Physics? puzzle

[FlFishOn]

Maybe more like fluid dynamics. This one comes from Feynman and I can't remember if the story comes with a solution or it's left hanging.

Picture a 'S' shaped lawn sprinkler. Two orifices, spins when water pressure is applied. 'S' this one would spin counter-clockwise. Now put the sprinkler under water and apply suction to the attached hose. Which way does it spin? It's said the Feynman could persuasively argue for either direction. I have no idea which is correct. The pay off to the story, as I remembeer it, was a large, wet lab accident.

[Evian]

mmmmmh, seems logicial that it would now go clockwise having a vacuum applied to it, it would be sucking water in no?

[chief444]

Looking at the forces at just the orifice of the nozzle and conservation of momentum that would definitely be true. But when the water enters the nozzle it exerts a force against the wall that theoretically equals the momentum force at the orifice. With a really low friction swivel an experiment probably could turn it opposite. But in reality that's just because with fluid flow the force against the wall of the right angle has losses and can actually be less than the force at the orifice. So it either will spin opposite or not spin I believe depending on the the swivel and the actual design of the tubing near the nozzle and the nozzle itself.

[chezlaw]

If you put your hand close to your mouth and blow you feel a force but if you suck you don't.

I hope this is illuminating because its extremely dull.

chez

[chief444]

Sure but in that analogy the sprinkler isn't comparable to your hand. It's comparable to your head. If you attached a baffle plate to the nozzle that diverted the flow perpindicularly just beyond the orifice then it wouldn't spin when it ran normally either for the reason you point out. But that doesn't mean there aren't forces occuring when you breath in just as there are when you breath out. The forces just aren't occuring (or are very minimal) outside of your mouth where your hand is when you breath in. They're occuring inside of your mouth.

[chezlaw]

[ QUOTE ]
Sure but in that analogy the sprinkler isn't comparable to your hand. It's comparable to your head. If you attached a baffle plate to the nozzle that diverted the flow perpindicularly just beyond the orifice then it wouldn't spin when it ran normally either for the reason you point out. But that doesn't mean there aren't forces occuring when you breath in just as there are when you breath out. The forces just aren't occuring (or are very minimal) outside of your mouth where your hand is when you breath in. They're occuring inside of your mouth.

[/ QUOTE ]
I was intending to draw an alalogy between the sprinkler and the head. If the air coming out when you blow is directional (you feel the force) then there must be a compensating force on the blowing head pushing it away from the air-flow (sprinkler spins). When you suck the air going in is not directional (you feel no force) so there is no compensating force on the sucking head (sprinkler stays still).

Does that make any sense?

chez

[chief444]

Well basically yes. But in reality it's better to look at it more localized because you could in fact design a nozzle that spins opposite. It would probably require a pivot for the sprinkler close to the nozzle and larger piping so that not all of the x-component (or whichever designation we choose) is removed from the fluid by the time it reaches the pivot. This is why some experiments have shown it to stay still while some have actually been able to rotate it opposite.

[chezlaw]

[ QUOTE ]
Well basically yes. But in reality it's better to look at it more localized because you could in fact design a nozzle that spins opposite. It would probably require a pivot for the sprinkler close to the nozzle and larger piping so that not all of the x-component (or whichever designation we choose) is removed from the fluid by the time it reaches the pivot. This is why some experiments have shown it to stay still while some have actually been able to rotate it opposite.

[/ QUOTE ]
ok. Sadly I don't do localized very well.

It makes me very happy to hear people have done the experiments. I've had a go making silly shapes with my mouth but no luck.
/images/graemlins/crazy.gif /images/graemlins/laugh.gif /images/graemlins/ooo.gif /images/graemlins/shocked.gif

chez

[Evian]

mmmmh, with small object aren't you able to lift them by sucking air in your mouth? Maybe I totally missed what you were saying...

[chief444]

Hey Chez,

FWIW I think your analogy does work fine for pretty much any sprinkler I've seen. I agree they won't spin. And I haven't seen the actual experiment data but I'm sure they spun much much slower than the normal direction. Really there are no exact equations for localized turbulent fluid flow. At least none that could be solved easily in this example without simulation software.

Blowing out and breathing in quickly makes you a bit dizzy doesn't it? /images/graemlins/smile.gif

Chief

[FlFishOn]

I googled this and it's all over the web. The experimental results show no motion. Some theories suggest unbalanced forces but all that is open to debate and debate and...

Saves me from buying a sprinkler.

[chezlaw]

[ QUOTE ]
Hey Chez,

FWIW I think your analogy does work fine for pretty much any sprinkler I've seen. I agree they won't spin. And I haven't seen the actual experiment data but I'm sure they spun much much slower than the normal direction. Really there are no exact equations for localized turbulent fluid flow. At least none that could be solved easily in this example without simulation software.

Blowing out and breathing in quickly makes you a bit dizzy doesn't it? /images/graemlins/smile.gif

Chief

[/ QUOTE ]
These profound scientific experiments are best done sitting down in a bar.

chez

[chezlaw]

[ QUOTE ]
mmmmh, with small object aren't you able to lift them by sucking air in your mouth? Maybe I totally missed what you were saying...

[/ QUOTE ]
Get in the bar and try to get ripples on your drink by sucking instead of blowing.

chez

[mindflayer]

what would happen if you put a funnel at the end of each nozzle? Now the suction force would be directional would it not?

[chief444]

A force is always directional.

[Sharkey]

Most, not all, of the problem can be reduced to the forces also experienced by a rigid vacuum cleaner hose extension while the machine is on.

At first glance, it seems that if suction is a form of acceleration, there should be a reaction force going the other way. On second thought, a vacuum is not merely a blower in reverse: the flow converges from all directions and moves differently. Then again, there is a net acceleration.

What an interesting problem.