Tag Archives: instability

Vortex street

Do you use a tide chart to find the best time for your Saturday walk, too?

I showed you a vortex street on a plate formed by pulling a paint brush through sugary water as an example. Now today I want to show you the real thing: Instead of stagnant water and a moving object, I bring to you the flowing Elbe river and a bollard!


Watch how vortices with alternating spin are shed every three or four seconds!

Confluence of Danube and Morava river

Confluence of Danube and Morava river.

Watching the shear flow on Elbe river the other day, I was reminded of another shear flow which I had watched a long time ago. In 2009, J and I went to Bratislava in Slovakia, and from there did a trip to Devín castle.

What you see below is the confluence of the Danube and Morava rivers (with the muddy water coming in from the right).


Confluence of Danube and Morava river near Bratislava

I found it fascinating to watch the muddy water coming in, and then being forced downstream by the much faster flowing Danube. In the picture above you can see the sharp corner and then the front carrying instabilities caused by the strong shear.

Unfortunately, this was at a time when I didn’t even dream of ever blogging, so I don’t have more pictures of the shear instabilities. But I have a better picture of the front in the more stagnant part of the flow:


Devín castle

Fascinating how such a sharp feature can persist! Both in almost stagnant water (wouldn’t boats going through, or fish, or something mix it up?) as well as fast-flowing (there are clearly huge instabilities on the front, why don’t they mix more efficiently?). Plus the muddy water should warm up faster than the green-ish water, so why doesn’t the muddy water form a surface layer, at least in the stagnant part?

Digging out these pictures really was a journey down memory lane. First, I had to dig out my old laptop. Which was the second laptop I ever owned, but still it’s huge. Then I had to remember how to get into the correct partition on that laptop. Funny how somehow my fingers remembered the password to the computer based on the different shape of keyboard, maybe? I could type it, but I would not have been able to spell it out. And then I had to somehow get the pictures off! Not easy, I can tell you. But it is incredible how fast technology advances. I did have a good digital camera then, and I uploaded the pictures at full resolution. So that is really all there is to look at. I am really curious what digital photography will be like in another 6 or so years…

Shear flow

Kelvin-Helmholtz instabilities in a shear flow in Elbe river.

Last week I talked about how I wanted to use the “Elbe” model in teaching. Here is another idea for an exercise:

On the picture below you see Kelvin-Helmholtz instabilities. They might be kinda hard to make out from the picture, but there is a movie below where they are a bit easier to spot.


Kelvin-Helmholtz instabilities the boundary layer of Elbe river

Anyway, this is what they look like: Kind of like the ones we saw off Jan Mayen in 2012.


Kelvin-Helmholtz instability off Jan Mayen

Kelvin-Helmholtz instabilities occur in shear flows under certain conditions. And those conditions could be explored by using a tool like Elbe. And once students get a feel for the kind of shear that is needed, why not try to reproduce a flow field that causes something similar to the instabilities seen in the movie below?

Internal waves in the atmosphere

A photo of internal waves in the atmosphere.

Internal waves exist on the interface between fluids of different densities. In the ocean they are mostly observed through their surface imprint. In the tank, we could also observe them by looking in from the side, but this is hardly feasible in the ocean. But luckily vision is easier in the atmosphere than in the ocean.

On our research cruise on the RRS James Clark Ross in August 2012, we were lucky enough to observe atmospheric internal waves, and even breaking ones (see image above). This is quite a rare sight, and a very spectacular one, especially since, due to the low density contrast between the two layers, the waves break extremely slowly.

It is really hard to imagine what it looked like for real. This movie shows the view of Jan Mayen – the volcano, the rest of the island and then the atmospheric waves. Please excuse the wobbly camera – we were after all on a ship and I was too excited to stabilize properly.