Tag Archives: tank experiment

Thermal forcing vs rotation tank experiments in more detail than you ever wanted to know

This is the long version of the two full “low latitude, laminar, tropical Hadley circulation” and “baroclinic instability, eddying, extra-tropical circulation” experiments. A much shorter version (that also includes the end cases “no rotation” and “no thermal forcing”) can be found here.

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 thermal forcing vs rotation. 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 showing the two full experiments: For small rotations we get a low latitude, laminar, tropical Hadley circulation case. Spinning faster, we get a baroclinic instability, eddying, extra-tropical case. And as you’ll see, I didn’t know which circulation I was going to get beforehand, because I didn’t do the maths before running it. I like surprises, and luckily it worked out well!

Thermal forcing vs rotation

The first experiment we ever ran with our DIYnamics rotating tank was using a cold beer bottle in the center of a rotating tank full or lukewarm water. This experiment is really interesting because, depending on the rotation of the tank, it will display different regimes. For small rotations we get a low latitude, laminar, tropical Hadley circulation case. Spinning faster, we get a baroclinic instability, eddying, extra-tropical case. Both are really interesting, and in the movie below I am showing four experimentsm ranging from “thermal forcing, no rotation”, over two experiments which include both thermal forcing and rotation at different rates to show both the “Hadley cell” and “baroclinic instability” case, to “no thermal forcing, just rotation”. Enjoy!

Ekman layers in my kitchen

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

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

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.)”

New rotating table on #FlumeFriday! Welcome to the family!

In addition to our four DIYnamics-inspired rotating tanks, we now have a highly professional rotating table with SO MANY options! And also so much unboxing and constructing and trouble-shooting to do before it works. But we finished the first successful test: wanna see some rotating coffee in which milk is added? Then check this out!

Luckily Torge is patient enough to deal with me bossing him around, but it took forever to get the whole thing to work and I wanted my movie ;-)

Before we got to that point, though, did I mention that we had a lot of unboxing and constructing to do? But it was a bit like Christmas… And I can’t wait to play with every last piece of equipment! So many new and fun options for experiments I’ve always been wanting to do!

Happy #FlumeFriday! :-)

Thermally driven overturning circulation

Today was the second day of tank experiments in Torge’s and my “dry theory 2 juicy reality” teaching innovation project. While that project is mainly about bringing rotating tanks into the theoretical teaching of ocean and atmosphere dynamics, today we started with the non-rotating case of a thermally driven overturning circulation.

Very easy setup: A rectangular glass vase filled with luke-warm water. A frozen cool pack for sports injuries draped over one end (which we’ll think of as the northern end) provides the cooling that we need for deep water formation. The deep water is conveniently dyed blue with food dye. Red food dye is warmed up and added to the “southern end” of the tank, and voilà! An overturning circulation is set up.

Watch the sped-up movie to see what happens:

As you will notice, this circulation won’t last for a very long time. Since we are adding neither warming nor mixing, the cold water will eventually fill up the tank. But it’s still quite a nice experiment!

(And should you have noticed the “salt fingers” forming towards the end of the movie, I’ll write about those tomorrow)

And here is the nice group of students that humoured me and posed for this picture. It’s fun with such a motivated group that comes up with new things to try all the time! :-)

Planetary Rossby waves on Beta-plane. A super easy tank experiment!

This is seriously one of the easiest tank experiments I have ever run! And I have been completely overthinking it for the last couple of weeks.

Quick reminder: This is what we think hope will happen: On a slope, melt water from a dyed ice cube will sink, creating a Taylor column that will be driven down the slope by gravity and back up the slope by vorticity conservation, leading to a “westward” movement in a stretched, cyclonic trajectory.

We are using the DIYnamics setup: A LEGO-driven Lazy Susan. And as a tank, we are using a transparent plastic storage box that I have had for many years, and the sloping bottom is made out of two breakfast boards that happened to be a good size.

Water is filled to “just below the edge of the white clips when they are in the lower position” (forgot to take measurements, this is seriously what I wrote down in my notes. We didn’t really think this experiment would work…)

The tank is then rotated at the LEGO motor’s speed (one rotation approximately every 3 seconds) and spun into solid body rotation. We waited for approximately 10 minutes, although I think we had reached solid body rotation a lot faster. But we had a lot of surface waves that were induced by some rotation that we couldn’t track down and fix. But in the end they turned out to not matter.

To start the experiment, Torge released a blue ice cube in the eastern corner of the shallow end. As the ice cube started melting, the cold melt water sank down towards the ground, where it started flowing towards the bottom of the tank. That increased the water column’s positive relative vorticity, which drove it back up the slope.

This was super cool to watch, especially since the ice cube started spinning cyclonically itself, too, so was moving in the same direction and faster than the rotating tank.

You see this rotation quite well in the movie below (if you manage to watch without getting seasick. We have a co-rotating setup coming up, it’s just not ready yet…)

Very soon, these amazing meandering structures appear: Rossby waves! :-)

And over time it becomes clear that the eddies that are being shed from the column rotating with the ice cubes are constant throughout the whole water depth.

It is a little difficult to observe that the structure is really the same throughout the whole water column since the color in the eddies that were shed is very faint, especially compared to the ice cube and the melt water, but below you might be able to spot it for the big eddy on the left.

Or maybe here? (And note the surface waves that become visible in the reflection of the joint between the two breakfast boards that make up the sloping bottom. Why is there so much vibration in the system???)

You can definitely see the surface-to-bottom structures in the following movie if you don’t let yourself be distracted by a little #HamburgLove on the back of the breakfast boards. Watching this makes you feel really dizzy, and we’ve been starting at this for more than the 8 seconds of the clip below ;-)

After a while, the Taylor column with the ice cube floating on top starts visibly moving towards the west, too. See how it has almost reached the edge of the first breakfast board already?

And because this was so cool, we obviously had to repeat the experiment. New water, new ice cube.

But: This time with an audience of excited oceanographers :-)

This time round, we also added a second ice cube after the first one had moved almost all the way towards the west (btw, do you see how that one has this really cool eddy around it, whereas the one in the east is only just starting to rotate and create its own Taylor column?)

And last not least: Happy selfie because I realized that there are way too few pictures like this on my blog, where you see what things look like (in this case in the GEOMAR seminar room) and who I am playing with (left to right: Torge, Franzi, Joke, Jan) :-)

Rossby waves in a rotating tank — three different demonstrations

For both of my tank experiment projects, in Bergen and in Kiel, we want to develop a Rossby wave demonstration. So here are my notes on three setups we are considering, but before actually having tried any of the experiments.

Background on Rossby waves

I recently showed that rotating fluids behave fundamentally differently from non-rotating ones, in that they mainly occur in the horizontal and thus are “only” 2 dimensional. This works really well as long as several conditions are met, namely the water depth can’t change, nor can the rotation of the fluid. But this is not always the case, so when either the water depth or the rotation does change, the flow still tries to conserve potential vorticity and stay 2 dimensional, but now displays so-called Rossby waves.

Here are different setups for Rossby wave demonstrations I am currently considering.

Topographic Rossby wave

For a demonstration of topographic Rossby waves, we want the Coriolis parameter f to stay constant but have the depth H change. We use the instructions by geosci.uchicago.edu as inspiration for our experiment and

  • build a shallow ridge into the tank. They use an annulus and introduce the ridge at a random longitude, we could also build one across the center of the tank all the way to both sides to avoid weird things happening in the middle (or introduce a cylinder in the middle to mimic their annulus)
  • spin up the tank to approximately 26 rpm (that seems very fast! But that’s probably needed in order to create a parabolic surface with large height differences)
  • wait for it to reach solid body rotation (ca 10 min)
  • reduce rotation slightly, to approximately 23 rpm so the water inside the tank moves relative to the tank itself, and thus has to cross the ridge which is fixed to the tank
  • introduce dye upstream of the ridge, watch it change from laminar flow to eddies downstream of the ridge (they introduced dye at the inner wall of their annulus when the water was in solid body rotation, before slowing down the tank).

What are we expecting to see?

In case A, we assume that the rotation of the tank is slow enough that the surface is more or less flat. This will certainly not be the case if we rotate at 26rpm, but let’s discuss this case first, anyway. If we inject dye upstream of the obstacle, the dye will show that the current is being deflected as it crosses the ridge, to one direction as the water columns are getting shorter as they move up the ridge, then to the other direction when the columns are stretched going down the obstacle again. Afterwards, since the water depth stays constant, they would just resume a circular path.

In case B, however, we assume a parabolic surface of the tank, which we will have for any kind of fast-ish rotation. Initially, the current will move similarly to case A. But once it leaves the ridge, if it has any momentum in radial direction at all, it will overshoot its circular path, moving into water with a different depth. This will then again expand or compress the columns, inducing relative vorticity, leading to a meandering current and eddies downstream of the obstacle (probably a lot more chaotic than drawn in my sketch).

So in both cases we initially force the Rossby wave by topography at the bottom of the tank, but then in case B we sustain it by the changes in water depth due to the sloping surface.

My assessment before actually having run the experiment: The ridge seems fairly easy to construct and the experiment easy enough to run. However what I am worried about is the change in rotation rate and the turbulence and Ekman layers that it will introduce. After all, slowing down the tank is what we do create both turbulence and Ekman layers in demonstrations, and then we don’t even have an obstacle stuck in the tank. The instructions suggest a very slight reduction in rotation, so we’ll see how that goes…

Planetary Rossby waves on beta-plane

If we want to have more dramatic changes in water depth H than relying on the parabolic shape of the surface, another option is to use a rectangular tank and insert a sloping bottom as suggested by the Weather in a Tank group here. We are now operating on a Beta plane with the Coriolis parameter f being the sum of the tank’s rotation and the slope of the bottom.

Following the Weather in a Tank instructions, we plan to

  • fill a tank with a sloping bottom (slope approximately 0.5)
  • spin it at approximately 15 rpm until it reaches solid body rotation (15-20 minutes later)
  • place a dyed ice cube (diameter approximately 5 cm) in the north-eastern corner of the tank

What do we expect to see?

Ice cube and its trajectory (in red) on a sloping bottom in a rotating tank. Note: This sketch does not include the melt water water column!

Above is a simplified sketch of what will (hopefully!) happen. As the ice cube starts melting, melt water is going to sink down towards the sloping bottom, stretching the water column. This induces positive relative vorticity, making the water column spin cyclonically. As the meltwater reaches the sloping bottom, it will flow downhill, further stretching the water column. This induces more positive relative vorticity still, so the water column, and with it the ice cube, will start moving back up the slope until they reach the “latitude” at which the ice cube initially started. Having moved up the slope into shallower water, the additional positive vorticity induced by the stretching as the water was flowing down the slope has now been lost again, so rather than spinning cyclonically in one spot, the trajectory is an extended cycloid.

My assessment here (before having run it): I find this experiment a little more unintuitive because there are the different components of stretching contributing to the changes in relative vorticity. And from the videos I’ve seen, we don’t really get a clear column moving, but there are cyclonic eddies in the boundary layer that are shed. So I think this might be more difficult to observe and interpret. But I am excited to try!

Planetary Rossby wave on a cone (cyclical beta-plane?)

Following the Weather in a Tank instructions, we plan to also do the experiment as above but with cyclical boundary conditions, by using a cone in a cylindrical tank instead of a sloping bottom in a rectangular one.

The experiment is run in the same way as the one above (except they suggest a slightly slower rotation of 10 rpm). Physics are the same as before, except that now the transfer to reality should be a little easier, since we now have Rossby waves that can really run all the way around the pole. Also the experiment can be run for a longer time, since we don’t run into a boundary in the west if we are moving around and around the pole.

Ice cube and its trajectory (in red) on a cone in a rotating tank. Note: This sketch does not include the melt water column!

My assessment before actually having run the experiment: This shouldn’t be any more difficult to run, observe or interpret than the one above (at least once we’ve gotten our hands on a cone). Definitely want to try this!

Tank demonstration of the circulation in a fjord

It has been a long time in the making, but finally we have a nice fjord circulation in our tank!

Pierre and I tried to improve it 6 years ago, Steffi, Ailin and I have been working on it for a couple of days last August, then finally this morning, Steffi and I tried again — and it worked beautifully right away!

We now have an experiment that shows how a fresh, yellow inflow (representing the freshwater input into fjords close to their heads by rivers) flows over a initially stagnant pool of salt water. As the freshwater plume flows out of the fjord, it entrains more and more salt water from below, thus thickening and setting up a return flow that brings in more salt water from the reservoir (representing the open ocean) on the right.

We drop dye crystals to visualize the surface current going out of the fjord and the return flow going in, and draw the profiles on the tank to be able to discuss them later.

Here is a movie of the whole thing:

But there is more to see!

When tipping the tank to empty it, a lot of turbulence was created at the sill (see movie below). While a fjord typically isn’t tipped very often, what we see here is basically what tides do on the sill (see the waves that keep going back and forth over the sill after the tank is initially lifted? Those are exactly like tides). This could purposefully integrated in teaching rather than only happen by accident, those waves could be created just by surface forcing rather than by tipping the tank. That’s a very nice demonstration to explain high mixing rates in the vicinity of steep topography!

And then there is also the issue of very low oxygen concentrations in some Norwegian fjords, and one proposed solution is to bring the river inflow deep down into the fjord. The idea is that the less dense river water will move up to the surface again, thereby creating mixing and oxygenating the stagnant deep water that, in some cases, hasn’t been renewed in many years.

We model this by putting the inflow (the hose) down into the tank and see the expected behaviour. What we also see: Since the water has a quite strong downward component as it enters the fjord, it stirs up a lot of old dye from the bottom. So possibly something to be aware of since there might be stuff dumped into fjords that you might not necessarily want to stir up…

And last, not least, a bonus picture: This is how we measure temperature at GFI. You would think it should be possible to find a smaller thermometer that isn’t an old reversing mercury one? But in any case, this worked very well, too :-)