The mystery of “51”, or: Tidal current at Straume Bru

My friend Pierre, who I went to Saltstraumen with in 2012, wrote me a text about a year ago and asked me to remind him to tell me about 51 next time we met. We met and, as we do, geeked out about some hydrodynamics stuff. And he told me about 51. It turns out that on his way to work, Pierre crosses Straume Bru on the 51 almost daily, and watches the strong tidal currents and whirlpools that form there. But it wasn’t until a couple of days ago that we managed to take bus no 51 to this specific spot — Straume Bru — and geek out about it together!

I got there about an hour before Pierre did. Not because he was late, but because I was so excited to finally see that tidal current I had heard so much about! And also because I had looked at the tidal forecast and wanted to make sure I would see the strongest current and not miss it by an hour.

Turns out that when I arrived, the current was indeed quite strong. But look at the water level relative to the structures. Pretty much half time between high water and low water, even though the current is strong, there is still a lot of water left in the reservoir!

I love watching the waves radiating from the edge in that wall, and the wedge of eddies that is separating the fast flow from the boundary.

Also look at how the waves are being deformed by both the eddy and the current in the picture below! Especially in the top left corner, where the “wake” of the edge follows the meanders.

As had been forecasted, it started to rain while I was wave watching. With the surface a little rougher now, I noticed these two long streaks. Not sure what was going on there?

It didn’t look like oil or any surface film, but I can’t really think of anything else right now. I was briefly considering Langmuir circulation, but I don’t think the wind was steady enough and also I don’t know if that would combine well with a strong current. Any ideas, anyone?

In any case, the stripes weren’t visible any more when the rain stopped. But look at all these amazing waves!

And, now looking downstream, some more eddies and whirlpools!

Looking upstream towards the bridge, we see the glossy V that is formed upstream of where the wakes meet that are formed by the walls on either sides of the outlet.

And downstream again — how awesome and cute are those little eddies? And how amazing is it that they can persist over long distances while maintaining a dip in the surface that is probably as long as my thumb?

And not two seconds look the same!

Below is a closer look of the two wakes of the sides of the outlet coming together and overlapping.

And here is another picture of the wedge of eddies that forms, separating the strong current from the more stagnant, turbulent boundary layer. Look at how irregular the wedge is, formed of eddies of different sizes that are being advected downstream! And also look at the waves that are being pulled with the current, leading to stripes along the current!

Here is another look at the wedge and the stripes of the waves that are being deformed by the current.

And another one, because the scenery is actually really pretty, too! Which I hardly noticed until I had taken about 500 pictures of the water ;-)

More eddies in the wedge.

Interesting how one side of the outlet forms a wedge while the other “just” forms a wake, isn’t it? I think it’s because the the left one, the one with the wedge, restricts the current a lot more.

And here is a new perspective: Looking at the wedge of a second, parallel outlet. You see really well how the boundary layers from both sides come together!

Looking upstream, the standing waves in the foreground give you an idea of how strong the current is!

And another beautiful wedge!

And more turbulence looking downstream. Funny how parts of the surface look so glossy and smooth, isn’t it? I think those are the areas outside of the current that aren’t turbulent.

Now a final excursion to the other side of the bridge, to look at the wakes of the structures. Notice how much less water there is now!

And here is the upstream part of the V.

And the beginning of the wedge.

Walking a little around the corner, we see that the wake begins already upstream of the corner!

And a final look at the wake.

This is how happy wave watching makes me, even when it’s cooold and raining! At least occasionally, not the whole 1.5 hours I was there… The rain, I mean. Happy I was all the time :-)

Two days later, I actually took the bus across the bridge (after another adventure with Pierre, more on that soon!) and the current went the other way! As it should, but it’s always nice to confirm theory.

Anyone taking this bus regularly? You should start taking pictures for a time series! :-)

#wavewatching: Standing waves on coffee on a train!

When I wrote the blog post on “wave watching in a bucket” a couple of days ago, it strongly reminded me of a movie I had filmed already back in March 2018. I was sitting on a train, still inside the train station, and noticed the pattern in my mug (also I just had gotten my awesome lighthouse thermos, hence the awkward angle of the camera).

The train is vibrating, and that vibration makes standing, concentric waves appear and disappear.

I noticed the same pattern on the lady-next-to-me’s coke zero on the bus yesterday, but felt weird leaning over and filming it. So I had to post the old movie instead. And also now I am wondering again what exactly determines the pattern in the standing waves that we get when vibrating buckets or cups with fluids in them…

Topographic Rossby waves in a tank

This experiment just doesn’t want to be filmed by me. Even though I spent more time on preparation of this experiment than on almost any other experiment I have ever done! I have written up the theory behind this experiment, run it with a blob of dye to visualize the wave, then with a ring of dye. But for some reason, something goes wrong every time. Like people opening the door to the lab to come and visit me just the very second I am about to put dye into the tank, resulting in me jumping and a lot of dye ending up in the wrong spots… Or the tank itself getting the hickups. Or the cameras not playing nicely if for once the experiment itself goes well.

Anyway, it is still a very cool experiment! So here are some pictures.

In all those pictures, the tank is rotating a lot more slowly than recommended in the instructions. I thought that might make it all easier to run (5rpm; dial at approximately 7 for GFI big tank, similar to Taylor column). And it looks just fine, except that the restoring force back to the middle isn’t really there (as was to be expected, since the surface is almost flat and the parabolic shape is needed for a difference in water depth).

Third attempt

Below, you see the “ridge”, a piece of hose that connects a solid cylinder in the middle of the tank to the tank’s outer wall. The tank is turning counter-clockwise.

The flow looks substantially different upstream and downstream of the ridge: Upstream, it is laminar and close to the middle cylinder. Downstream, it’s meandering (the Rossby waves!) and diffusive.

Fifth attempt (same as above)

In this experiment, the difference between the flow up- and downstream of the ridge are even more obvious. Look at those eddies!

It’s quite amazing to see how a small disturbance can make the entire system unstable.

 

Ice cubes melting in fresh water and salt water

Today we are doing the melting ice cubes experiment in fancy glasses, because Elin is giving a fancy lecture tonight: The Nansen Memorial Lecture of the Norwegian Science Academy in Oslo! Cheers!

We each had green ice cubes in our glasses, but one of our glasses contained fresh water and the other one salt water, both at room temperature. Can you figure out who got which glass?

This time lapse might give you a clue…

To read more about this experiment, check out this blog post!

Planetary Rossby waves — a side view

What I find really fascinating about the planetary Rossby wave experiment is to look from the side and see the columns develop and move around the tank.

Here is a side-view camera plus the top view, both cameras rotating with the tank. The movie is sped up 20x so in about 22 seconds, you will have a good idea of what happens:

And here is the same movie in real time. Here you can really beautifully watch the plumes of dense water sinking to the bottom while the whole column is rotating.

One thing to avoid when running this experiment: Don’t put the ice cube too close to the side of the tank, otherwise it will get stuck there. I don’t know if it was surface tension keeping it so close to the wall or if, since it couldn’t rotate, it decided not to move at all, but in any case: If the ice cube is too close to the wall, it will get stuck. In our case, the dense water then sank down in the small gap between the sloped bottom and the wall of the tank (as you see in the picture below, which is looking under the sloping bottom towards the deep end of the tank).

You still see columns forming underneath the sloping bottom, but that wasn’t quite what we were aiming to do…

 

Wave watching in a bucket

On the GEOF105 student cruise that I was lucky enough to join like I did last year, I happened to observe what you see in the picture above: Standing waves in a bucket! And this isn’t a staged photo, this is me taking a picture of a student at work.

We are looking at the bucket the students use to take surface water samples which they measure on deck. The bucket happens to stand just above the engine room. Which leads to vibrations. Which, in turn, leads to waves. Many different kinds of waves! In addition to what you see above, we find, for example, plain circular waves. They might look like they do in the picture below:

And here is a short movie of the waves, first in real time, then in slow motion.

Sometimes the circular waves also have other wave lengths.

The next pattern that develops from a monopole (like the one you see above) is my favourite: A monopole with higher order stuff developing at the edge of the bucket.

Watch the movie below to see it in motion (first at real speed, then in slow motion).

The next step, then, is water that almost looks as if it was boiling. Like so:

Here is a movie of the bucket with the “boiling” wave pattern, again in real time first and then in slow motion.

The movie below shows a close-up of some of the waves in the “boiling” state, when there was enough energy in the system to throw drops up in the air. The movie goes from real time to slow motion. Careful when you play it, I left the sound in in order to show how the frequency of the waves is the same as the frequency of the engine. (And because of the annoying sound, it doesn’t start up automatically, so you have to click to play)

Here is a movie that shows the bucket in different positions, shot continuously to show how quickly the wave pattern develop and also how close together the different spots with the different pattern are located. Thanks for playing along, Kjersti!

So clearly the location has an influence on what wave pattern develops. But what are other important factors? We tested material, shape and size of the container.

A small plastic bucket which is almost cylindrical, for example. Guess what happens?

We can get the same wave pattern as in the large bucket! The movie below shows three different wave pattern. When the frequency suddenly changes that’s because the movie is in parts played in slow motion.

As to material: It seems to be important that it’s flexible. Iron cast pans don’t work (yes, there is water in it!), neither do metal lunch boxes…

And round shapes make nicer waves. But the rectangular vanes of the surface drifters (aka paint roller trays) also make pretty pattern! But now the waves are, unsurprisingly, only parallel to the edges of the tray.

Yep, this is the kind of stuff that makes me really happy! :-)

Kiel to Bergen, the mini series. Part 16, in which I am almost home in Bergen

And now we have arrived in the part of Norway that, to me, feels like home rather than “oh wow, impressive landscape”. Even though the landscape is still impressive! But also very familiar now.

And very beautiful!

Especially with these low clouds randomly hanging out above the fjords.

And are you going to be disappointed if I tell you that I will be taking the night train on my way back home?

Kiel to Bergen, the mini series. Part 15, in which the weather gets worse but the rivers more interesting

I love the little meandering river in the picture above!

We are now approaching the west coast of Norway and all prejudices when it comes to weather over there are being confirmed. It’s grey and overcast. Still, there is a lot of cool water watching to be done on this train ride!

I think it was around this point of my 7 hour train ride, from which I sent a minutt for minutt live broadcast to my friend Kristin, that she pointed out that she had just noticed the common thread in the pictures I was sending: Water!

What a surprise… ;-)

Anyway, it’s getting darker outside, so the quality of the pictures is rapidly decreasing, but I will still show you some rapids in the rivers.

Because they just look super awesome!

And again, waves breaking upstream because the river is flowing so fast, it’s ripping their bases away from underneath them.

And some impressive gorges!

…and very low-hanging clouds.

But the landscape is a lot lusher and greener again!

And I love water falls.

And rapids.

And more water falls!

Kiel to Bergen, the mini series. Part 14, in which we are in the snowy part of Bergensbanen minutt for minutt

And now we’ve reached the heights where there is fresh snow! And also where there are wild rivers. Don’t know which I find more exciting :-)

Ok, yeah, it’s definitely the rivers that I find more exciting. Also, isn’t it funny that at first glance, the river seems to be flowing left-to-right, because that’s the direction in which the waves break? It’s flowing right-to-left, though, and the waves are breaking in an upstream direction because the water is just flowing so fast, it’s ripping the carpet out from underneath their feet, so to speak.

And now we have arrived in winter wonder land :-)

…where there is some new ice on the more sheltered parts of the lakes, too!

And now we on our way back down on the west side of the mountains. Snow only on the higher peaks, not at ground level any more.

And now it’s properly overcast and occasionally also snowing!

But let’s end this part of the journey with this beautiful outflow of the lake, and the standing waves that look as if they were braided into each other. So pretty!

Kiel to Bergen, the mini series. Part 13, in which we start Bergensbanen minutt for minutt — the wave watching edition

While I was on the train from Oslo to Bergen for almost 7 hours, I sent a lot of pictures to my friend Kristin who had expressed an interest in virtually joining the trip.

And it is a very pretty trip indeed!

Also really interesting wave watching, do you notice the white caps on the lake below? Must be quite windy here, too!

And an accidental tractor in the wave watching pic, but it looks actually quite interesting for a change!

I find it also pretty fascinating how everything looks more autumn-y as we go higher up the mountains.

And, of course, I love watching the rivers up here with all their rapids!

We are slowly approaching the tree line. See how vegetation is already very different from where we started out?

But it’s still quite windy, so lots of waves to be seen! Some even with white caps.

And here we see waves traveling around this little headland. Isn’t it fascinating to see how the wave direction is different on either side of the headland?

And some more waves changing direction after they have propagated out of the wind and around another little headland.

And some places are sheltered from the wind so that you see individual gusts of wind quite clearly!

Also — beautiful weather for the train ride! That is, at least, until…

…we are getting closer to the highest point. It’s starting to get overcast…

Oh, and there is the first snow on the mountain peaks in the back!