I’d love your input: If your student lab for GFD tank experiments had to downsize, but you had to present a “wish list” for a smaller replacement, what would be on that list? Below are my considerations, but I would be super grateful for any additional input or comments! :-)
Background and “boundary conditions”
The awesome towing tank that you have come to love (see picture above) will have to be removed to make room for a new cantina. It might get moved into a smaller room, or possibly replaced all together. Here are some external requirements, as far as I am aware of them:
- the (new) tank should ideally be movable so the (small) room can be used multi-purpose
- since the new room is fairly small, people would be happy if the new tank was also smaller than the old one
- the rotating table is kept (and a second, smaller one, exists in the building)
- There are other, smaller tanks that will be kept for other experiments, dimensions approximately 175x15x40cm and smaller
- the whole proposal needs to be inexpensive enough so that the likelyhood that it will actually be approved is moderate to fair ;-)
Here are a couple of things I think need to be definitely considered.
Dimensions of the tank
If the tank was to be replaced by a smaller one, how small could the smaller one be?
The dimension of the new tank depend, of course, on the type of experiment that should be done in the tank. Experiments that I have run in the tank that is to be replaced and that in my opinion should definitely be made possible in the new location/tank include
- “Dead water”, where a ship creates internal waves on a density interface (instructions)
- Internal lee waves & hydraulic jumps, where a mountain is moved at the bottom of the tank (instructions)
- Surface imprints of internal waves (example)
- Surface waves (example)
- Intrusions (example)
- Waves in a density stratification (example)
- Surface waves running up on a slope (I haven’t blogged about that yet, movies waiting to be edited)
If we want to be able to continue running these experiments, here is why we should not sacrifice the dimensions of the tank.
Why we need the tank length
The first reason for keeping the length of the tank is that the “mountains” being towed to create the lee waves are already 1 and 1.5m long, respectively. This is a length that is “lost” for actual experiments, because obviously the mountain needs space inside the tank on either end (so in its start and end position). Additionally, when the mountain starts to move, it has to move for some distance before the flow starts displaying the features we want to present: Initially, there is no reservoir on the “upstream” side of the mountain and it only builds up over the first half meter or so.
The second reason for keeping the length of the tank are wave reflections once the ship or mountain comes close to the other side of the tank. Reflected surface waves running against the ship will set up additional drag that we don’t want when we are focussing on the interaction between the ship and the internal wave field. Reflected internal waves similarly mess things up in both experiments
The third reason for keeping the length of the tank is its purpose: as teaching tank. Even if one might get away with a slightly shorter tank for experiments when you just film and investigate the short stretch in the middle of the tank where there are no issues with either the push you gave the system when starting the experiment or the reflections when you get near the end, the whole purpose of the tank is to have students observe. This means that there needs to be a good amount of time where the phenomenon in question is actually present and observable, which, for the tank, means that it has to be as long as possible.
Why we need the tank width
In the experiments mentioned above, with exception of the “dead water” experiment, the tank represents a “slice” of the ocean. We are not interested in changes across the width of the tank, and therefore it does not need to be very wide. However, if there is water moving inside the tank, there will be friction with the side walls and the thinner the tank, the more important the influence of that friction will become. If you look for example at the surface imprint of internal wave experiment, you do see that the flow is slowed down on either side. So if you want flow that is outside of the boundary layers on either side, you need to keep some width.
Secondly, not changing the tank’s width has the advantage that no new mountains/ships need to be built.
Another, practical argument for a wide-ish tank (that I feel VERY strongly about) is that the tank will need to be cleaned. Not just rinsed with water, but scrubbed with a sponge. And I have had my hands inside enough tanks to appreciate if the tank is wide enough that my arm does not have to touch both sides at all times when reaching in to clean the tank.
Why we need the tank depth
The first reason for keeping the height is that for the “dead water” experiment, even the existing tank is a lot shallower than what we’d like from theory (more here). If we go shallower, at some point the interactions between the internal waves and the ground will become so large that it will mess up everything.
Another reason for keeping the depth is the “waves running up a slope” experiment. If you want waves running up a slope (and building up in height as they do), you have the choice between high walls of the tank or water spilling. Just sayin’…
And last not least: this tank has been used in “actual” research (rather than just teaching demonstrations, more on that on Elin’s blog), so if nothing else, those guys will have thought long and hard about what they need before building the tank…
Without getting too philosophical here about models and what they can and cannot achieve (and tank experiments being models of phenomena in the ocean), the problem is that scaling of the ocean into a tiny tank does not work, so “just use a mountain/boat half the size of the existing ones!” is actually not possible. Similarly to how if you build the most amazing model train landscape, at some point you will decide that tiny white dots are accurate enough representations of daisies on a lawn, if you go to a certain size, the tank will not be able to display everything you want to see. So going smaller and smaller and smaller just does not work. A more in-depth and scientific discussion of the issue here.
Other features of the tank
When building a new tank or setting up the existing tank in a new spot, there are some features that I consider to be important:
- The tank needs a white, intransparent back wall (either permanently or draped with something) so that students can easily focus on what is going on inside the tank. Tank experiments are difficult to observe and even more difficult to take pictures of, the better the contrast against a calm background, the better
- The tank should be made of glass or some other material that can get scrubbed without scratching the surface. Even if there is only tap water in the tank, it’s incredible how dirty tanks get and how hard they have to be scrubbed to get clean again!
- The tank needs plenty of inlets for source waters to allow for many different uses. With the current tank, I have mainly used an inlet through the bottom to set up stratifications, because it allowed for careful layering “from below”. But sometimes it would be very convenient to have inlets from the side close to the bottom, too. And yes, a hose could also be lowered into the tank to have water flow in near the bottom, but then there needs to be some type of construction on which a hose can be mounted so it stays in one place and does not move.
- There needs to be scaffolding above the tank, and it needs to be easily modifiable to mount cameras, pulleys, lights, …
- We need mechanism to tow mountains and ships. The current tank has two different mechanisms set up, one for mountains, one for ships. While the one for the ship is home-made and easily reproducible in a different setting (instructions), the one to tow the mountain with is not. If there was a new mechanism built, one would need to make sure the speeds at which the mountain can be towed matches the internal wave speed to be used in the experiment, which depends on the stratification. This is easy enough to calculate, but it needs to be done before anything is built. And the mechanism does require very securely installed pulleys at the bottom of the tank which need to be considered and planned for right from the start.
The “source” reservoirs (plural!) are the reservoirs in which water is prepared before the tank is filled. It is crucial that water can be prepared in advance; mixing water inside the tank is not feasible.
There should be two source reservoirs, each large enough to carry half the volume of the tank. This way, good stratifications can be set up easily (see here for how that works. Of course it works also with smaller reservoirs in which you prepare water in batches as you see below. But what can happen then is that you don’t get the water properties exactly right and you end up seeing stuff you did not want to see, as for example here, which can mess up your whole experiment)
Both reservoirs should sit above the height of the tank so that the water can be driven into the tank by gravity (yes, pumps could work, too, more on that below).
Depending on the kind of dyes and tracer used in the water, the water will need to be collected and disposed of rather than just being poured down the drain. The reservoir that catches the “waste” water needs to
- be able to hold the whole volume of the tank
- sit lower than the tank so gravity will empty the tank into the reservoir (or there needs to be a fast pump to empty the tank, more on that below)
- be able to be either transported out of the room and the building (which means that doors have to be wide enough, no steps on the way out, …) or there needs to be a way to empty out the reservoir, too
- be able to either easily be replaced by an empty one, or there needs to be some kind of mechanism for who empties it within a couple of hours of it being filled, so that the next experiment can be run and emptied out
If the waste water is just plain clear tap water, it can be reused for future experiments. In this case, it can be stored and there need to be…
If reservoirs cannot be located above and below tank height to use gravity to fill and empty the tanks, we need pumps (plural).
- A fast pump to empty out the tank into the sink reservoir, which can also be used to recycle the water from the sink reservoir into the source reservoirs
- One pump that can be regulated very precisely even at low flow rates to set the inflow into the tank
- Ideally, a second pump that can be regulated very precisely, so the double bucket method of setting up a stratification in a tank can be done automated rather than relying on gravity.
Preferable the first and the latter are not the same, because changing settings between calibrating the pump for an experiment, setting it on full power to empty the tank, and calibrating it again will cause a lot of extra work.
Inlets for dyes
Sometimes it would be extremely convenient if there was a possibility to insert dyes into the tank for short, distinct periods of time during filling to mark different layers. For this, it would be great to be able to connect syringes to the inlet
Hoses and adapters
I’ve worked for years with whatever hoses I could find, and tons of different adapters to connect the hoses to my reservoir, the tap, the tank. It would be so much less of a hassle if someone thought through which hoses will actually be needed, bought them at the right diameter and length, and outfitted them with the adapters they needed to work.
Space to run the experiment
The tank needs to be accessible from the back side so the experimenter can run the experiment without walking in front of the observers (since the whole purpose of the tank is to be observed by students). The experimenter also needs to be able to get out from behind the tank without a hassle so he or she can point out features of interest on the other side.
Also, very importantly, the experimenter needs to be able to reach taps very quickly (without squeezing through a tight gap or climbing over something) in case hoses come loose, or the emergency stop for any mechanism pulling mountains in case something goes wrong there.
Space for observers
There needs to be enough room to have a class of 25ish students plus ideally a handful of other interested people in the room. But not only do they need to fit into the room, they also need to be able to see the experiments (they should not have to stand in several rows behind each other, so all the small people in the back get to see are the shoulders of the people in front). Ideally, there will be space so they can duck down to have their eyes at the same height as the features of interest (e.g. the density interface). If the students don’t have the chance to observe, there is no point of running an experiment in the first place.
Ideally, when designing the layout of the room, it is considered how tank experiments will be documented, i.e. most likely filmed, and there needs to be space at a sufficient distance from the tank to set up a tripod etc..
Both for direct observations and for students observing tank experiments, it is crucial that the lighting in the room has been carefully planned so there are minimal reflections on the walls of the tank and students are not blinded by light coming through the back of the tank if a backlighting solution is chosen.
In my experience, even though many instructors are extremely interested in having their students observe experiments, there are not many people willing to run tank experiments of the scale we are talking about here in their teaching. This is because there is a lot of work involved in setting up those experiments, running them, and cleaning up afterwards. Also there are a lot of fears of experiments “going wrong” and instructors then having to react to unexpected observations. Running tank experiments requires considerable skill and experience. So if we want people using the new room and new tank at all, this has to be made as easy as possible for them. Therefore I would highly recommend that someone with expertise in setting up and running experiments, and using them in teaching, gets involved in designing and setting up the new room. And I’d definitely be willing to be that person. Just sayin’ ;-)