In my last post, I showed you the legendary overturning experiment. And guess what occurred to me? That there is an even easier way to show the same thing. No gel pads! (BUT! And that is a BIG BUT! Melting of ice cubes in lukewarm water is NOT the process that drives the “real” overturning! For a slightly longer version of this post check this out).
So. Tank full of luke warm water. Red dye on one end. Spoiler alert: This is going to be the “warm” end.
Now. Ice cubes on the “cold” end. For convenience, they have been dyed blue so that the cold melt water is easily identifiable as cold.
A very easy way to get a nice stratification! And as you see in the video below, awesome internal waves on the interface, too.
And because I know you like a “behind the scenes”:
I took this picture sitting on my sofa. The experiment is set up on the tea table. The white background is a “Janosch” calendar from 15 years ago, clipped to the back of a chair. And that is how it is done! :-)
For one of my side-projects I needed higher-resolution photos of the overturning experiment, so I had to redo it. Figured I’d share them with you, too.
You know the experiment: gel pads for sports injuries, one hot (here on the left), one cold (here on the right). Blue dye on the cold pad to mark the cold water, red dye on the warm pad as a tracer for warm water.
A circulation develops. If you drop dye crystals in the tank, the ribbon that formed gets deformed by the currents for yet another visualization of the flow field.
Lighting is a problem this time of year. I chose natural light over artificial, and it came out ok, I think.
Did you ever notice how when certain ferries dock, they stop, already parallel to the dock, a couple of meters away from the dock and then just move sideways towards the dock? Usually they don’t even move passenger ferries any more, just use thrusters to keep them steady while people get on and off.
But why this weird sideward motion?
One reason is the Coanda effect – the effect that jets are attracted to nearby surfaces and follow those surfaces even when they curve away. You might know it from putting something close to a stream of water and watching how the stream gets pulled towards that object, or from a fast air stream that can lift ping pong balls. So if the ship was moving while using the thrusters, the jets from the thrusters might just attach themselves to the hull of the ship and hence not act perpendicularly to the ship as intended.
But I think there is a secret second reason: Because it just looks awesome :-)
The experiment presented in this post was first proposed by Marsigli in 1681. It illustrates how, despite the absence of a difference in the surface height of two fluids, currents can be driven by the density difference between the fluids. A really nice article by Soffientino and Pilson (2005) on the importance of the Bosporus Strait in oceanography describes the conception of the experiment and includes original drawings.
The way we conduct the experiment, we connect two similar tanks with pipes at the top and bottom, but initially close off the pipes to prevent exchange between tanks. One tank is filled with fresh water, the other one with salt water which is dyed pink. At a time zero we open the pipes and watch what happens.
As was to be expected, a circulation develops in which the dense salt water flows through the lower pipe into the fresh water tank, compensated by freshwater flowing the opposite way in the upper pipe.
We measure the height of the interface between the pink and the clear water in both tanks over time, and watch as it eventually stops changing and equilibrates.
Usually this experiment is all about density driven flows, as are the exercises and questions we ask connected to it. But humor me in preparation of a future post: Comparing the height of the two pink volumes and the two clear volumes we find that they do not add up to the original volumes of the pink and clear tanks – the pink volume has increased and the clear volume decreased.