Wave watching Sunday in Grenoble

Today I went on a wave-hunt expedition to take pictures for posts on the Froude and Reynolds number over at Elin & team’s blog (which you should totally check out if you haven’t done that yet! I am actually proof-reading my posts there and that is saying something ;-))

Anyway. Let’s look at the picture below. Do you see how there are two qualitatively different flow regimes in the Isère? Closer to the banks, you see waves that look like normal waves, happily propagating wherever they want to. And towards the middle of the river, you see that there is a lot of turbulence, but disturbances don’t propagate wherever they want, they are being flushed downstream.

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For comparison below a picture of a part of the Isère where it is turbulent all the way to the sides:

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And below a nice example of how phase velocity of waves depends on wave length. See all the small, choppy stuff being flushed downstream and then standing waves caused by some obstacle in the middle of the river? That’s because the longer the wavelength, the faster the wave propagates (assuming that we are in deep water, which I think is a safe assumption in this case). So the river is so fast that the slower waves get flushed away and only waves of the length of those created by the obstacle (or longer) can stay in one place (or even propagate against the current). I think that’s pretty cool.

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Below is one of my favourite wave-watching sights: A half slit.

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And in the picture below, we can kinda see vortices detaching behind the obstacle (or is that just me)?

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And what I really liked: see the spot below where there are all of a sudden standing waves appearing in the middle of the river? Clearly there is a sill below, but I like that you cannot see the obstacle, just deduce that it must be there from how the waves look :-)

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It’s not a hardship to be here, I can tell you ;-)

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It is quite a beautiful place! And, by the way, this is my 600th blog post on this blog. Can you believe this?

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Total reflection and fishies

Do you know the phenomenon that once you start noticing something, you see it everywhere? That’s been the case with me and total internal reflection. Not quite as impressive as last time, but still there:

And what I found really interesting this time: a swarm of tiny fishies making wave rings! I only noticed them because of those tiny waves. And if you look closely you can see so many of them just below the surface right where the wave rings are!

So funny to see the water almost boiling with fish on such a calm morning.

And another thing that fascinated me: how it’s so much easier to see into the water in places that are shaded (or dark) from the reflection of that pier. Not quite sure yet why it’s so much easier to see here, maybe just because there isn’t any glare? Any ideas?

Total internal reflection

Have you ever wondered why at some angles the sea looks blue (or whatever the color of the sky that day) and at others you can actually look into the water? That’s the phenomenon of total internal reflection. There is a critical angle at which you switch from “being able to look into water” to “total internal reflection”, i.e. the sky being reflected off the water’s surface and reaching your eye. Below you see a nice example of this: The more perpendicular you look at the water surface (i.e. those sides of the wave facing you), the better you can look into the water. Whereas all those parts of the sea surface that face away from you look blue and you can’t look into the water there.

I think this is totally fascinating! Don’t those pictures look almost fake?

And, btw, this doesn’t only happen if you look in parallel to the direction of wave propagation. Although it looks even weirder at an angle:

Can you see how all those tiny ripples on the wave each show the same phenomenon of either reflecting the sky or being transparent and showing the sea floor underneath? How cool is that? :-)

Amplification of waves when they run into shallower water

It’s one thing to know that waves build up as they run from the open ocean over the shelf onto a beach, and that they build up as the water gets shallower and shallower. But we are so used to seeing exactly that (because that’s what the sea is supposed to look like!) that we don’t really notice it any more, at least most of the time. But below is an interesting case: Waves are running onto the beach, but there is also a headland going out into the sea. So in addition to running onto a beach, there is also a depth gradient along each wave crest (do you know what I mean?).

Watch the movie below to see that there are hardly any waves visible when you look at the open water on the left, but that the crests and troughs become clearly visible as soon as you are close enough to the headland where the water is shallow enough: One and the same wave crest builds up a lot more the closer you look to the headland than it does in open water.

Awesome interference pattern in a Norwegian fjord

“Hei Mirjam.

Bergen had it’s two days of allocated summer during the weekend of 22 – 23 July 2017 and Elsa and I decided to – in true Norwegian style – take advantage of the rare occasion and go for a hike. A colleague of mine has a “hytte” near Langhelle and had invited us over for the day. So we each packed our “matpakke”, hiking boots and got on the train from Bergen to Vaksdal, where my colleague had arranged to pick us up.

Anyway, long story short, apart from the spectacular view over Sørfjorden, I thought that the following would make you smile. Pointed it out to Elsa and, as if on cue, in unison we said your name out loud.

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I’m afraid the resolution is not that great though – had to zoom quite a bit to capture what was much more clearly visible with the naked eye.

I’m including a map to show where it is. The arrow indicates more or less where we were standing when I took the picture; the circle around the area. Opposite Vaksdal, on the western bank (does a fjord have a “bank”? What’s the correct term? “Wall”?) of Sørfjorden is Olsneset and the little isle, Olsnesøyna, you see in the pic. There’s apparently an “open air” prison on the island. Not a bad place to be incarcerated!

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One of the wave trains was made by the little ferry that runs to and fro between Vaksdal, Olsnesøyna and Osterøy.

I’m sure that the readers of your blog would also enjoy the pic, so please feel free to use it.”

Wow! Thanks, Pierre! :-)

I obviously love it when my friends think of me, but it makes me even more excited when they think of me in connection to cool stuff related to water and send me pictures. But clearly the first thing I had to do upon receiving this email was to try and interpret the picture.

So I know there were two ships causing the waves. But which way were they going? So my first guess was two ships going in opposite directions. I’ve drawn the edges of their wakes into the picture below (ship 1 green, ship 2 red), the ships would now be more or less at the pointy end of each of the Vs.

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But then I noticed the waves that I drew in blue in the picture below. Could they be part of the wake if a ship? And could that white spot in the picture actually be said ship? Then ship 1 would actually be going in the opposite direction of what I first thought. So one side of the wake would be what I have indicated in red below, and that side I can actually see in the picture (and I am fairly confident now that that’s the correct interpretation, judging from the shape of the feathery winglets). The green second part of the wake is just my guess of where it would have to be if my idea of where the ship is is correct.

Ship 2 (now shown in yellow) is still going the way I thought it was. Phew ;-)

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But there is one part of the picture that I think is especially cool: The actual interference part where parallel wave crests seem to appear out of nowhere (crests marked in red below, troughs in blue). This is a possible mechanism for the creation of those parallel wave crests marked in blue above, too, but I don’t think that that’s what had happened there. But I am confident that that is what happened for those waves marked below.

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Now it’s your turn, Elsa and Pierre. Do you remember what was going on? How well am I doing interpreting waves? ;-)

This is SO MUCH HARDER than seeing stuff in pictures I took myself and remember the situation! You poor guys always seeing my pictures without good explanations of what is going on on them. I think I might have learned my lesson here…

Feathery wavelets and wakes

My sister took this amazing picture — and from a train no less! And I got super excited. Can you see the feathery wavelets* of the bow wave of that large ship? And then the wakes of both ships, spreading out at the same angle? I should definitely start spending time on high bridges going across canals, there is so much unused potential for wave photography!

*I was super convinced at first that they were called “winged wavelets”. But then doubt started to kick in, so I asked google. Turns out they are called “feathery”. However, apparently “winged wavelets” is a very poetical expression, all google hits are in some poem or other! And one that I really liked by Mary Bamburg, where it goes like this (as part of a longer poem):

“… waves wring sand from the shore,
strew shells, strech after them
white wash and wild winged wavelets
glass green, blaze blue, slick silver …”

Does it create the same beautiful image before your inner eye as it does for me?

More than 2/3rds need to be water

I’m in this weird phase right now: In landscape pictures that I take, I like more than 2/3rds of the total area to be of water. That I like water shouldn’t really come as a surprise to anyone, but this perspective — looking from right above the water level towards the horizon — is new to me and I am totally into it. Maybe because I’m swimming almost every morning these days?

Anyway, here is a bunch of pictures taken in Ratzeburg recently when I helped out in their sailing school (ratzeburger-segelschule.de — totally recommended! Clearly awesome instructors there ;-))

And obviously the first one of the bunch had to deviate from my 2/3rds-water rule…

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