About neutrally buoyant particles, popcorn, and more bubbles

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When you see all our pretty images of currents and swirling eddies and everything, what you actually see are the neutrally buoyant particles that get lit by the laser in a thin sheet of light. And those particles move around with the water, but in order to show the exact movement of the water and not something they are doing themselves, they need to be of the exact same density as the water, or neutrally buoyant.

But have you ever tried creating something that just stays at the same depth in water and does neither sink to the bottom or float up to the surface? I have, and I can tell you: It is not easy! In fact, I have never managed to do something like that, unless there was a very strong stratification, a very dense lower layer in which stuff would float that fell through a less dense upper layer. And in a non-stratified fluid even the smallest density differences will make particles sink or float up, since they are almost neutral everywhere… One really needs stratification to have them float nicely at the same depth for extended periods of time.

But luckily, here in Grenoble, they know how to do this right! And it’s apparently almost like making popcorn.

You take tiny beads and heat them up so they expand. The beads are made from some plastic like styrofoam or similar, so there are lots of tiny tiny air bubbles inside. The more you heat them up, the more they expand and the lower the density of the beads gets.

But! That doesn’t mean that they all end up having the same density, so you need to sort them by density! This sounds like a very painful process which we luckily didn’t have to witness, since Samuel and Thomas had lots of particles ready before we arrived.

Once the particles are sorted by density, one “only” needs to pick the correct ones for a specific purpose. Since freshwater and salt water have different densities, they also require different densities in their neutrally buoyant particles, if those are to really be neutrally buoyant…

Below you see Elin mixing some of those particles with water from the tank so we can observe how long they actually stay suspended and when they start to settle to either the top or the bottom…

Elin experimenting with the buoyancy of our particles

Elin experimenting with the buoyancy of our particles

Turns out that they are actually very close to the density of the water in the tank, so we can do the next experiment as soon as the disturbances from a previous one have settled down and don’t have to go into the tank in between experiments to stir up particles and then wait for the tank to reach solid body rotation again. This only needs to be done in the mornings, and below you see Samuel sweeping the tank to stir up particles:

Samuel sweeping particles from the topography that sank to the bottom over night

Samuel sweeping particles from the topography that sank to the bottom over night

Also note how you now see lots of reflections on the water surface that you didn’t see before? That’s for two reasons: one is because in that picture there are surface waves in the tank due to all the stirring and they reflect light in more interesting pattern than a flat surface does. And the other reason is that now the tank is actually lit — while we run experiments, the whole room is actually dark except for the lasers, some flashing warning signs and emergency exit signs close to the doors and some small lamps in our “office” up above the rotating tank.

But now to the “more bubbles” part of the title: Do you see the dark stripes in the green laser sheet below? That’s because there are air bubbles on the mirror which is used to reflect the laser into the exact position for the laser sheet. Samuel is sweeping them away, but they keep coming back, nasty little things…

Samuel sweeping particles from the topography that sank to the bottom over night

Samuel sweeping particles from the topography that sank to the bottom over night

I actually just heard about experiments with a different kind of neutrally buoyant particles the other day, using algae instead of plastic. I find this super intriguing and will keep you posted as I find out more about it!

#scipoem on an Darelius et al. article about ice shelves

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“Observed vulnerability of Filchner-Ronne Ice Shelf to wind-driven inflow of warm deep water”*

Let’s talk ab’t a favourite paper
“Observed vulnerability of Filchner-
Ronne Ice Shelf to
wind-driven inflow
of wa(-a-a-a-a)rm deep water”

An ice shelf is ice that is floating
on top of the sea as it’s flowing
down from a continent
this one is prominent
more ar’onl’ the Ross Shelf is coating.

In oc’nographers’ jargon, “deep water”
(as we learned by heart at my alma mater)
are defined by their propertie’
and live in the deep, deep sea
and currently they are getting hotter.

But “warm” is a relative measure
bathing in it would be no pleasure
it’s temperature typically
less than just one degree!
Go measure yourself at your leisure!

As winds weaken now during summer
warm water, like led by a plumber,
climbs up the continent
and can now circumvent
sills and reach ice from under.

If temperatures rise as projected
a lot of the ice will be ‘ffected.
Raising the lev’l o’ sea,
changing hydrography,
which needs to be further dissected.

Because of its climatic impact
which Elin has now shown to be fact
we need close observation
of deep water formation
so all changes can carefully be tracked.

*that’s the title of an article by (Elin) Darelius et al. (2016) which served as inspiration for this poem.

Whale watching on the Azores

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Whale watching on Terceira was A-MAH-ZING. We saw three different kinds of dolphins: Bottle-nose, common and spotted. But what you should really be doing once you are done swooning over all the pictures below: go over to Elin Darelius & team’s blog and read about what is going on with the 13-m-diameter rotating tank! :-)
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And here is a video. Be careful you don’t get sea sick — it was impossible to hold the camera steady on a moving boat, plus I was too excited to care much about it ;-)

No, the edge of our tank is not “the equator”

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A very common idea of what goes on in our tank is that we have a tiny Antarctica in the center and that the edge of our tank then represents the equator. We are rotating in Southern Hemisphere direction, clockwise when looked from above the pole. And when looking at the Earth that way, where the Earth seems to end is at the equator. It makes sense to intuitively assume that the edge of the tank then also represents “the end of the world”, i.e. the equator.

But then it is confusing that our Antarctica is so big relative to a whole hemisphere and that we don’t have any other continents in our tank. And it’s confusing because the idea that the edge of our tank represents the equator is actually wrong.

Let’s look at the Coriolis parameter. The Coriolis parameter is defined as f=2 ω sin(φ). ω is the rotation of the Earth, which is  so constant everywhere. φ, however, is the latitude. So φ is 90 at the North Pole, -90 at the South Pole, and 0 at the equator. And this is where the problem arises: The Coriolis parameter depends on the latitude, which means that it changes with latitude! From being highest at the poles (technically: Being highest at the North Pole and the same value but opposite sign at the South Pole) to being zero at the equator. And with the latitude φ obviously changes also sin(φ), and f with both of those.
Sketch of f as a function of latitude

Sketch of f as a function of latitude

In our tank, however, we don’t have a changing latitude, it’s constant everywhere. You can imagine it a little like sketched below: As if the top of the Earth was cut off at any latitude we chose, and then we just put our tank on the new flat surface on top of the Earth: the latitude is constant everywhere (at least everywhere on the shaded surface where we are putting our tank)!

How we simulate f in a tank

How we simulate f in a tank

Since the latitude is constant throughout our tank, so is the Coriolis parameter. That means that if we want to simulate Antarctica, we will match our f to match the real Antarctica’s, except scaled to match our tank. And if we wanted to simulate the Mediterranean*, we would match our f to the one corresponding the Mediterranean’s latitude.

This means that we actually cannot simulate anything in our tank that requires a change in f, much less half the Earth! So currently no equator in our tank (although that would be so much easier: No need to rotate anything since f=0 there! :-)

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*which, in contrast to my sketch above, is well in the Northern Hemisphere and not at the equator, but I am currently sitting at Lisbon Airport and this sketch is the best I can do right now… Hope you appreciate the dedication to blogging ;-)

First full week of experiments ended successfully! :-)

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The lab is rotating a lot faster now!

The lab is rotating a lot faster now!

As you see, we have increased the rotation rate of the tank! From 1 rotation in 50 seconds to now 1 rotation in 30 seconds. Which means that at the edge of the platform, where we get on and off, the difference in speed between the room and the moving platform is 5,6 km/h. For security reasons we don’t have any movies of people getting on or off: people really need to concentrate on where they are going! And even though Elin recently said that we don’t get sick by the rotation of the platform (link), I can say confidently that that doesn’t hold for all of us any more.

But with two days and nights per minute now, it’s not surprising that time flies! Our first full week of experiments is over, and it was quite a success! We’ve been in Grenoble for 1.5 weeks out of our 2 months now, and it’s time for some changes in the team: Elin and I are going to leave for a while (we’ll be back soon!) and Nadine and Lucie will be joined by new team members soon! But of course, we will keep you updated on what is happening here in Grenoble!

For now: Happy weekend, everybody!

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Your team in Grenoble — for the first 1.5 weeks: Mirjam, Elin, Lucie and Nadine (from left to right). Photo: Samuel Viboud

How the strength of the current influences which path it takes. First observations!

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Depending on how strong a current we introduce in the 13-m-diameter rotating tank to simulate the strength of the coastal current in Elin et al.’s 2016 article (link on our blog, link to the article), it takes different pathway along and across our topography.

According to theory, we expected to see something like what I sketched below: The stronger the current, the more water should continue on straight ahead, ignoring the canyon that opens up perpendicular to the current’s path at some point. The weaker the current, the more should take a left into the canyon.

What we expect from theory

What we expect from theory

We have now done a couple of experiments, and here you get a sneak preview of our observations!

Small disclaimer beforehand: What you see below are pictures taken with my mobile phone, and the sketched pathways are what I have observed by eye. This is NOT how we actually produce our real data in our experiments: We are using cameras that are mounted in very precisely known positions, that have been calibrated (as described here) and that produce many pictures per second, that are painstakingly analysed with complex mathematics and lots of deep thought to actually understand the flow field. People (hi, Lucie!) are going to do their PhDs on these experiments, and I am really interpreting on the fly while we are running experiments. Also we see snapshots of particle distribution, and we are injecting new particles in the same tank for every experiment and haven’t mixed them up in between, so parts of what you see might also be remnants of previous experiments. So please don’t over-interpret! :-)

So here we go: For a flow rate of 10 liter per minute (which is the lowest flow rate we are planning on doing) we find that a lot of the water is going straight ahead, while another part of the current is following the shelf break into the canyon.

First observations - low flow rate (10l/min)

First observations – low flow rate (10l/min)

For 20 liter per minute, our second lowest flow rate, we find that parts of the current is going straight ahead, parts of it is turning into the canyon, and a small part is following along the coastline (Which we didn’t expect to happen). However it is very difficult to observe what happens when the flow is in a steady state, especially when velocities are low, since what jumps at you is the particle distribution that is not directly related to the strength of the current which we are ultimately interested in… So this might well be an effect of just having switched on the source and the system still trying to find its steady state.

First observations - higher flow rate (20l/min)

First observations – higher flow rate (20l/min)

The more experiments we run in a day after only stirring the particles up in the morning, the more difficult it gets to observe “by eye” what is actually happening with the flow. But that’s what will be analysed in the months and years to come, so maybe it’s good that I can’t give away too many exciting results here just yet? ;-)

How our experiments relate to the real Antarctica

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After seeing so many nice pictures of our topography and the glowing bright green current field around it in the tank, let’s go back to the basics today and talk about how this relates to reality outside of our rotating tank.

Figure 1 or Darelius, Fer & Nicholls (2016): Map. Location map shows the moorings (coloured dots), Halley station (black, 75°350 S, 26°340 W), bathymetry and the circulation in the area: the blue arrow indicates the flow of cold ISW towards the Filchner sill and the red arrows the path of the coastal/slope front current. The indicated place names are: Filchner Depression (FD), Filchner Ice Shelf (FIS), Luipold coast (LC) and Ronne Ice Shelf (RIS).

Figure 1 or Darelius, Fer & Nicholls (2016): Map. Location map shows the moorings (coloured dots), Halley station (black, 75°350 S, 26°340 W), bathymetry and the circulation in the area: the blue arrow indicates the flow of cold ISW towards the Filchner sill and the red arrows the path of the coastal/slope front current. The indicated place names are: Filchner Depression (FD), Filchner Ice Shelf (FIS), Luipold coast (LC) and Ronne Ice Shelf (RIS).

Above you see the red arrows indicating the coastal/slope front currents. Where the current begins in the top right, we have placed our “source” in our experiments. And the three arms the current splits into are the three arms we also see in our experiments: One turning after reaching the first corner and crossing the shelf, one turning at the second corner and entering the canyon, and a third continuing straight ahead. And we are trying to investigate which pathway is taken depending on a couple of different parameters.

The reason why we are interested in this specific setup is that the warm water, if it turns around the corner and flows into the canyon, is reaching the Filchner Ice Shelf. The more warm water reaches the ice shelf, the faster it will melt, contributing to sea level rise, which will in turn increase melt rates.

In her recent article (Darelius, Fer & Nicholls, 2016), Elin discusses observations from that area that show that pulses of warm water have indeed reached far as far south as the ice front into the Filchner Depression (our canyon). In the observations, the strength of that current is directly linked to the strength of the wind-driven coastal current (the strength of our source). So future changes in wind forcing (for example because a decreased sea ice cover means that there are larger areas where momentum can be transferred into the surface ocean) can have a large effect on melt rates of the Filchner Ice Shelf, which might introduce a lot of fresh water in an area where Antarctic Bottom Waters are formed, influencing the properties of the water masses formed in the area and hence potentially large-scale ocean circulation and climate.

The challenge is that there are only very few actual observations of the area. Especially during winter, it’s hard to go there with research ships. Satellite observations of the sea surface require the sea surface to be visible — so ice and cloud free, which is also not happening a lot in the area. Moorings give great time series, but only of a single point in the ocean. So there is still a lot of uncertainty connected to what is actually going on in the ocean. And since there are so few observations, even though numerical models can produce a very detailed image of the area, it is very difficult how well their estimates actually are. So this is where our tank experiments come in: Even though they are idealised (the shape of the topography looks nothing like “real” Antarctica etc.), we can measure precisely how currents behave under those circumstances, and that we can use to discuss observations and model results against.

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Darelius, E., Fer, I., & Nicholls, K. W. (2016). Observed vulnerability of Filchner-Ronne Ice Shelf to wind-driven inflow of warm deep water. Nature communications, 7, 12300.

Water jet pumps, and why we don’t like them in our experiments

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Just a quick update from the lab tonight: We are fixing more bugs by the hour :-)

First: the bubble-free source.

I have previously written about how we thought we were going to get our source bubble-free (link here). Turns out, it’s not quite as easy as we thought — there were still plenty of bubbles everywhere! But luckily, Thomas came to our rescue and put some foam inside the source so the water has to pass through there before leaving the source through the honeycomb.  That effectively gets rid of all the bubbles since they just don’t fit through and surface inside the source box instead of outside of it in our experiment. We were really concerned about all those bubbles for two reasons: A) They might show up in the pictures we want to analyse and destroy any correlations we are hoping to find since they are there one second and then burst and disappear the next. And B) since the bubbles left the source below water level, they popped up to the surface and introduced vertical flow where we really didn’t want it.

But anyway, the bubbles are gone now! It’s amazing how well that works and all our (ok, my) prophecies of doom (the water is never to go through the foam! It is going over it and then enter the tank as a water fall! And even if it does go through, the particles we need to visualize the flow with, won’t!) were completely unnecessary.

Thomas kneeling on Antarctica, fixing the source

Thomas kneeling on Antarctica, fixing the source

Second: The unwanted water jet pump.

We don’t actually know where it happened, but somewhere there was a leak and air got pulled into our inflow. Thomas and Samuel fixed this problem, too, but this is what happened: We have quite a fast flow from a reservoir sitting high above the tank down to the source. The faster the flow, the lower the pressure in it, which means that it sucks stuff (in our case air) from the surroundings, and entrains it. And that’s exactly the effect that is used in water jet pumps, except there people want it to happen…

Below you see an example of the Isère here in Grenoble, where a rather fast flow is causing a return flow as soon as the river bed widens a little.

Sketch of how a water jet pump works, using the Isere as example

Sketch of how a water jet pump works, using the Isere as example

Anyway, now it’s almost dinner time in our shared flat. But we’ll be back tomorrow with first results from our experiments! :-)

Why the Isère reminds me of a water jet pump

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This is what the way to and from the 13-meter-diameter rotating tank in Grenoble looks like (and you should really visit Elin & team’s blog to learn about all the exciting stuff we are doing there!!!)

And the best part is the Isère right next to the bike path:

And one thing that I find really impressive with this river (coming from a much flatter part of the world than Grenoble, where rivers aren’t typically as fast-flowing as the Isère) is how all these return flow pools form everywhere.

Watch the movie below to spot them yourself, or my annotated picture below:

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It seems really counterintuitive that a strong current would make water on it’s side flow upstream instead of flushing everything downstream or even just going downstream through stagnant water, doesn’t it? But when I thought about why that is, it reminded me of the way a water jet pump works: You flush water from a tap down through a hose, and that hose is connected with another hose through which you want to suck something (usually some gas out of some container). So there it’s the same: The fast-flowing water pulls things in from the side and takes them with it. Now for continuity reasons, the water that is entrained in the jet needs to come from somewhere, so water has to be brought upstream in order to get sucked into the jet. That’s also similar to playing with Venturi tubes where the thinner the tube, the faster the flow, the lower the pressure… Anyway, riddle solved and I can think about other stuff again ;-)

But it is a really beautiful place to be:

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I like water so much better than mountains, but mountains still have their charms, can’t deny that…