Tag Archives: fronts

Front-spotting in muddy waters

On a recent flight from Hamburg to London City Airport, I ended up on one of the tiniest planes I’ve ever been on. Which meant that we flew super low, I took tons of pictures out of a not-very-clean window, and all my pictures have at least one propeller blade in them.

But look at what we saw!

For example in the picture below, a plume of muddy water coming from some canal into a river (and I should probably know where this is, but I have no idea. Somewhere between Hamburg and London?). I’m not sure whether the inflowing water itself was muddy to begin with, but I would guess that it is stirring up mud from the bottom of the river since it seems to be low tide and the inflowing water is maybe moving a lot faster than the water in the river itself?

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Closer to England we flew across this wind farm, where turbines have mud stripes in their lee. Also pretty interesting. Maybe they change direction with tides?

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And then coming to the mouth of the River Thames, there is quite a clear front between outflow and muddy North Sea water.

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Going upstream on the River Thames, boats stir up a lot of mud!

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So you can clearly see where they went for a pretty long time.

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On this flight, I sat next to a professional photographer who rolled his eyes at me taking pictures pretty much non-stop. And yes, they might not be the best quality. But at least you see what I saw, right?

Hetonic explosion

Or, an experiment on this blog often known as “slumping column”. (deutscher Text unten)

If you don’t scale your tilting of frontal surfaces under rotation experiment correctly, you get a phenomenon called “hetonic explosion”: the formation of a cloud of baroclinic point vortices. From the densities, the rotation rate, the dimensions etc you can calculate the Rossby radius and determine how many eddies you will generate. In our case, though, the calculation went wrong by a factor 10 (9.81, to be precise) and what we ended up getting is shown below.

Watch the movie below for the whole experiment (though most of it in time lapse).

Heute haben wir ein sehr spannendes Experiment gemacht. In einem Drehtank hatten wir in der Mitte einen Zylinder mit gefärbten Salzwasser und außen herum klarer Süßwasser ins Gleichgewicht gedreht. Dann wurde der Zylinder entfernt und die Säule blauen Wassers musste ein neues Gleichgewicht finden.

Im Film oben zeigen wir das Experiment – zum Teil allerdings im Zeitraffer. Viel Spaß!

Tilting of a frontal surface under rotation

Eddy in a rotating tank.

This is an experiment that Pierre and I ran two years ago in Bergen but that – as I just realized – has not been featured on this blog before. Which is a pity, because it is a pretty cool experiment.

Under rotation, vertical fronts with different densities on either side can persist for a long time without leading to the density-driven adjustment shown in the non-rotating Marsigli experiment. This is what we demonstrate with this experiment.

In a not-yet-rotating tank, dyed salt water is filled into a centered cylinder while, at the same time, fresh water is filled in the tank outside of the cylinder.

This setup is then spun up for approximately half an hour. Then, the cylinder can be carefully removed and the column of dense water can adjust to the new conditions.

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The rotating tank just as the cylinder is being removed

When the cylinder is being removed, disturbances are being introduced. Hence, several columns with sloping fronts develop in the rotating system.

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Dense columns developing towards an equilibrium state in the rotating system.

This is what the rotating tank looks like from the side several minutes after the cylinder has been removed.

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Side view of the sloping front around the dense column

Here are a couple of movies of this experiment. First a top view (note how you can see the deformation of the surface when you focus on the reflection of the ceiling lights on the water’s surface!):

Then a side view:

And finally (just because it’s fun) this is what it looks like when you switch off the rotation of the tank when you are done with the experiment: