Category Archives: observation

Refraction of waves

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I remember being on a looooong walk on some Danish dike when my sister was small and really didn’t want to walk any more, telling her about how phase velocity of shallow water waves depended on water depth and how you could observe that when waves are refracted towards the coast (assuming the sea floor has the right slope). And whenever I see this happening now I have to think of that freezing cold and windy day a long time ago.

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Wave fronts turning towards the shore

Watch how the angle of the wave fronts changes as they come closer to the shore:

 

Reading the water

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As you might know, I really enjoy reading the water – watching the water trying to figure out what processes caused the patterns I see. So here are two more movies from my recent Birthday trip.

First, look at the Este and tell me: Which way does the water go?

And then a second look at the Este shortly before it flows into the Elbe. Watch the oscillating flow. Can you guess what’s going on underneath the surface?

What you see is one of the two Este flood barriers.

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The other one, by the way, has an awesome flap bridge, that happened to open right when we arrived there, so I jumped out of the car to watch:

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Oh, and it will be a great year for pears :-)Screen shot 2015-05-15 at 6.34.08 PM

Thank you, Archimedes!

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I really like hydrostatics. Of course I like moving water even better, but even static water is great. And there are so many things to explore! If I was to teach hydrostatics any time soon, there are so many little teasers I would use.

For example this one:

A sailor is standing on the bottom step of a rope ladder, painting the outside of his ship. The bottom step is 50 cm above the water, the distance between steps is 30 cm. The flood is coming in, and the water is expected to rise by 1.5 m. How many steps will the sailor have to climb in order to keep his feet dry?

Or this one:

How much heavier will a trough in a ship lift get when a ship is inside?

A: the weight of the ship
B: the weight of all parts of the ship above the water line
C: not at all
D: I don’t know*

You might think that these are really easy questions, but then you might be surprised! Funnily enough I drafted this post weeks ago, and then last week a colleague of mine talked about how this was a really difficult question, so I had to post it now ;-)

Another question that he mentioned that students found really difficult is similar to this one:

If an anchor is dropped from a boat into a pond, what will happen to the water level?

A: It will rise
B: It will sink
C: Nothing
D: I don’t know

Answer to that one in this post…

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*Remember why we always include the “I don’t know” option? If not, check out some more posts on multiple choice questions under the MCQ-tag!

Ship lift Scharnebeck

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Today I’m going to share a long movie with you, but I’m planning to talk about ship lifts in more detail soon. But just how awesome is it that they can lift ships (SHIPS!) 40 meters up just like that? Each of the troughs carries 5,800 t water. You see the counterweights move when the troughs move, and it is totally fascinating.

I went there with my parents, but this is pretty much all they saw of me for a very long time :-) Weirdly enough I was the only person standing right at the railing. Well, maybe not so weird considering how wet I got. But you’ve gotta do what you’ve gotta do and I definitely enjoyed watching!

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Fascination.

The movie below is sped up by a factor 3.5 because there was so much footage that I wanted to show…

Waves radiating from an object

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In the last post, I showed you flow separation on a pylon in Elbe river. Remember?

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Flow separation at a pylon in Elbe river

Today, we are back at the same pylon, only that this time the tidal current is a lot less strong, but there is a lot of wind, so our focus is on wind-generated waves.

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Waves running towards the pylon and radiating radially away from the obstacle.

It might be admittedly a bit hard to see, but if you watch closely and use your imagination, you might be able to see the waves propagating towards the pylon and then being reflected and radiating radially outward from where they hit the pylon. Pretty fascinating!

Can you see the locally generated waves to the left of the pylon? All those tiny waves where the wind is funneled around the pylon?

 

Flow separation

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On the way to the pool I cross over the Elbe river on this pretty bridge.

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Which is pretty spectacular, just because the structure itself is so amazing.

IMG_0901 IMG_0906 IMG_0923But what is even more spectacular is how every time I am there I see new things in the flow field. And the example I want to show to you today is the flow field around one of the pylons of the bridge that runs in parallel to the one I am on.

In the movie below you see a classical flow separation, similar to what might happen at an airplane’s wing. The water flowing towards you under the bridge arrives (pretty much) laminar, but then on contact with the pylon turbulence develops, eddies form and the flow separates from the boundary of the obstacle. Nice! :-)

 

Eddy generation and optics.

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Eddies. Dips in the surface and shadows on the ground.

I always get really fascinated by watching how eddies are generated by obstacles in a fluid. But it is especially exciting when you don’t only see the eddies because you see how they deform the surface, but when the water is clear enough so you can see the “shadows” on the ground!

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Of course, the dark spots you see aren’t shadows, strictly speaking. As light enters the water from the air, it is being refracted. And since the eddies’ surface imprints are dips in the surface, light is being refracted away from the perpendicular, leading to a less-well lit area – the dark spots.

But isn’t it fascinating to watch how eddies form when the water passes the stick and stones in the water when there is absolutely nothing going on upstream?

Waves on a current

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When you throw a stick in the water and the waves don’t form circles.

Throwing something in the water usually results in waves traveling out in circles from the point of impact. But if you throw your stick into a current, the waves get distorted. Watch the movie below!

Slightly confusing that the stick drifts away, too, so that it doesn’t mark the center of the circle. But still it is clear that waves travel a lot faster downstream than upstream – at least relative to the whole system, not the water ;-)

Currents caused by thrusters

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Or: fast inflow into nearly stagnant water body

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.

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Currents caused by thrusters of a harbor ferry in the port of Hamburg

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 :-)

Thermoplastic ketchup bottles

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After all those text-heavy posts this week, here is something that really fascinated me recently. I had an empty ketchup bottle lying in my sink when I strained some pasta. Some time later I picked up the bottle and noticed how the label was all wrinkly. So this is how much a ketchup bottle shrinks if you pour hot water over it:

IMG_0829And funnily enough, the thread didn’t shrink and the lid still fits perfectly.