Using art in your science teaching. The why and the how. This is a blog post that I wrote for the EGU’s blog’s “educational corner”. Check it out here.
Whenever I’m in a canoe or kayak, I love watching the two eddies that form behind the paddle when you pull it through the water. It looks kinda like this:
Instead of pulling a paddle through more or less stagnant water, we could also use a stationary paddle in a flow. And that is the setup I want to discuss today: A stationary, round paddle perpendicular to an air flow.
A very cool feature of the paddle – which we know has to exist from the sketch above – is shown below: There is a point somewhere downstream of the paddle, where the direction of the air flow changes and a return flow towards the paddle starts. You can see that the threads on the stick I am placing in the return flow go partly towards, partly away from the paddle. So clearly the stick is in the right spot!
Another visualization that my dad came up with below: Threads are pulled back towards the paddle in the return flow.
Doesn’t it look awesome?
Another way to visualize the change in flow direction is to take a rotor and move it from far downstream of the paddle towards the paddle and back.
All of this is shown in the movie:
Don’t you wish you had all this stuff to play with? :-)
(And do you now understand why I was so excited about the diving duck? :-))
Another early morning crossing this bridge.
And the current and the sun glint were perfect for this kind of photos:They almost look like schlieren photography images in those super old papers, don’t they?
And I find it extremely fascinating how you can see the boundary layer between the flow and the stagnant water, and how wind waves don’t manage to cross that boundary.
See the tiny capillary waves on the right side of the boundary? Those are locally generated because the larger waves on the top left just don’t make it over the strong shear.
You want to watch a movie? Sure!
On the way to the pool I cross over the Elbe river on this pretty bridge.
Which is pretty spectacular, just because the structure itself is so amazing.
But what is even more spectacular is how every time I am there I see new things in the flow field. And the example I want to show to you today is the flow field around one of the pylons of the bridge that runs in parallel to the one I am on.
In the movie below you see a classical flow separation, similar to what might happen at an airplane’s wing. The water flowing towards you under the bridge arrives (pretty much) laminar, but then on contact with the pylon turbulence develops, eddies form and the flow separates from the boundary of the obstacle. Nice! :-)
You might think that three hours of canoe polo on a Saturday morning would be enough water for the day, but no. As when I did the experiment for the “eddies in a jar” post a while back, sometimes I just need to do some cool oceanography. So last Saturday, this is what I did:
I took a plate, mixed some sugar, silvery water color, and water, pulled some stuff through the water and that was pretty much it. As a first order approximation, pulling an object through a stagnant water body is the same as the water body moving past a stationary object. And since it is usually pretty difficult to visualize flow around stationary objects (at least if you don’t want to pollute that little creek nor waste a lot of water). So this is really exciting.
The plate I am filming is the one underneath the camera (I love my gorilla grippy). My water colors from back when I was in primary school, a paint brush, a chop stick, the plate I tried first that turned out to not have enough contrast with the silver paint, a blanket because the tiles are cold to sit on. Oh, and the flowers that I have been meaning to put into nice pots for a couple of days now. So – no big mystery here! Go try! And let me know how it went.