Tag Archives: waves

More interference of waves, this time as seen “at sea”

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“At sea” in quotes, because technically we were at the mouth of the Weser river… But still! (deutscher Text unten)

The really exciting thing working with the kids here at the JuniorAkademie is that they are really good at transferring things that we talk about theoretically to what they see in a tank experiment, and even to what they see on the “real ocean”.

The day before we went on the student cruise, we did the wave interference experiments described here and here. But then “at sea” they saw a situation similarly to the one I filmed and posted below and they got so excited to see the same phenomena for real. One kid said that before, he couldn’t see the waves for the wave (alluding to not being able to see the wood for the trees), but that it was so cool to look at the water and see so much physics. Those are the moments we teach for, aren’t they? :-)

Das wirklich spannende daran, hier bei der JuniorAkademie zu unterrichten ist, dass die Jugendlichen sehr gut darin sind, das theoretisch erlernte Wissen (wobei wir uns sehr bemühen, nicht über Theorie an die Dinge heran zu gehen!) auf praktische Situationen zu übertragen und die Phänomene wieder zu erkennen – sowohl im Tank als auch “auf See”. “Auf See” in Anführungszeichen, weil die Wesermündung natürlich noch nicht so richtig zur See zählt, aber immerhin.

Am Tag vor der Ausfahrt haben wir die Interferenzexperimente gemacht, die ich hier und hier beschrieben hatte. Aber dann “auf See” waren mehrere Jugendliche wirklich begeistert, als sie eine Situation beobachtet haben die so ähnlich war wie die, die ich gefilmt habe (Siehe Film unten). Ein Teilnehmer sagte, dass er vor unserem Workshop “die Wellen vor Wellen nicht” gesehen habe, in Anlehnung an “den Wald vor Bäumen nicht sehen”. Jetzt aber sei er total begeistert und fasziniert davon, aufs Wasser zu gucken und so viel Physik zu sehen. Das ist ja eigentlich das beste Kompliment, das wir als Lehrer bekommen können.

[vimeo 104339641]

Wave interference in a tank

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Creating waves and watching them interfere. (deutscher Text unten)

You might not have guessed it from reading about our waves meeting over a sandbank experiment, but we weren’t doing in purely for its entertainment value. Our goal was to see how waves interfere, because the theory of interfering waves seems to be counter-intuitive in some cases. A second experiment we have been doing on this topic is shown below. We create waves by dripping water drops on the water surface and film (and in some cases also watch) from below. Movie at the end of this post!

Obwohl es sicherlich nicht danach aussah, haben wir das  Experiment mit den Wellen auf der Sandbank nicht nur aus Spaß veranstaltet, sondern durchaus mit einem wissenschaftlichen Hintergrund: Wir wollten uns ansehen, wie sich mehrere Wellen überlagern.

Von oben werden Wassertropfen in den Tank getropft, das daraus entstehende Wellenfeld wird von unten gefilmt (und in einigen Fällen auch beobachtet).

 

Waves and sandbanks

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Creating waves in a tank. (deutscher Text unten)

In the big tank, we have a paddle that is really good for making big waves. And if you create them in just the right way, they reflect at the back of the tank to meet up with the incoming waves right above the “sandbank” we put in (that’s what the label is you see in the movie below: it says “this is a sandbank”).

Im großen Wellentank können wir mit einem Paddel Wellen erzeugen. Wenn man das genau richtig macht, treffen sich die primären und die reflektierten Wellen direkt über der Sandbank und es spritzt schön. Viel Spaß beim Anschauen des Films!

Hair-dryer driven surface waves

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Looking at wave length, frequency and speed. (deutscher Text unten)

The wave group played with a tank and a hair dryer (the hair dryer safely away from the water, obviously) and different modes of recording. high definition, slow motion and what have you. They also did a really cool data-model comparison, which is still top-secret, but we might reveal it tomorrow. Stay tuned!

Die Wellengruppe hat mit einem Fön (der natürlich in sicherer Entfernung vom Tank war!) Wellen erzeugt und sich die Wellenlänge, Frequenz und Geschwindigkeit angesehen. Sie haben außerdem mit unterschiedlichen Modi der Kamera gespielt: High Definition, Zeitlupe und noch mehr. Sie arbeiten außerdem an einem geheimen Daten-Modellvergleich, von dem wir wahrscheinlich morgen schon berichten werden. Stay tuned!

Standing waves

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Standing waves caused by rocks in a current.

I am incredibly fascinated by standing waves.

Screen shot 2014-04-20 at 4.04.06 PM

Standing waves.

The standing waves are caused by rocks sitting in a current. From the pictures below it is not really clear where those rocks are situated, whether they are upstream of all this wave action or in the focal point of the wave fronts.

Screen shot 2014-04-20 at 4.03.53 PM

More standing waves.

Having stood there with my mom for quite some time the other weekend, just watching the water, I can tell you that it’s the upstream obstacle. You can see for yourself here:

What you also see in that video is that not all of the waves are, in fact, standing waves. The lower-amplitude waves to the left on both the image above and below are not – they are radiating away from some obstacle.

Screen shot 2014-04-20 at 4.03.31 PM

More standing waves.

Just from looking at that image it is clear that the bathymetry is very irregular and that the current speed is quite inhomogeneous, too. So maybe it is not surprising that the condition for a standing wave – that the current speed and the wave speed are the same, but going in opposite directions – is not met everywhere. Particularly, in many cases it is hypercritical and the waves are just flushed away. Note the current speed in the video below.

And all of this action is happening on an exciting river called … wait for it … Pinnau. In Mölln. And this is what it looks like to most people: Tiny little rapids somewhere in a forest.

 

Pinnau

P.S.: I just realized that when I’ve talked about standing waves before on this blog, I’ve always talked about the see-sawing kind. When obviously this kind is so much cooler!

Reading the water

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Just because it’s fun! :-)

I’ve mentioned before that I tend to stare at water when nobody else seems to find anything interesting to look at. So just because I’m weird, let’s look at some more water.

For example here. What could have caused waves like those below?

ruderwelle

What could have caused this pattern?

Yes. These guys went past and what we see are both the circular waves caused by the oars and the stern wave of the boat.

ruderwelle2

Rowing boat. Seriously, why would anyone want to go backward all the time???

Ok. So on to the next riddle: What could cause what we see below?

alsterdampferwelle

Bubbles on water. What could have caused them?

Right, that was him:

alsterdampferwelle2

Alsterdampfer!

And this?

alsterdampferwelle3

More waves.

Yes! Him again!

alsterdampferwelle4

Alsterdampfer.

Does anyone see where we are going with this?

Correct. Here.

Screen shot 2014-04-12 at 9.38.08 PM

Hamburg town hall.

And a last glimpse on the way back:

Screen shot 2014-04-12 at 9.39.42 PM

Lombardsbrücke.

Isn’t this the most beautiful city in the whole wide world? :-)

Wind waves meet current

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Wind waves on one side of the current – no waves on the other.

Recently in Bergen, I was walking to meet up with a friend at the kayak club, and I had to cross a bridge that has always fascinated me. Underneath the bridge, there is only a very narrow opening connecting basically the ocean on one side and a small bay on the other side. On this part of the Norwegian coast, the tidal range is easily of the order of a meter, so this narrow opening under the bridge makes for some pretty strong currents. In fact, when paddling through that opening, when the tide is right you can really see how the surface elevation changes from one side of the bridge to the other.

So when I was walking there recently, this is what I saw:

Strong current from the lower left to the upper right of the picture, wind blowing from the right, hence waves on the right side of the current and no waves on the left side.

This might be difficult to see on this picture, but there is a strong current going from the lower left corner of the picture towards the upper right. And on the right side of that current there are a lot of wind waves. But on the left side there are hardly any, even though there is nothing blocking the wind, just the current blocking the propagation of waves. Wind is coming from the right here.

I found it really fascinating how this current acted as a barrier to the waves and stood a couple of minutes watching. A couple of people stopped and looked, too, but didn’t find anything interesting to see and were slightly puzzled. But what I see is fetch (or that there isn’t enough of it on the left side of the current) and hydraulic jumps (or that the current is clearly going faster than the waves are). Which means that I start wondering how fast that current would have to be in order to stop waves from propagating across. Which then means I start estimating the wave lengths in oder to estimate the waves’ velocities to answer the previous question. So that’s reason enough to stand there for quite some time, just watching, right?

Rogue waves in a bath tub

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Trying to create rogue waves in the bath tub of the infamous “red house”.

As a part of their projects, students in the CMM31 in Isafjördur course had to conduct an experiment, document and interpret it. One of the students, Silvia, chose to create rogue waves in the bath tub of the “red house”, one of the student houses, and I was invited to participate and eat delicious cupcakes.

Since rogue waves can have devastating effects on ships they encounter, clearly we had to have a ship. None were to be found, so we had to make our own.

Since most studies of rogue waves in wave tanks had a hard time actually producing the waves (and a bathtub might not be the most ideal setup) we did not have high hopes that our experiment would be successful. And we did not manage to produce rogue waves in the strict sense – but we managed to avoid major spillage of the tub and still sink a couple of the paper boats, so at least we were getting some results.

Great to see students do experiments on a Sunday afternoon!

Ship-generated internal waves

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A tank experiment showing ship-generated internal waves.

When entering a fjord from the open ocean by ship, it can sometimes be noted that the speed of the ship changes even though apparently nothing else changed – the wind didn’t change, the position of the sails didn’t change, the settings on the engine didn’t change – whatever was driving the ship didn’t change. And yet, the ship slowed down. How can that be?

According to the legend (that I like to propagate in my classes), when this phenomenon was first noticed, people attributed it to sea monsters latching onto the ship and slowing it down. Or if not monsters, than at least mollusks and other not-quite mostery monsters. But then Bjerknes came along and, together with Ekman, set up experiments that explain what is taking all the energy away from propulsion. I’ll give you a hint:

Yes – the ship excites internal waves at a density interface. Since the stratification in a fjord is much stronger than in the ocean, driving into a fjord means loosing a lot more energy towards the generation of internal waves.

See the movie here:

Langmuir circulation, take 2

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Attempt at mechanistic understanding of Langmuir circulation.

After  complaining about how I didn’t have mechanistic understanding of Langmuir circulation recently, and how I was too lazy to do a real literature search on it, my friend Kristin sent me a paper that might shed light on the issue. And it did! So here is what I think I understand (and please feel free to jump in and comment if you have a better explanation).

First, let’s recap what we are talking about. My friend Leela (and it was so nice to have her visit!!!) and I observed this:

Long rows of foam on the surface of the fjord, more or less aligned with the direction of the wind (we couldn’t tell for sure since we were on a moving boat, and since it was a tourist cruise we couldn’t ask them to stand still for a minute to satisfy our oceanographic curiosity). Foam is – and so much makes sense – accumulated in regions of surface convergence.

But let’s see. The explanation that Kristin forwarded me is from the paper “Upper ocean mixing” by J.N. Moum and W.D. Smyth for Academic Press Encyclopedia of Ocean Sciences, 2000. According to my understanding of their paper and others, Langmuir circulation is related to Stokes drift.

Stokes drift is the small current in the direction of wave propagation that is caused by orbital wave motions not being completely closed (even though they are as a first order explanation, and that’s what you always learn when you think about rubber ducks not being laterally moved by waves).

As the wave orbital motions decrease with depth, there is a shear in the Stokes drift, with strongest velocities being found at the surface. At the same time, if there are small disturbances in the wind field, there are small inhomogeneities in the resulting surface current, hence shear that generates vertical vorticity.

The combination of horizontal and vertical vorticity causes counterrotating vortices at the ocean surface. The convergences between two adjacent rows concentrate the wind-driven surface current into a jet at the convergence, hence providing a positive feedback.

Voila: Stokes drift!