Tag Archives: Kelvin-Helmholtz instability

#wavewatching in the sky

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On Elin’s student cruise (read more about that here) very nice wave watching was to be had, both on the water as well as in the sky.

In the picture below, if you look slightly left of the mountain top in the right of the picture, you see five parallel cloud stripes — evidence of the air moving in a wave motion after going over that mountain top! This motion results in clouds being there for certain phases of the waves and then no clouds for others, and since the movement is periodic, this results in cloud stripes. Now if I knew more about cloud formation I could probably tell you what changes with height except for pressure, and how that results in cloud formation or no cloud formation, and hence whether the cloud stripes indicate wave crests or wave troughs. My gut says troughs. Does anyone know?

Another very nice wave pattern is seen below: Kelvin-Helmholz instabilities! Those are shear instabilities that will eventually start breaking. Unfortunately I went back to work and next time I looked I didn’t find them again.

Kelvin-Helmholtz instabilities

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I’m back at my happy place — the teaching lab at GFI in Bergen! :-)

Here a quick look at what we’ve been doing today: Filling the large wave tank! With clear fresh water and then salty pink water that forms a layer below. As the pink water flows underneath the clear water, there is shear between the two layers, waves form and then they break. Beautiful Kelvin-Helmholtz shear instabilities!

Why have we filled the large tank? Just you wait and see… ;-)

Totally not the focus of our experiments, but so beautiful! Kelvin-Helmholtz instabilities

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This is really not the focus of our experiments here in Grenoble, but they are too nice not to show: Kelvin-Helmholtz instabilities!

Sheer instabilities in the flow

Sheer instabilities in the flow

They showed up really nicely in our first experiment, when we only had neutrally-buoyant particles in our source water (and not yet in the ambient water). The water that shows up as the lighter green here is thus water that originally came from the source (and at this point has recirculated out of the canyon again).

Sheer instabilities in the flow

Sheer instabilities in the flow

I get so fascinated with this kind of things. How can anyone possibly not be interested in fluid dynamics? :-)

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Watch the movie below to see them in motion! The scanning works as explained here.

 

Shear flow

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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.

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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.

Breaking_wave_Jan_Mayen

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

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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.