# 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? :-)

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

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! :-)