Tag Archives: turbulence

Visualizing flow around a paddle

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:

Screen shot 2015-06-27 at 4.03.20 PM

Flow around a paddle

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!


Visualizing the flow field behind a paddle with a threaded stick

Another visualization that my dad came up with below: Threads are pulled back towards the paddle in the return flow.


Visualizing the return flow behind a paddle with threads

Doesn’t it look awesome?


Visualizing the return flow behind a paddle with threads

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.


Visualizing the change in flow direction by moving a rotor towards and away from a paddle blocking an air stream

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? :-))

Shear flow

Another early morning crossing this bridge.

IMG_1544And the current and the sun glint were perfect for this kind of photos:IMG_1581They almost look like schlieren photography images in those super old papers, don’t they?
IMG_1587And 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.
IMG_1592See 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!

And another thing I love on those early morning trips? Being completely alone in a pretty park, with dew on the grass and flowers in the sun :-)

Flow separation

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.

IMG_0901 IMG_0906 IMG_0923But 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! :-)


Vortex streets on a plate

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:
Screen shot 2015-02-21 at 4.38.32 PMI 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.

Screen shot 2015-02-21 at 5.01.02 PM

Depending on the size of the object you pull through the water, and its speed, all kinds of different eddies develop. So fascinating! Watch the movie below to get an impression. (It’s really only an impression – it’s 2 minutes out of the 40 or so that I filmed ;-))

And for those of you who are always like “oh, I would love to, but I couldn’t possibly do this at home!”: This is what it looked like in my kitchen when I filmed the video above:Screen shot 2015-02-21 at 4.27.15 PM

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.