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:

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? :-))