Feynman Sprinkler [video]

Circa 1986, I read Surely You're Joking, Mr. Feynman and became mildly obsessed with the fact that Feynman doesn't present the result of the sprinkler experiment (see the first few seconds of the video for context). A number of colleagues (at the software startup where I was working) and I tied ourselves up in knots debating what the answer must be.

I wanted to perform the experiment, but lacking materials, skill, and (frankly) ambition, I settled for a laughably primitive apparatus involving a couple of bendy straws, the bathroom sink, and my mouth. This was enough to reproduce the well-known fact that if you push water out through the sprinkler, it will spin. When I tried sucking water in, it would give a momentary rotational jerk and then stop moving... but perhaps that was due to my mouth tightening up on the straw?!?

After I shared this inconclusive result, one of my co-workers decided to get to the bottom of the matter. He called the Pasadena operator, asked for the home phone of a Mr. Richard Feynman, and – to my utter horror – dialed the number.

It seemed impossible that this was a viable procedure for making contact with a Nobel Prize-winning physicist, but through the speakerphone, we could all hear a gruff voice that certainly sounded like it might be him. All doubt was removed when my friend explained that we were looking for the answer that isn't presented in the book, and the gruff voice said, "Why should I tell you?"

Feynman then asked whether we had tried the experiment, and to my redoubled horror, the phone was handed to me. I stumbled through some explanation of what I had tried and what result I had observed. Fenyman then relented, and explained the entire situation in ten short words:

"The sprinkler cannot rotate, because no angular momentum is transferred."

(Not an exact quote, but something very much like that)

https://www.youtube.com/watch?v=z3scTRJCm7w

The jitter is likely due to the suction pulling the air from the outside of the tubes, which then get pulled in the gap between the things - which does so in a buffeting fashion due to the movement of the pipes, so it finds the oscillation... (the tubes are a Foil - so you have to accomodate the flow diff on the external and internal of your receiving tube - and how the flows from the outside of the foil are a diff speed/flow/rate than the center of the tube - thus the light suction guy gets enough strength for the outside air to push it back - once it gets far enough from the horizontal tube, the pressure from the outside flow drops and the lever goes back to its resting position and begins again)

The other thing, as he mentioned eddys -- the inlet and outlet cones should be turbine-spiral shapes - as the air/fluid comes down the pipe, if you give it a spiraling/riffling it will keep the flow somewhat cohesive as it spins it.

Further, as a YT comment mentioned - stuffing a bunch of small straws in the main tube helps, because if you have a lot of smaller tubes - (especially with water) you can get the effective "orientation of the molocules" the same - so your stream can be more cohesive (this method was (invented?employed) by Disney Land Engineers when they were building the fountains that shoot clean, cohesive lasers of water.

They run the streams to the fountain through pipes that have a ton of staws inside - so the water gets orientated in such a way that when the stream leaves the nozzle, the water stays together.

Also - you can test all this in FluidX3d: https://github.com/ProjectPhysX/FluidX3D

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The Eddy Pump was first invented by Leo Davinci... and now we have great pumps like:

https://eddypump.com/pump-videos

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The eddy is the most important thing to understand in fluid dynamics IMO. (its what your Heart is, BTW)

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@Btrettel:

THANKS! I couldn't think of the term laminar...

And I was simplifying "stuffing straws"..

Appreciate the input. Have you tried FluidX3D?