Category Archives: observation

More wave-spotting in “urban environments”

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I recently started looking at waves in “urban environments” (in contrast to “on the sea”) with a new found fascination. The reason why will be revealed soon, but for now just know that there are more waves coming up on this blog!

Today, let’s start by looking at more waves on Store Lungegårdsvannet, like we did before.

Here, you look downwind and see the flat water right in front of you, shaded from the wind by the walls around the lake. And then the further away you look, the larger the waves grow.

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Another very funny picture of a similar situation below: See how parts of the lake’s surface reflect the buildings and mountains and clouds really well (since that part of the surface is really flat), whereas other parts are way too choppy and appear a lot greyer on the picture?

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Yes, I admit, the purpose of this blog post was not so much to talk about waves as to show you how beautiful Bergen is in May. I miss this city… And my AMAZING Bergen friends!!! <3

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Walking around a lake to look at waves from all sides

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The most awesome thing about being on vacation is that I have the time to stare at water as much as I like.

For example the other day, I walked around Lille Lungegårdsvannet on a windy day.

Looking downwind, one sees a very smooth surface right in front of us, and then waves start developing further away. Looking at the fountain, you see that it is actually pretty windy.

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Lille Lungegårdsvannet in Bergen

Walking a quarter of the way around the lake, we now look at the fountain at a 90 degree angle to the wind: it is blown over to the right. We now see wave crests traveling and see the shape of the waves much more clearly.

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Lille Lungegårdsvannet in Bergen

Walking further, we see the waves coming directly towards us; the fountain is also blown in our direction. All of a sudden the water looks a lot more rough. And of course it feels a lot more windy, too, when the wind is coming right towards us and not in our back.

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Lille Lungegårdsvannet in Bergen

And bonus picture: A rainbow in the fountain when we’ve gone 3/4 of the way around the lake. Beautiful day in a beautiful city!

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Lille Lungegårdsvannet in Bergen

 

 

Ships sailing through trees

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Ships sailing through trees? What?

The other day I went on a trip to Husum with my sister and her family. While walking along the sea, we saw the weirdest thing: Birch trees growing in the middle of the water! From their positions it was clear that they were some kind of marker for the waterway, but it looked very strange. But google suggests that this kind of marker is very common in wadden seas, where the water is too shallow for traditional buoys.

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“Pricke” – birch trees serving as markers for the water way

When coming from the sea, you’ll see the birch trees on the port side of the waterway, and on the starboard side there will be poles with branches which are tied together on the very top of the pole, branching out below. Apparently this is called “Pricke” in german. You live and learn! :-)

Diffraction and reflection of waves

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Last night, we saw really nice wave phenomena on the Schlei in Schleswig.

Do you see the waves being diffracted by the pier in the picture below?

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Waves are coming in from the right (see the three lines on the right in the picture below) and at the head of the pier they get bent around (all other lines).

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Now look at the line on the very left. What happens where that wave hits the pier?

This.

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Awesome criss-crossing of wave crests!

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An annotated picture of what happens below: The red lines show the incoming original wave crests, and the green lines show the wave crests of the wave that got reflected by the shore.

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If all those lines are a tad confusing, thankfully a ducky in a fairly wave-less spot made a single wake which also got reflected on the sea wall:

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Same picture as above, this time with the original wake marked in red, and the reflected wake marked in green:

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Nice evening, isn’t it?

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And since everybody else is asleep, I put together some short video clips into a movie for you:

What I learned from the movie-making? I need to take longer footage and practice my editing-skills! :-)

Foam stripes on the water.

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Sometimes you need to look at the bigger picture to understand what is going on, especially when looking at phenomena on the water.

My dad recently sent me the images below from Schleswig: Weird foamy stripes on the water.

They don’t really make a lot of sense until you look at it from a different angle:

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Now you see how the foam is forming in the waves all over, but that only some of that foam makes it through the gaps in that floating pier, forming a stripe behind every single gap. Cool, isn’t it?

What I found also really interesting in one of the pictures was this:

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The different wave fields upwind and downwind from the pontoons. On the upwind side (right side of the picture above), you see really choppy water. On the downwind side, though, close to the pontoons, the water is pretty calm, and only with increasing distance from the pontoons waves start to build again. And we can see that the waves at the far left of that picture are still a lot smaller than those coming in on the right side, just right of the pontoons!

Rain on water — why does each raindrop cause several concentric waves?

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When we watch rain falling on a water surface, we observe that each raindrop causes several concentric waves with different radii. In my post on Tuesday I just stated that that was what we observe, but today I want to look into the explanation.

This is what it looks like when it rains on a water surface. Not much surprise here!

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But when I was visiting my parents last weekend, it started to rain with nice and heavy drops that were few and far between. So I saw my chance, grabbed my camera and ran outside to try and capture exactly what happens when a rain drop hits the water surface. Not an easy task, since everything happens very fast and it’s impossible to anticipate where the next drop will fall, so I had to rely on my camera’s auto focus and just press the trigger as often as possible. And guess what? It stopped raining within a minute! How annoying is that?

But I still managed to capture enough pictures to show you what I wanted to show (see image below):

First, a raindrop just causes a dent in the surface, starting the first circular wave. But if the raindrop was sufficiently large and fast, the surface will bounce back, throwing a secondary (and sometimes tertiary) droplet up into the air. Those droplets will fall in the same spot as the first one, causing the smaller waves.

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Isn’t this amazing? I’ll definitely work on better pictures in the future, but I am not sure it can be done with my camera.

[Edit 20.4.2016, 12:24. We don’t actually need the secondary and/or tertiary droplets, as Martin pointed out. It is sufficient that the surface gets deformed by the first rain drop, then bounces back and overshoots. When the water that overshot falls back down, this has the same effect as a secondary droplet: to cause a new circular wave just inside of the first one. And of course, the overshooting and triggering of new waves can happen several times, depending on the impact of the initial drop. In a way, my secondary / tertiary drops are just the extreme case of this more moderate version of wave formation.]

To wrap up this post — a bonus picture: Four stages of wave development all captured in one (lucky) shot!

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Are you looking forward to the next rainy day now because then you can go outside and observe all this cool stuff?

Taking the hydrostatic paradox to the next (water) level

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How well do people understand hydrostatics? I am preparing a workshop for tomorrow night and I am getting very bored by the questions that I have been using to introduce clickers for quite a lot of workshops now. So I decided to use the hydrostatic paradox this time around.

The first question is the standard one: If you have a U-tube and water level is given on one side, then what is the water level like on the other side? We all know the typical student answer (that typically 25% of the students are convinced of!): On the wider side the water level has to be lower since a larger volume of water is heavier than the smaller volume on the other side.

Clearly, this is not the case:

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However, what happens if you use that fat separator jug the way it was intended to be used and fill it with two layers of different density (which is really what it is intended for: to separate fat from gravy! Your classical 2-layer system)?

Turns out that now the two water levels in the main body of the jug and in the spout are not the same any more: Since we filled the dense water in through the spout, the spout is filled with dense water, as is the bottom part of the jug. Only the upper part of the jug now contains fresh water.

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The difference in height is only maybe a millimetre, but it is there, and it is clearly visible:

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Water level 1 (red line) is the “main” water level, water level 2 (green line) is the water level in the spout and clearly different from 1, and water level 3 is the density interface.

We’ll see how well they’ll do tomorrow when I only give them levels 1 and 3, and ask them to put level 2 in. Obviously we are taking the hydrostatic paradox to the next (water) level here! :-)

A string of bubbles

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Have you ever noticed champagne bubbles that form as a string right in the middle of the glass and hardly anywhere else? This leads to the very cool pattern you see here:

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Astrid and I recently happened to notice how differently bubbles in champagne and in mineral water behaved. In the mineral water, bubbles formed in random spots along the sides of the glass. In the champagne, they mainly formed in the middle; and formed a string of rapidly forming bubbles.

So now I was hoping for a really interesting explanation of why the bubbles behave so differently. They form at different rates, but that makes sense if the partial pressure of CO2 in both drinks is different. After a bit of research on the web it turns out that fancy champagne glasses have tiny scratches right in the center of the glass to serve as condensation nuclei — in other words: to cause exactly what we observed: A nice string of pearls instead of bubbles forming randomly along the sides of the glass. So theoretically, if we had had our mineral water from the same glasses, we would have observed the same thing in mineral water. What a disappointing explanation!

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Refraction of light in water

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How deep is the pool? Really looks like it is super flat, doesn’t it? It’s not, it’s almost shoulder-deep (if you aren’t taller than me ;-))

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Even though I’ve known about the effect since I was a small child and we did this kind of experiments, I still find it super fascinating to see a pool like that one, climb in and confirm that both ends are, in fact, the same depth. Don’t you?