Flow direction reversing downstream of obstacle

One of the things that fascinated me most when playing with the huge fan we used to look at the flow downstream of a paddle was how the flow direction reverses.

Unfortunately (alas, it was to be expected) we didn’t really see this on the paper towel stream line test I did the other day.

But here is another way to visualize it: using a propeller!

Propeller in air flow

Depending on the direction the air flows at the propeller, its direction changes. So as we move it towards and away from the paddle, when the flow direction changes, so does the direction of rotation of the propeller, too.

Watch!