What are we actually trying to measure in our rotating swimming pool?

You’ve heard us talk a lot about rotating swimming pools. Nadine has written about why we care about Antarctic ice shelf melting (link), why the ice shelf is melting (link) and how we are going to investigate it (link). Today, I am going to bring those explanations together with all that you’ve seen so far about our experiments in Grenoble. I hope! :-)

Let’s start with a technical drawing of our “Antarctica”, the topography we have in the middle of the tank. You have seen it in many of our previous posts: It’s the thing that used to be in clear plastic, but that one early morning got painted black.

Sketch of our topography and what currents we expect to see in our experiment

What we are going to investigate is how a current, introduced at the “source”, will behave. We expect that it will flow along the shelf break and that some of it will flow around the corner into the canyon, while some of it will continue on straight ahead. How large a portion of the current takes which part depends on several parameters, which we will systematically change over the next couple of weeks: How large the source’s flow rate is (we are starting with 50 liter per minute), whether there is a density difference between the source water and the ambient water (we are starting with no density difference) and what happens if we add a sharp corner to the nice and smooth corner of Antarctica and the shelf.

So far, so good. When we look down from the rotating first-floor office, things look a little different (an annotated version further down this page):

Woah! It is happening! Laser sheets and currents are on!

You see (parts of) the topography in the lower right corner of the image. And then you see a lot of green light: Our laser sheets! The laser sheets will illuminate thin layers of water, which we can take pictures of with cameras mounted perpendicularly to the sheets. That in itself isn’t so exciting, but we will have neutrally buoyant particles in the water, which light up when lit with the laser. If we take enough pictures of the particles, we can track individual particles as they get advected by the currents, and thus get a good idea of the flow field that is illuminated by the lasers.

Details of the experimental setup: Position of laser sheets, source, topography

And the cool thing is that we have not only one, but six vertical laser sheets, that are used sequentially. Below you see our first experiment and each picture in the animated gif is showing you a different layer, so you get an idea of the vertical shape of the flow field.

And these are the first current fields we are observing!

Isn’t this amazing? How lucky are we that we got the opportunity to travel to Grenoble to see all this? :-)

Getting rid of bubbles in our jet

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Sometimes the devil is in the details…

On our first day at the Coriolis platform in Grenoble, I took a picture of the “source” in our experiments (see above): The plastic box that is fed by a hose from above and that has one open side with a “honeycomb” (or: a make-the-outflowing-water-nice-and-laminar thingy, technical term) that introduces the water into the tank that we want to follow around Antarctica.

This source is sitting against our topography, and will be partly submerged so that we introduce the jet at water level and below (instead of having a waterfall going in). The idea is to get a nice and bubble-free flow because — as we talked about yesterday — bubbles reflect the laser very strongly and disguise the signal that we are actually interested in.

So when we were doing our first tests last night, the first step was to flush out all the bubbles from all the pipes and hoses that supply the water to our source. Except that bubbles kept coming. And coming. Until, at some point, we realised that this was the problem:

Sketch of “the bubble issue”

The inflowing water was free-falling through air before hitting the water inside the source box, thus entraining a lot of air bubbles directly inside of the source. Good luck flushing them out… The solution was to add an extra piece of hose to just below the water surface so no air can be entrained.

So when we arrived at the lab this morning to an empty tank* we were delighted to see that the amazing Samuel and Thomas had already fixed the source!

“Antarctica” in the tank which is empty yet again…

*Yes, the tank really was empty again. Turns out that the reflection of the laser off the topography is so strong that it’s both a problem for data quality and that too much of the light gets scattered out of the water to be safe when we use the laser at it’s real setting for the experiments rather than at the super low setting we used for the tests… Disappointing, yes, but we were so surprised and pleased when we arrived this morning and the topography had already been painted AND the source been fixed! We are super impressed with and grateful to the awesome team here in Grenoble! :-) And we are happy to report that there is water in the tank again and we can start measuring after lunch!

We have water in our rotating tank! Now testing the lasers

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Above you see the very first water coming into our tank. Only a couple of hours, and the tank was full! And in solid body rotation (since it has been spinning all the time while being filled) which means that we can start doing the real experiments very soon! :-)

Most of the afternoon has been spent testing the lasers that will be used later to measure flow velocities inside the water around our topography. Laser testing isn’t something where we can help with, but that doesn’t keep us from having fun with safety goggles! Although it took us a little while to figure out that while the goggles made the laser invisible (or, hopefully, blocked it from coming anywhere near our eyes) we could see on the displays of our cameras whether the laser was on or off!

Safety first! Fancy goggles protecting Mirjam, Elin and Nadine’s eyes from the lasers

Below you see the laser going through the water and illuminating the topography in the lower right corner of the image.

Green laser beams in the water!

What needed to be done then was to make sure that the laser sheet is actually at exactly the position we want it to be.

When you look in from the side through the water, you see the shape of our topography illuminated and the vertical laser sheet coming in from the right.

Green laser beams in the water, reflecting off of our topography

Same if you look in from the top: Do you recognise our little Antarctica? Below we see a vertical laser sheet.

Green laser beams in the water, illuminating “Antarctica”

What the “official” camera sees can be observed on a screen in our second-floor office:

Checking out the recordings from the second floor (co-rotating) office

And what we saw is that there are way too many bubbles on the topography still, that show up as bright spots (which distract from the particles that we specifically seed to visualize currents). So: Someone needed to go in and clean…

Samuel cleaning bubbles off the topography so they don’t distort the laser beam

We could observe on the screen how the bubbles were swept away!

Bubble-sweeping being observed on the screen

Next, it was time to set the exact positions we want the laser sheets at.

Nadine helps Samuel program the laser sheets to the perfect positions

For the horizontal sheets, this is done by having someone stand in the tank and actually measure the height at which the laser hits a ruler for a given setting.

Thomas measuring the height of the laser sheets in the tank

But now I am going to pick up Lucie, or new team member, at the tram stop and hope that we are ready to start the real experiments first thing tomorrow morning! :-)

Tale of arctic melting and deep water formation #scipoem

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Tale of arctic melting and deep water formation

Freshwater freezes long before saltwater does,
and it also floats on top of saltwater.
In the Nordic Seas, deep waters are formed.
If there is a lot of freshwater,
less deep water can be formed.
The sea freezes over.
Ice then insulates,
prevents heat flux,
shutting down
ocean’s
pump.

But
this is
too simple.
Influencing
fresh water layers
are also the currents.
East of Greenland, to name one,
flows fast the East Greenland Current,
taking away all the freshwater
through the Denmark Strait south, and further south,
where the freshwater mixes with saltwater
until anomalies return decades later,
starting the circle again. Now what if Greenland melts?*

*I don’t actually have an answer to the question what will happen if there is a large input of freshwater into the Nordic Seas (which seems unavoidable under global warming when both Arctic sea ice and Greenland glaciers melt). My own research, interpreting measurements taken in the region between 1950 and 2000, shows that during that period the fresh meltwater got transported south, out of the Nordic Seas, as suggested in the poem (Glessmer, Eldevik, Våge, Nilsen, & Behrens, 2014). However, even the newest of those measurements are almost a decade old now, and the debate among experts about what will happen is wide open. Exciting times!

Why do we go to all the hassle of rotating our swimming pool?

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This blog post was written for Elin Darelius & team’s blog (link) which you should totally follow if you aren’t already!

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We have started rotating and filling water into our 13-meter-diameter rotating tank! So exciting! Pictures of that to come very soon.

But first things first: Why do we go to the trouble of rotating the swimming pool?

The Earth’s rotation is the reason why movement that should just go straight forward (as we learned in physics class) sometimes seems to be deflected to the side. For example, trade winds should be going directly towards the equator from both north and south, since they are driven by hot air rising at the equator, which they are replacing. Yet we see that they blow towards the west in addition to equatorward. And that is because the Earth is rotating: So even though the air itself is only moving towards the equator, when observed from the Earth, the winds seem to be deflected by what is called the Coriolis force.

The influence of the Coriolis force becomes visible when you look at weather systems, which also swirl, rather than air flowing straight to the center where it then raises. Or when you look at tidal waves that propagate along a coastline rather than just spreading out in all directions. Or when you look at ocean currents. But all of these effects are fairly large-scale and not so easy to observe directly by just looking up in the sky or out on the ocean for a short while.

There are however easy ways to experience the Coriolis force when you play on a merry-go-round or with a record player or with anything rotating, really. Those are obviously spinning much faster than the Earth, and that’s exactly the point: The faster rotation makes it easy for us to see that something is going on. And obviously, Nadine and I had to test just that on the best merry-go-round that I have ever seen:

And that is what we’ll use in our experiments, too: Since our topography is a lot smaller than the real world it is representing, we also have to turn the tank faster than the real world is turning in order to get comparable flow fields. How to exactly calculate how fast we need to turn we’ll talk about soon. Stay tuned! :-)

Nadine demonstrating the — southern hemisphere! — Coriolis defliection

Wave watching Sunday in Grenoble

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Today I went on a wave-hunt expedition to take pictures for posts on the Froude and Reynolds number over at Elin & team’s blog (which you should totally check out if you haven’t done that yet! I am actually proof-reading my posts there and that is saying something ;-))

Anyway. Let’s look at the picture below. Do you see how there are two qualitatively different flow regimes in the Isère? Closer to the banks, you see waves that look like normal waves, happily propagating wherever they want to. And towards the middle of the river, you see that there is a lot of turbulence, but disturbances don’t propagate wherever they want, they are being flushed downstream.

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For comparison below a picture of a part of the Isère where it is turbulent all the way to the sides:

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And below a nice example of how phase velocity of waves depends on wave length. See all the small, choppy stuff being flushed downstream and then standing waves caused by some obstacle in the middle of the river? That’s because the longer the wavelength, the faster the wave propagates (assuming that we are in deep water, which I think is a safe assumption in this case). So the river is so fast that the slower waves get flushed away and only waves of the length of those created by the obstacle (or longer) can stay in one place (or even propagate against the current). I think that’s pretty cool.

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Below is one of my favourite wave-watching sights: A half slit.

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And in the picture below, we can kinda see vortices detaching behind the obstacle (or is that just me)?

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And what I really liked: see the spot below where there are all of a sudden standing waves appearing in the middle of the river? Clearly there is a sill below, but I like that you cannot see the obstacle, just deduce that it must be there from how the waves look :-)

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It’s not a hardship to be here, I can tell you ;-)

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It is quite a beautiful place! And, by the way, this is my 600th blog post on this blog. Can you believe this?

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Calibrations and decisions

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This blog post was written for Elin Darelius & team’s blog (link). Go check out that blog, more cool stuff being added there every day! :-)

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Remember this little office on top of the tank? Exciting stuff going on there: Testing of the cameras that were just being installed in the last post! (Of course it will get even more exciting once we start rotating! :-))

The office — co-rotating with the tank. Important decisions being taken down there!

Of course we need to know which part of the tank the camera captures exactly, and what any given length on the pictures relates to in real life. That’s why we measured the topography in the last blog post.

Below, you see that office again and additionally get a glimpse one floor down into the tank, and people moving a large grid there:

The office — co-rotating with the tank. More important discussions!

This grid, with exactly known dimensions and positioned in different ways above our topography, lets us calculate the focal length of the camera, which will ultimately give us the possibility to calculate back from pictures taken from far above to actual lengths in the tank. This is necessary to calculate the currents inside of the tank.

The calibration grid on our topography. This will help us connect the pictures we are taking to actual length scales inside the tank.

Easy as this sounds, I can assure you that it is not. So many considerations need to go into this now, for example where do we want to put the origin of the coordinate system when we interpret the data? And how should the axes be oriented? We are in the Southern Hemisphere, so should we use this to determine the direction of the x-axis? Or should the direction of the jet that we will be introducing into the tank give us our x-axis orientation? The source (where we will have the inflow of the jet later) seems like an obvious origin of a coordinate system, but we will move it around on the topography, so it really is not.

Decisions made now will make it a lot more convenient (or inconvenient) to work with the data for years to come, so lots of different considerations going on right now…

First critical looks at the calibration. This is such an important step!

And there are two cameras that need to play well together, or at least the data coming from them needs to be connected seamlessly to each other…

Rotating a whole swimming pool

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This blog post was written for Elin Darelius & team’s blog (link) and is just reposted here to keep my archives complete. If you aren’t already following Elin’s blog, you should really do it — more awesome blog posts coming in every day! :-)

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We have arrived at the Coriolis platform in Grenoble and it is seriously impressive. When you have heard us talk about a 13-meter-diameter swimming pool that is being rotated, you could not have possibly imagined this GIANT 13 METER DIAMETER SWIMMING POOL THAT WILL BE PUT INTO ROTATION! At least I know that even though I theoretically knew the dimensions, I had absolutely no idea of how massive the structure would actually be when you stand in front of it or even climb in.

Let me show you around a little.

This is what the whole thing looks like when seen from the outside. It’s very difficult to imagine the scale of it all, but you can see the dark floor at the bottom of the tank, then on the left, there is a second level, and a little further up a third one. Those are all normal floors — on the second level there even is a little office!

The Coriolis platform in Grenoble: A rotating, 13-m-diameter tank, complete with several co-rotating offices above!

When we climb down as low as possible, we see how the whole tank rests on all kinds of very heavy duty structures. And it must be, considering that it is supporting not only a lot of moving water, but also an office! I can’t wait to see everything rotating! Here you can maybe gauge the scale a little from the stair cases and handrails?

The Coriolis platform in Grenoble: A rotating, 13-m-diameter tank, complete with several co-rotating offices above! And here you see below the tank the engineering stuff that is actually moving it.

Below, the green metal wall (inside of that black-and-yellow striped zone, which is what will be turning, and the black safety guard) is the outside of the actual tank, that will contain the water later.

This is the office space on the second floor I talked about: you can kinda imagine the size of the tank underneath from the curvature of this room. And this will be rotating with the tank!

The co-rotating office on the Coriolis platform in Grenoble

And here you see Elin down in the tank. The clear structure to her feet is the topography of Antarctica that we will use in the first experiment (more on that later).

Elin is excited about the topography that will soon be surrounded by water!

When you look up from the tank, you see a lot of scaffolding and two very nice technicians (can you spot both?) installing fancy camera equipment for us (more on what we are going to do with those in later posts).

This is the roof above the rotating platform, and all this is rotating with us!

So this is where we’ll be for the next couple of weeks!

The first thing we did today was to measure the topography so we have a reference for what is actually in the water later (rather than what we thought there should be). You see Elin (on the left) sitting on the bottom of the tank, and Nadine (on the right) climbing on the topography.

Elin and Nadine are measuring the topography so we know exactly what we are working with.

And now we are busy sorting out all the things like access to the servers so we can see the data we’ll be measuring, VPN connections so Matlab finds its licence back home, and all the other fun stuff. But we will obviously keep you informed of every exciting new development, the super awesome science, and we are hoping to start calibrating the cameras later today! :-)

Of a pool that sits on a merry-go-round and how we use it to investigate ocean circulation in Antarctica

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You know I like tank experiments, but what I am lucky enough to witness right now is NOTHING compared to even my wildest dreams. Remember all the rotating experiments we did with this rotating table back in Bergen?

Those were awesome, no question about that. But the rotating tank I am at now? 13 meters diameter.

Yes, you read that correctly. 13 METERS DIAMETER!

I’m lucky enough to be involved in Elin Darelius & team’s research project on topographically steered currents in Antarctica, and I will be blogging on her blog about it:

Follow the blog, or like us on Facebook!

In any case, don’t miss the opportunity to see what is going on in a tank this size:

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Yes, they are both INSIDE the tank. Elin (on the left) is sitting on the tank’s floor, Nadine (on the right) is climbing on the topography representing Antarctica. For more details, head over to the blog!

Adventures in Oceanography and Teaching

Dr. Mirjam S. Glessmer

What are we actually trying to measure in our rotating swimming pool?

Getting rid of bubbles in our jet

We have water in our rotating tank! Now testing the lasers

Tale of arctic melting and deep water formation #scipoem

Tale of arctic melting and deep water formation

Why do we go to all the hassle of rotating our swimming pool?

Wave watching Sunday in Grenoble

Calibrations and decisions

Rotating a whole swimming pool

Of a pool that sits on a merry-go-round and how we use it to investigate ocean circulation in Antarctica