/s. That means that for a typical ocean depth of 4 km, it takes 66 minutes for the instrument to go down, and then another hour to be brought back up to the ship. And then we haven’t even stopped the winch on the way up, which we usually do each time we want to take a water sample. So yes, the ocean is very deep!

Jens and Arnt recovering the CTD

Jens and Arnt recovering the CTD

And yet, it is not deep. At least not compared to its horizontal extent. The fastest crossing of the Atlantic, some 5000km, took something like 3 days and 10 hours. And according to a quick google search, a container ship typically takes 10 to 20 days these days. So there is a lot of water between continents! And it is really difficult to imagine how large the oceans really are.

One way to describe the extent of the ocean is to use the L/H aspect ratio. It is just the ratio between a typical length (L), and a typical depth (H). A typical east-west length in the Atlantic are our 5000 km used above, and a typical depth are 4 km. This gives us an aspect ratio L/H of 5000/4 which is 1250. That is actually a really large aspect ratio — the horizontal length scales are a lot wider than the vertical ones.

Now think about the kind of tank experiments we typically do. Here is a picture of a very simple Denmark Strait overflow experiment (more on that experiment here). You see the tank in the foreground, and a sketch of the same situation on the wall in the background. What you notice both for the experiment and the depiction is that in both cases the horizontal length scale is only about twice as much as the vertical one, leading to a L/H of 2.

Tank experiments at GFI

Tank experiments at GFI

This L/H of 2, however, is supposed to represent a situation that in the real world has a horizontal scale of maybe 1000 km and a vertical one of maybe 1 km, which leads to a L/H of 1000. So you see that the way we typically depict sections through the ocean is very distorted from what they would look like if they were geometrically similar, meaning that they had the same L/H ratio, which means that they could be transformed into the real world just by uniformly stretching or shrinking.

Below I have sketched a couple of duck ponds. The one on the left (with an aspect ratio L/H of 1) is geometrically shrunk below: Even though L and H become smaller, they do so at the same rate: their ratio stays the same. However going along the top row of duck ponds, the aspect ratio increases: While L stays the same, H shrinks. This means that the different ponds along the top of the picture are not geometrically similar. However, the one on the top right is geometrically similar to the one in the bottom right again (both have an L/H of 6). Does this make sense?

Sketch of different kinds of similarity

Sketch of different kinds of similarity

So in case you were wondering about why we need a tank that has 13 meters diameter — maybe now you see that it allows us to maintain geometric similarity a lot better than a smaller tank would, at least when we want to have water depths that are large enough that allow us to neglect surface tension effects and all that nasty stuff.

More on how Elin actually designed the experiments soon! :-)