I just found this picture that I took back in May near my friend Elin’s cabin on an island in western Norway, and it’s a really nice illustration of how the same wind will cause very different waves depending on whether it’s blowing over the sea for many kilometres, or over a puddle for only a couple of centimetres.
This is a really good way to train your eyes to spot reflections of waves: watch a single wave train hit a sea wall:
Please don’t get sea sick when — about halfway through — I’m moving the camera around :-)
After having talked about wakes quite a bit recently, I’m going to show you a couple more pics today (bit not as many as I did two days ago, sorry! ;-))
And today we are focussing on the wake of one specific ship, the Stena Germanica that we sailed on for a mini cruise to Gothenburg (highly recommended!!). And even though that cruise sadly ended a month ago, I am still puzzled by what we saw:
Look at the turbulent wake directly behind the ship. Do you see how the two sides are markedly different from each other?
One side seems to be a lot more turbulent than the other.
And I didn’t find an explanation for that. According to the internet, the ship has two propellers and on each of those propellers the pitch of the blades can be changed so that the propeller can always run at a constant speed and the ship’s velocity is changed by changing the propeller blades’ angles.
Maybe that’s what is going on? But would they run two propellers at different pitches even when just steaming straight ahead?
Btw, look at that beautiful wake pattern! You can see both the feathery V going out from the ship and those half circles filling in the V. I don’t think I have ever seen that this clearly on a ship this size before. But then maybe I just wasn’t observing well enough.
I love it!
Now. The next morning: It is quite windy now, and the difference between the two sides of the wake is quite pronounced. Also very interesting: The foam on both sides is behaving very differently! One side (the upwind side) has a much sharper edge than the other.
But this surely can’t be the effect of just the wind?!
Looks great though!
Also very interesting: When arriving in Gothenburg, we met another Stena Ferry, and the same thing could be seen. Here she is after she passed by:
And her wake stays visible for quite a while with this marked foam stripe right in the middle of the wake, as if she had counter-rotating propellers that set up a convergence zone there?
Same phenomenon again when leaving Gothenburg later that day:
And arriving in Kiel the next morning, we could beautifully observe the different parts of the wake again:
What do you think is going on here? Why is the wake of the Stena Germanica not symmetric? I’m having sleepless nights over this :-)
And now you thought I was done showing you wakes? Ha, I was not nearly done! I told you, watching waves makes me happy :-)
And from Stena Germanica’s sun deck we had an excellent view over the Kiel fjord and later the Port of Gothenburg…
When I said that wake watching made me happy last week, did you really think those were all the wakes I was going to show you? Ha! No, I have plenty more! :-)
Today, I want to show you a couple that have one thing in common: the way that they show up against the sun’s reflection and thus become a lot more visible than they would be if they were just reflecting a uniformly blue or grey sky.
The best ship-watching of the year happens during Kiel Week (even if I do a fair bit of ship-watching year round ;-))
But this year, I was absolutely fascinated with wake-watching. Look at the sailing ship below and its beautiful wake!
You very clearly see the streak directly behind the boat, caused by turbulence where the hull pushed through the water. And then there is the actual wake, fanning out from the ship.
And then that wake gets reflected on a sea wall as the ship is sailing past!
Watching things like this makes me happy :-)
A 1.5 hour walk around a lake — and 242 photos of said lake — later I can tell you one thing: You definitely don’t need to live close to the coast in order to observe wave phenomena!
The idea to go on a “wave hunt expedition” is actually not mine (although it definitely sounds like something I could have come up with!), it’s Robinson’s idea. Robinson had students go on wave hunt expeditions as part of their examination, and present their results in a poster. I was so impressed with that, that I had to do it myself. Obviously. So the second best thing about work travel (right after the best thing, again, obviously!) is that I find myself in a strange place with time on my hand to wander around and explore. Not that Münster might not have been a nice city to explore, but the lake…
Anyway. I only want to show you 53 out of the 242 pictures. I was going to annotate all of them so you actually see what I mean. And I started annotating. But since I am giving a workshop tomorrow (which is all prepared and ready, but I do need my beauty sleep!) I only drew the key features in the pictures, and you will have to come up with the correct keywords all by yourself (have your pick: refraction! diffraction! fetch! interference! :-)) So click through the gallery below and see first the original photo and then one that I drew in. Do you spot the same stuff that I saw, or what else do you see? Let me know!
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If you think it would be useful to see all those pictures with proper annotations and descriptions at some point please let me know. I might still be excited enough to actually do it, who knows…
P.S.: I actually really enjoy work travel for the work parts, too. For example, I went to a great workshop in Dortmund earlier this year to learn about a quality framework for quantitative research, and that workshop was amazing. And a week ago, I went to Stuttgart for a meeting with all the fellows of the Stifterverband für die Deutsche Wissenschaft, which was also great. And now I am giving this workshop in Münster, that I am actually really excited about because I managed to condense pretty much all I know about “active learning in large groups” into a 2.5 hour workshop. Just so you don’t get the wrong idea about my priorities. Obviously water comes first, but then work is a very close second ;-)
When you look at waves, do you sometimes notice the train of smaller waves being pushed forward by the “main” wave? That has always fascinated me. Kind of like in the center of this picture:
wave train
When we were sailing in Ratzeburg earlier this year, one day there were hardly any wind waves on the lake, so putting a foot in the water from a sailing boat resulted in exactly the phenomenon that had puzzled me for so long.
In the movie below you see it “occurring naturally” and then afterwards “created” like in the picture above. I’m pretty sure it’s the “group velocity is only half the phase velocity” thing, with small waves passing forwards through the group and vanishing, only to be replaced by waves coming from the back of the group. Is this what is happening here? Anyone?
And really high order ones, too!
On Monday, I showed you a movie on wave generation in Hamburg Ship Model Basin (HSVA)’s wave tank. At the end of that movie, we see that the wave energy is being dissipated by a “beach”. Well, we actually see that some of the energy is reflected in those cute little baby waves. And there is another fraction of the total energy that passes through the beach into another part of the tank. And that’s what I want to show you today.
When I’ve talked about standing waves in a tank before, that always meant the simplest form: Only one node. We have always tried to avoid higher-order modes before, partly because they are a lot more difficult to generate, at least using our method.
But here is what happened in the wave tank:
Isn’t it beautiful?
A simple visualization of two types of waves.
The FIFA world cup has been over for a while now, but I still need to share an idea I had watching one of the games when the audience got bored and started doing a wave around the stadion: this would be a great in-class demonstration of how waves do not transport matter! I usually show demos of waves travelling on ropes, but this could be much more fun – to see the shape of the wave travelling when clearly the students are not moving away from their spots.
Depending on how easy it is to calm that particular class down again you might even consider letting them do a longitudinal wave, too.
Have fun and let me know how it goes!