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.