Tag Archives: tank experiment

Observing waves in a tank

So you thought filling water into a tank was boring? Not on my watch!

This is how we fill up the tank: Through a hole at the bottom. Which leads to a very nice fountain that slowly submerges as the water level rises:

…and to tons of nice waves, which are great to observe!

Propagation of waves

Below you see waves propagating. Can you spot the water’s orbital movement, i.e. water particles moving in circles, even though the wave phase is propagating from left to right?

Standing waves

After a while, waves are reflected at the end of the tank and propagate back, setting up a different, very cool, pattern:

Now the wave phase does not seem to travel any more! Instead, there are fixed points in space where water levels oscillate between maximum and minimum, and in between there are other points where the water level stays more or less the same. How cool is that?!

…And this is just filling the tank. Just wait how cool it gets when we are actually running our demonstrations! :-)

Experiment: Double-diffusive mixing (salt fingering)

On the coolest process in oceanography.

My favorite oceanographic process, as all of my students and many of my acquaintances know, is double-diffusive mixing. Look at how awesome it is:

Double-diffusive mixing happens because heat and salt’s molecular diffusion are very different: Heat diffuses about a factor 100 faster than salt. This can lead to curious phenomena: Bodies of water with a stable stratification in density will start to mix much more efficiently than one would have thought.

In the specific case of a stable density stratification with warm, salty water over cold, fresh water, finger-like structures form. Those structures are called “salt fingers”, the process is “salt fingering”.

IMG_4233_sehr_klein

Salt fingering occuring with the red food dye acting as “salt”.

Even though salt fingers are tiny compared to the dimensions of the ocean, they still have a measurable effect on the oceanic stratification in the form of large-scale layers and stair cases, and not only the stratification in temperature and salinity, but also on nutrient availability in the subtropical gyres, for example, or on CO2 drawdown.

Over the next couple of posts, I will focus on double diffusive mixing, but less on the science and more on how it can be used in teaching. (If you want to know more about the science, there are tons of interesting papers around, for example my very first paper)

How to easily set up the stratification for the salt fingering process.

Setting up stratifications in tanks is a pain. Of course there are sophisticated methods, but when you want to just quickly set something up in class (or in your own kitchen) you don’t necessarily want to go through the whole hassle of a proper lab setup.

For double diffusive mixing, there are several methods out there that people routinely use.

For example the hose-and-funnel technique, where the less dense fluid is filled in the tank first and then the denser fluid is slid underneath with the help of a hose and a funnel. And a diffuser at the end of the hose. And careful pouring. And usually a lot more mixing than desired.

Or the plastic-wrap-to-prevent-mixing technique, where the dense fluid is put into the tank, covered by plastic wrap, and then the lighter fluid is poured on top. Then the plastic wrap is removed and by doing so the stratification is being destroyed. (No video because I was frustrated and deleted it right away)

Or some other techniques that I tried and didn’t find too impressive. (No videos either for the same reason as above)

But then accidentally I came across this method (as in: I wanted to show something completely different, but then I saw the salt fingers and was hooked):

Granted, this is not a realistic model of an oceanic stratification. But as you can see towards the end of that movie, that turns out to be a blessing in disguise if you want to talk about the process in detail. As you see in the movie, the salt fingers inside the bottle are much smaller than the salt fingers outside the bottle. Because, clearly, inside the bottle the warm water is cooled both at the interface with the cold water inside the bottle, and by heat conduction through the walls of the bottle, since the water is surrounded by cold water. The warm water that flowed out of the bottle and up towards the water’s surface is only cooled at the interface with the water below (the air above is warmer than the cold water). So this gives you the perfect opportunity to discuss the scaling of salt fingers depending on the stratification without having to go through the pains of actually preparing stratifications with different gradients in temperature or salinity.

IMG_9084

Self-portrait with salt fingers :-)

In my experience, the best salt fingers happen when you use hot water with dye (as the warm and salty top layer) and cold fresh water below. Salt fingers develop quickly, you don’t have the hassle of hitting the exact temperatures or salinities to make the density stratification statically stable, yet unstable in salinity, and it ALWAYS works.

 

IMG_9079

Double-diffusive mixing. Scale at the bottom is centimeters.

 

IMG_9054

Salt fingering in a tank. Scale at the bottom is centimeters.

And look at how beautiful it looks! Do you understand why I LOVE double diffusion?

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Tank experiment: Lee waves in a fancy density (and dye) stratification

Did you seriously think we’d stop tank experiments with only 2-layer systems? Nooo!

Today, the plan was to set up a continuous stratification, which I have been planning to do for many years. After fiddling with the setup all morning (do you have any idea how many fittings on all kinds of hoses are needed to get that to work well?), reality set in and we ended up doing a quasi-continuous stratification, i.e. 12 density layers dyed in 6 different colors*.

And this is what it looks like when you tow a mountain through that stratification (and try to ignore the excited audience being reflected in the tank): Still very nice lee waves and surprisingly little turbulence!

*We set up the tank to contain the same amount of salt as our 2-layer system yesterday, so instead of one big density jump from about 1000g/l to 1026g/l, this now happened in 5 smaller, more or less regular, jumps. And here is how we did it in the end: Two large reservoirs (unfortunately of different diameters), one containing freshwater, the other one filled up to the same height, containing as much salt as we had in our experiment yesterday. Now the height of the reservoirs was divided in 12 equal dzs, and for each dz that went out of the “freshwater” tank into the experimental tank, we added salt water of the same dz to the “freshwater” tank, which thus continued to increase in salinity. The water that we mixed that way went through a hose and entered the experimental tank through the bottom of the tank through a hole over which we had put the mountain (to contain mixing to a small volume and also so we didn’t have to watch water shooting out of that hole in our nice stratification). So as the water we added became increasingly dense, it nicely layered itself underneath the other water in the tank. And we just had to add more and more dye for the color gradient. Easy peasy :-)

The one where it would help to understand the theory better (but still: awesome tank experiment!)

The main reason why we went to all the trouble of setting up a quasi-continuous stratification to pull our mountain through instead of sticking to the 2 layer system we used before was that we were expecting to see a tilt of the axis of the propagating phase. We did some calculations of the Brunt-Väisälä frequency, that needs to be larger than the product of the length of the obstacle and the speed the obstacle is towed with (and it was, by almost two orders of magnitude!), but happy with that result, we didn’t bother to think through all the theory.

And what happened was what always happens when you just take an equation and stick the numbers in and then go with that: Unfortunately, you realize you should have thought it through more carefully.

Luckily, Thomas chose exactly that time to come pick me up for a coffee (which never happened because he got sucked into all the tank experiment excitement going on), and he suggested that having one mountain might not be enough and that we should go for three sines in a row.

Getting a new mountain underneath an existing stratification is not easy, so we decided to go for the inverse problem and just tow something on the surface rather than at the bottom. And just to be safe we went with almost four wavelengths… And look at what happens!

We are actually not quite sure if the tilting we observed was due to a slightly wobbly pulling of the — let’s use the technical term and go for “thingy”? — or because of us getting the experiment right this time, but in any case it does look really cool, doesn’t it? And I’ll think about the theory some more before doing this with students… ;-)

Dead water — the fancy experiment including Nansen himself

Now that we do have a really awesome 12-layer 6-color stratification, we obviously had to do the dead water experiment again. This time we chose to include a not-too-happy-looking Nansen on the ship, too!

I love this even more than the one we did yesterday!

“Dead water” or: ship-generated internal waves

And here is another experiment that can be done with the same stratification as the lee waves: Towing a ship to explore the phenomenon of “dead water”!

Dead water is well known for anyone sailing on strong stratifications, i.e. in regions where there is a shallow fresh or brackish layer on top of a much saltier layer, e.g. the Baltic Sea of some fjords. It has been described as early as 1893 by Fridtjof Nansen, who wrote, sailing in the Arctic: “When caught in dead water Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm. In calm weather, with a light cargo, Fram was capable of 6 to 7 knots. When in dead water she was unable to make 1.5 knots. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose.” (cited in Walker,  J.M.; “Farthest North, Dead Water and the Ekman Spiral,” Weather, 46:158, 1991)

Finding the explanation for this phenomenon took a little while, but in 1904, Vilhelm Bjerknes explained that “in the case of a layer of fresh water resting on the top of salt water, a ship will not only produce the ordinary visible waves at the boundary between the water and the air, but will also generate invisible waves in the salt-water fresh-water boundary below” — a lot of the ship’s work is now going towards generating the internal waves at the interface rather than for propulsion.

It’s hard to imagine how a ship will generate waves somewhere in the water below, so we are demonstrating this in the tank:

Isn’t it fascinating to think about how far oceanography has come in only a little over a hundred years? And despite all the extremely powerful instrumentation and modelling that we have available now, how cool are even such simple demonstrations in a tank? These are the moments where I know exactly why I went to study oceanography in the first place, and why it’s still the most fascinating subject I can think of…

Lee waves in the tank

Did you guess what we needed the stratification for? Yes — we are moving mountains again! :-)

What we want to look at: How a current reacts to an obstacle in its way, especially a current in a stratification. But since it is really difficult to set up a current in a tank, let alone a stratified one, we are doing the next best thing: Moving the obstacle relative to the water rather than the other way round.

And this is what it looks like:

Et voilà: Beautiful lee waves!

And look at the paper bits floating on the surface and how they visualize convergences and divergences in the upper layer!

The three layers in the pink all have (more or less) similar densities, and are only dyed slightly differently because we had to make several batches of dyed salt water to be able to fill the tank. But look how well they show that the wave is really happening at the interface, and that the other layers are phase locked. What would happen if the stratification inside the pink layer was stronger? Just wait and see…. ;-)

Kelvin-Helmholtz instabilities

I’m back at my happy place — the teaching lab at GFI in Bergen! :-)

Here a quick look at what we’ve been doing today: Filling the large wave tank! With clear fresh water and then salty pink water that forms a layer below. As the pink water flows underneath the clear water, there is shear between the two layers, waves form and then they break. Beautiful Kelvin-Helmholtz shear instabilities!

Why have we filled the large tank? Just you wait and see… ;-)

Experiment: Oceanic overturning circulation (the easiest version)

“The easiest” in the title of this page is to show the contrast to a “slightly more complicated” version here.

Background

One of the first concepts people hear about in the context of ocean and climate is the “great conveyor belt”. The great conveyor belt is a very simplified concept of the global ocean circulation, which is depicted as a single current that spans the world oceans (see Figure 1 below). In this simplified view of the global circulation, water flows as a warm, global surface current towards the North Atlantic, where it cools, sinks and finally returns southward and through all the world oceans near the bottom of the ocean. Water is transported back to the surface through mixing processes and starts over its journey again as a warm surface current. While in reality some part of the conveyor belt is wind-driven and many processes come to play together, a large part of the circulation can be explained by the water sinking due to cooling at high latitudes.

Conveyor_belt

Figure 1: The great conveyor belt. My sketch on top of a map from http://www.free-world-maps.com (used with permission)

The experiment

Since the global conveyor belt is such a basic concept that we come across in many different contexts, it is very useful to have a good demonstration of what is happening in the world ocean. Plus demonstrations and experiments are always fun!

I here present a very simple experiment that can be used for many different purposes. In science outreach, for example on a fair or in a talk, to catch people’s attention and raise an interest in oceanography. In schools to do the same, or to connect the fascination of the ocean to school physics and talk about density, convection, heat. At university to do all of the above, as well as to practice writing lab reports, talk about the scientific method or the validity of simplifications in theoretical or physical models.

Materials needed

All we need to run this experiment is

  • a clear plastic container
  • lukewarm water
  • red and blue food dye
  • an ice cube tray and
  • access to a freezer.

Ideally we’d also have a thermos or some other kind of insulation to keep the ice cubes frozen until we start running the experiment. To prepare the experiment, all we need to do a half a day ahead is mix some blue food dye into the water that we put in the ice cube tray, and freeze the ice cubes.

Running the experiment

To run the experiment, we start out by filling our “tank” with lukewarm water. Let it settle for a bit. Now we decide for one end of your tank to be the “equator” end. There, we add some red food dye (see Figure 1).

overturning-ice-1

Figure 2: Tank with luke warm water. Red food dye added to the “warm” end of the tank.

Then we add the blue ice cubes to the “poleward” end of our tank (see Figure 3).

overturning-ice-2

Figure 3: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”.

The cold melt water from the ice cubes is denser than the lukewarm water in the tank and therefore sinks to the bottom of the tank where it spreads “equatorward”, pushing below the warmer water. This can be seen where the red water is pushed upwards and “poleward”.

Discussion

Of course, the processes at play here are not exactly the same as in the real ocean.

For one, deep water formation is NOT due to ice cubes melting in lukewarm water. In fact, melting of sea ice will in most cases not lead to any kind of sinking of water, since the melt water is fresh and the surrounding ocean water is salty and hence denser than the melt water. Cooling in the ocean happens through many processes at the surface of the ocean, like radiation into space and evaporation.

Heating is also represented in an extremely simplified way in this experiment. Heating in the ocean occurs mainly (with the negligible exception of thermal springs in the ocean) by radiative heating from the sun, and at the surface only. We “heat” throughout the whole depth of the ocean by filling the whole tank with lukewarm water.

Also, the mixing processes that, in the real ocean, bring deepwater back to the surface are not represented here at all. Our tank will eventually fill with a layer of cold water at the bottom (See Figure 4), and the circulation will stop once all the ice has melted.

overturning-ice-3

Figure 4: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”. Slowly, the tank fills with cold water.

Why use the experiment?

Even with all the simplifications described above, this experiment is a great first step in becoming intrigued by the ocean, and towards understanding ocean circulation. Seeing the melt water sink from the ice cubes is fascinating no matter how little interest one might have in the physics that cause it. Sliding a finger up and down the side of the tank lets you experience – feel! – how the temperature changes from warm near the surface to very cold near the bottom. Actually physically feeling this is a lot more impressive than just watching the experiment or even just being shown temperature sections of the ocean. And the experiment invites you to play: What if you added little pieces of paper on the water surface, would you see them move with the flow towards the cold end of the tank? Or if you dropped a dye crystal in the middle of the tank, would the dye ribbon that forms be deformed by the currents in the tank? And what if you added twice as many ice cubes, would the currents be twice as fast?

This is pretty much the easiest experiment you can imagine. If you are afraid of what food dye might do in the hands of your participants, you don’t even need to let them handle it themselves, even when they are working in small groups with individual tanks: just go around dripping the dye in and then add the dyed ice cubes yourself. While someone might still tip over a tank and spill the water, this has yet to happen to me. Especially since, before running the experiment, you will have pointed out that they need to make sure not to bang against the tables as to not disturb the experiment. And now apart from making sure that the ice cubes are frozen when you want to run this experiment, there is nothing that can go wrong. So why not try this experiment next time you want to talk about global ocean circulation?

Watch a video of the experiment here:

What would I do differently next time?

Next time, I would pay attention to which end of my tank will represent the equatorward and poleward side of the ocean. Not that it matters much, but in most graphics of sections through the North Atlantic, the northern end will be on the right side and the southern end on the left. If the experiment is set up the other way round (as on all pictures and movies above) you will need to remember to point it out (or even mark it on the tank with a sharpie or such).

Still scared of the hassle of running experiments?

And for all of you who hesitate doing awesome experiments because it looks like you need so much equipment: No you don’t. Here is a “making of” shot from how I did this experiment on my coffee table while sitting on my couch. The background is the back of an old calendar sheet, clipped to the back of a chair. And that’s it.

Screen shot 2015-11-02 at 3.41.24 PM

The setup for my experiment.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Eddy in a jar

Rotating experiments in your kitchen.

Eddies, those large, rotating structures in the ocean, are pretty hard to imagine. Of course, you can see them on many different scales, so you can observe them for example in creeks, as shown below:

IMG_1266

Eddies in the Pinnau river, and their dark “shadows”.

If you can’t really spot them in the image above, check out this post for clues and a movie.

So how can you create eddies to observe their structure?

MVI_0698

Dye spiral caused by an eddy in a jar

I took a large cylindrical jar, filled it with water, stirred, let it settle down a little bit and then injected dye at the surface, radially outward from the center. Because the rotating body of water is slowed down by friction with the jar, the center spins faster than the outer water, and the dye streak gets deformed into a spiral. The sheet stays visible for a very long time, even as it gets wound up tighter and tighter. And you can see the whole eddy wobble a bit (or pulsate might be the more technical term) because I introduced turbulence when I stopped stirring.

Watch the movie below if you want to see more. Or even better: Go play yourself!

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.