Tag Archives: continuity

Ships displace huge amounts of water, and it doesn’t come back immediately when they are gone!

Last weekend, while wave watching at one of my favourite spots, I observed something curious.

Look at the movie below, this is what the turbulent wake of a ship usually looks like right behind the ship: As the ship moves forward through the water, it displaces water and right behind it, it creates a hole that has to be filled in again by water from behind the ship and from the sides. So right behind a ship, water is sucked towards the ship. So far, so good.

But now look, there is a ship coming out of the locks at Kiel Holtenau.

What that means is that it is pretty much coming out of a dead end, since, in order to keep the water level inside the Kiel canal stable, the lock closes it off from Kiel fjord at all times by either set of doors.

The wake of the ship looks pretty normal so far:

But any ship displaces a huge amount of water. The one above is 115m long, 19m wide and has a drought of 5.4m (according to my favourite app). That means that it displaces almost 12000m3 of water! And this water has to come from somewhere (otherwise the ship would leave a trench in the water where it went, much like your hand leaves a trench when you pull it through sand at the beach).

But since the ship is coming out of a dead end, there is only so much water that can fill in said trench from behind and the sides. So even after the ship has sailed, there is still water moving back into the narrow entrance of the locks!

See below: (Surface) water on either side of the wake is moving to the right, driven by the wind. But in the wake itself, water is moving left, still filling in what was sucked out of the entrance of the lock! And that for quite some time after the ship is gone. At the end of the movie below, the camera turns and you can see the (white-and-green) ship sailing away just to give you an idea of how long this is after it left the locks…

Isn’t that cool?

P.S.: Yes, I though about whether there might have been water pumped out of the end of the dead end to fill up the lock chamber again, but I don’t think that’s it. Do you?

Why the Isère reminds me of a water jet pump

This is what the way to and from the 13-meter-diameter rotating tank in Grenoble looks like (and you should really visit Elin & team’s blog to learn about all the exciting stuff we are doing there!!!)

And the best part is the Isère right next to the bike path:

And one thing that I find really impressive with this river (coming from a much flatter part of the world than Grenoble, where rivers aren’t typically as fast-flowing as the Isère) is how all these return flow pools form everywhere.

Watch the movie below to spot them yourself, or my annotated picture below:


It seems really counterintuitive that a strong current would make water on it’s side flow upstream instead of flushing everything downstream or even just going downstream through stagnant water, doesn’t it? But when I thought about why that is, it reminded me of the way a water jet pump works: You flush water from a tap down through a hose, and that hose is connected with another hose through which you want to suck something (usually some gas out of some container). So there it’s the same: The fast-flowing water pulls things in from the side and takes them with it. Now for continuity reasons, the water that is entrained in the jet needs to come from somewhere, so water has to be brought upstream in order to get sucked into the jet. That’s also similar to playing with Venturi tubes where the thinner the tube, the faster the flow, the lower the pressure… Anyway, riddle solved and I can think about other stuff again ;-)

But it is a really beautiful place to be:


I like water so much better than mountains, but mountains still have their charms, can’t deny that…




Last week I went to beautiful Lüneburg with a group of climate scientists to continue working on a very exciting project I’ve been involved in over the last year or so (see “scales in the climate system” funded by CliSAP here). I so enjoyed being with a group of people who talk about converging solutions of discretized differential equations over dinner! I have really spent way too little time with people like that ever since I left oceanographic research and went for instructional design. So it was great to discover that I haven’t lost that side of my life but that I can still happily talk about climate models and eddy covariance measurements!


Waterfalls in Lüneburg

But the “continuity” in the title of this post is actually referring to something else which I saw during a break we took. In the picture above you see the river going through Lüneburg, which is clearly going downhill, just like every good river should. You also see a couple of fronts, so clearly something is going on there. Watch:

[vimeo 138396707]

I find it super fascinating. Where does the water that comes down the wide waterfall go to if the sea grass (hey, I’m not a biologist! You know, the green stuff in the river!) is going towards the waterfall, too? Is there a vertical circulation involved? But then where does the water actually sink? Yet it doesn’t really look like it could all go in the current along the front. What is going on there?

Isn’t it weird how I always look for continuity? :-)


What do you see when you look at an aquarium?

When I was in Gothenburg last year for EMSEA14, one night we got to hang out at the Sjöfartsmuseet Akvariet there, and, even cooler, had the whole place to ourselves. A lot of the staff was around and happy to chat, including people who actually designed the exhibitions, so that was really exciting. But those are the times when I realize that I am really a physical oceanographer at heart. I like looking at colorful anemones or fishies or sea horses, sure!


But what I am most excited about is stuff like this: When there is enough suspended stuff in the water to visualize a flow field to recognize hydrodynamic principles, like in this case continuity.