As part of my blog’s 6th Birthday celebration, I asked you to submit your favourite ocean quotes so I could illustrate them for you. This is what Benjamin suggested (and I love it!).
P.S.: The quote might be by Rumi, but in total, my internet research was inconclusive as to the original source. Since I had already taken the pictures of my drawing anyway, we’ll have to live with a non-attributed quote…
Even though I’ve been looking at waves for years now, wave watching is still full of surprises. Yesterday I showed you capillary waves that a jellyfish made, and today I’m showing you a helpful seagull.
What I found most fascinating walking through this marina were the long straight wave crests that form in parallel to the step in the foreground as waves leap up on the step and then the water flows back. I can watch this kind of thing for a very long time!
But I was also pointing out the ring waves around the bollards to my friend, which are a lot more difficult to spot. But, as if it had just been waiting for the opportunity to help out, the seagull on one of the bollards dropped something and created ring waves for us! Can you spot it?
Sometimes the best wave watching doesn’t happen how you expect it to happen. Look at this jellyfish, bobbing about in the surface of Kiel fjord. Can you spot the circular wave pattern all around where it breaks the surface? I find this so amazing. Would you have thought that you would spot waves that a jellyfish makes?
Btw, there are other places with capillary waves in this picture, too: In several spots you see thin wave crests, parallel to each other, running in front of a larger wave crest. Those are capillary waves, and the cool thing is that the shorter their wavelength, the faster their propagation. Therefore the larger crests seem to be pushing the smaller ones in front of them, bunching them up over time.
When you stop a rotating tank, lots of stuff happens and it is usually very impressive to watch. Sometimes we stop tanks on purpose to show for example the development of Ekman layers, but sometimes we are just done with an experiment and then get to see cool stuff to see just as part of cleaning up.
Like below: When the tank stops, the water inside continues to spin, but friction with the sides and the bottom of the tank starts slowing the water down, inducing shear. Shear in turn produces turbulence and the structures cause smaller and smaller eddies. Very cool to watch!
One of the first exercises Torge and I plan on doing with the students in our “dry theory to juicy reality” project is to bring a water-filled tank to solid body rotation and measure rotation, surface height at the center of the tank and the sides, as well as water depth before rotation, and then have them put those together according to theory.
Setup of the experiment as we did it using a glass vase my mom gave me as tank (diameter 24.5 cm). The non-rotating water depth was 9.2 cm. Once we rotated the tank with 10 rotations per 8.6 seconds, the maximum water level at the outside edge of the tank was approximately 10.8 cm, and the minimum 7.9 cm.
Seeing how difficult it is to “measure” the surface heights while the tank is rotating (we chose to draw circles on the outside of the tank at the heights where we thought the water levels were, in order to measure them later on a non-rotating tank), we were quite pleased with those results once we plugged them into the equations.
Calculating the resting water level as arithmetic mean between the rotating maximum value at the rim and the minimum value in the center, we are only off by 0.1 cm, so not too shabby!
And calculating the height difference between resting water level and rotating maximum level from the tangential velocity and radius of the tank, we are only off by 0.4 cm. So all in all, that’s working well!
Btw, below you see the resting water level and above the mark for the rotating maximum value. Quite impressive difference, isn’t it?
Anyway, looking at rotational surfaces and volumes and stuff this way is a lot more fun than doing it the dry theoretical way only! At least that’s what I think ;-)
Celebrate with me by sending me your favourite ocean-themed quote and I will illustrate my three favourites and mail them to you as postcards!
To motivate myself to start sketching regularly, I bought myself this new pen with a soft tip. Which I am still getting used to, as you can see. But I’m having fun! :-)
Lots of demonstrations being prepared for Torge’s and my “dry theory to juicy reality” project. Shown here today: rotating vs non-rotating turbulence. Because the only way to really appreciate how amazing rotating flows are is to compare them with non-rotating ones. And not everybody does have a clear idea what non-rotating flows would even look like.
So here we are dropping dye into a non-rotating tank. Top view shows it forming tons of small eddies and spreading to the sides.
Side view shows that most of the dye sank to the bottom of the tank and is spreading there, showing 3-dimensional turbulence.
Now, for comparison, the rotating case!
Top view shows one single, clean eddy.
And side view shows that the structure is coherent all the way from surface to bottom. Now doesn’t this look really fascinatingly different from the non-rotating case?
To show the difference even more clearly, check out the movie below. Speed of both movies is the same!
With all the rotating tank experiments I’ve been showing lately, one thing that comes up over and over again is the issue of solid body rotation.
On our DIYnamics-inspired turntable for our “dry theory to juicy reality” project, Torge and I came up with a fun way to illustrate the importance of full body rotation in tank experiments, again inspired by the DIYnamics team, this time their youtube channel.
For the spinning dye curtain experiment, we start up the rotating table, and then pretty much immediately add in some dye. Below, you see what happens when you add in the dye too late (we waited for 2 minutes here before we added it): The water is so much in solid body rotation already, that we only form columns and 2D flow.
But if we add in the dye right away after starting up the tank, we form these spirals where the water further away from the center is spinning faster than the water right at the center, thus distorting the dye patches into long, thin filaments (Btw, I’ve shown something similar in my “eddies in a jar” experiment earlier, where instead of starting up a turntable I just stirred water in a cylindrical tank).
But as the tank continues to spin up, the eddies eventually stop spinning and the tank turns into solid body rotation. If new dye is added now, only columns form, but they stay intact as if they were, indeed, solid bodies.
But seeing the behaviour of a fluid change within half a minute or so is really impressive and something we definitely want to do in class, too!
Early morning Kiel fjord — today even featuring a hot-air balloon!
But, more interestingly, the wake of this police boat. I find it already pretty cool in the picture below: The fjord is calm and mirror-like, but inside the ship’s V-shaped wake the surface changes completely and the reflections look totally different (now only reflecting the sky back, not the cranes). And, of course, the V-shaped wake itself has quite a large amplitude, too.
A little while later, the wake has not only reached the sea wall, it is already being reflected back away from the wall. See the original wake at the bottom of the picture below, and the reflection further away, near the five bollards?
Looking slightly further right, we see the concave shape of the sea wall here, and how waves are being focussed similarly to how radio waves are focussed towards the receiver with parabolic antennas.
So as the reflected waves propagate out further and further, they little by little reach a focal point.
Which you see in the picture below: An area of higher waves in the middle of the water, seemingly for no reason.
And the area where waves interfere and amplitudes are so high moves a bit over time, but it’s a quite persistent pattern.
Had I just come across this pattern without seeing it develop, I don’t think I would have been able to explain what is going on here.
And see how, now that the wake has passed, the checkerboard pattern of interfering waves in the foreground is a lot more prominent again?
Whenever I get out of my house and it looks like this, I am slightly disappointed because it means that the wave watching that morning will not be ideal. I mean, I like colorful sunrises as much as the next person, buuuut…
Today, at least, the fog was kinda interesting, also because there was a large cruise ship driving through.
There was a low layer of fog, but look at what happens as the ship passes through: It lifts up! Visualizing the stream lines around the obstacle. Pretty cool! (And thank you, little police boat, for making at least some waves for me today!)
Even better visible below, but check out the smoke coming from the ship’s chimneys. Do you see how it is propagating forward? Or does it just look like that to me? At least below the fog layer there was pretty much no wind. So what’s going on up there? Anyone care to explain?
