[HN Gopher] Centrifugal flows drive reverse rotation of Feynman'...
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Centrifugal flows drive reverse rotation of Feynman's sprinkler
Author : sohkamyung
Score : 28 points
Date : 2024-01-25 12:30 UTC (2 days ago)
(HTM) web link (journals.aps.org)
(TXT) w3m dump (journals.aps.org)
| aredox wrote:
| "The issue of reversibility in hydromechanical sprinklers that
| auto-rotate while ejecting fluid from S-shaped tubes raises
| fundamental questions that remain unresolved" - I thought that
| was resolved already, by the fact in one case the fluid is
| expelled outside of the sprinkler and doesn't interact with it,
| whereas in suction the fluid "bumps" into to curve of the
| sprinkler and cancels most of the pull of the suction.
| leephillips wrote:
| There's a lot in classical physics that remains puzzling and
| even controversial:
|
| http://arstechnica.com/science/2014/08/the-never-ending-conu...
| eig wrote:
| The most intuitive explanation for the Feynman sprinkler I can
| think of is conservation of angular momentum.
|
| In the pump-out case, the water adds positive angular momentum
| when it flies out, so the sprinkler body must add negative
| angular momentum (constant torque) to have conservation.
|
| In the Feynman suck-in case, the water drains from the base of
| the sprinkler without any angular momentum, so the sprinkler head
| does not have to add an opposite angular momentum (feel a
| constant torque).
| MatthiasWandel wrote:
| regardless of what happens inside the S-curve, the opening ends
| up getting a slight vacuum from suction, but there is no such
| suction on the opposite side.
|
| Imagine the S is straight, but a vane attached to the side of
| the pipe blocks suction from one side.
|
| I tried to illustrate this in ascii art, but it appears HN has
| an algorithm to destroy ascii art.
|
| That vane will now have a slight vacuum on the side of the
| pipe, and it seems logical that it should want to move in that
| direction.
|
| Now imagine that vane curved around the pipe so it forms the
| end of the S-bend. Same thing.
| toxik wrote:
| Hi Matthias, love your channel. You can indent text by four
| spaces to show preformatted text: hello
| world
| MatthiasWandel wrote:
| Ok, ascii diagram for my previous comment, indented:
| =============== Pipe <---Suck
| ===============
| -----------vane-----------
| verteu wrote:
| In the reverse case, why doesn't the water add negative angular
| momentum when it "flies in"?
|
| IMO, that's the crucial asymmetry -- fans "blow out" air in a
| straight line, but "suck in" air from the entire surroundings.
| eig wrote:
| You got it- the sucking in comes from all directions.
|
| In response to your first question, the water doesn't add
| negative angular momentum because the whole system we are
| conserving is the sprinkler body and the moving water
| together. The sum must just be zero in both cases. We can
| track a single drop of water in case two (Feynman) to see how
| the conservation works. At the start the drop is at zero
| angular momentum since it is stationary in the tank. It then
| passes through the whole Sprinkler mechanism (and whatever
| s-curves there are) and ends up at the suction drain at the
| center of the sprinkler also at zero angular momentum.
| Therefore no matter what happened it could not have applied a
| net torque to the sprinkler, no matter the path it took to
| get to the drain.
|
| Contrast with case 1, the regular sprinkler, where the water
| drop starts at zero angular momentum (pumped from the center
| of the sprinkler) and ends with positive angular momentum
| (spinning away from the center) therefore it must have
| applied an opposing net torque at some point in its journey
| to the body of the sprinkler, which spins.
| thehappypm wrote:
| I think it's a different thing actually.
|
| When you're forcing the water in, you've got a pipe with
| pressure greater than the water. Pressure is a measurement
| of force per unit area. The pressure of the water presses
| evenly on the tube walls, but there is no tube walls at the
| opening. So, there is a spot where there is no pressure on
| the tube. This means a net force because that lack of
| pushing on the opening is not canceled out. Since the
| pressure differential is in the direction of the rotational
| axis you get rotation.
|
| In the reverse, there is also more pressure in the tubes
| than in the water. However, the spot where there is a net
| difference in pressure is the drain. The drain has an
| opening putting less pressure, so we get a net force.
| However the direction of the net force is perpendicular to
| the rotational axis--so we get no rotation.
| mhuffman wrote:
| Matthias Wandel did some experiments on this recently and came to
| some conclusions of his own it seems.
|
| [1]https://www.youtube.com/watch?v=ued2cEcfAio
|
| [2]https://www.youtube.com/watch?v=z3scTRJCm7w
| mhb wrote:
| Some discussion: https://news.ycombinator.com/item?id=38985308
| GuB-42 wrote:
| Note that the problem the paper is about is that in real life,
| using precision instruments, the Feynman's sprinkler _does_
| rotate the other way under suction.
|
| The counterintuitive asymmetry between blowing and sucking is
| already well understood.
| eig wrote:
| You can interact with a working demonstration booth of this at
| MIT. That apparatus had an initial backwards torque when you
| turned on the suction, but no constant torque pushing it the
| other way once everything hit steady state. I wonder why these
| authors seem to have gotten a different result...
| Sharlin wrote:
| Nominative determinism: the name of the second author is Brennan
| _Sprinkle_.
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