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Topic: Sprinkler (Read 28523 times) |
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SWF
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Re: Sprinkler
« Reply #50 on: Jul 27th, 2006, 10:37pm » |
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Most of the proposed explanations are not accounting for the relevant factors in this problem. This is a challenging problem, and not just a Physics 101 exercise (if it was so simple Feynman and the other great physicists at Los Alamos would not have been confused by it).
The submerged sprinkler is acted upon by all the fluid that touches it. To evaluate the forces and moments, the flow field in the fluid must be found, which is governed by the Navier-Stokes equations. Solving these equations is not easy and simplfying assumptions are often made. If it is assumed that viscous effects are negligible compared to intertia of the fluid ("ideal fluid"), the Navier-Stokes equations are simplfied, and the forces acting on the sprinkler are unchanged if the direction of the fluid flow is reversed. If instead it is assumed that inertia effects are neglible compared to inertia effects ("Stokes flow") then all forces are reversed when direction of fluid flow is reversed. The above comments include some other assumptions, such as the fluid is incompressible, steady state flow conditions, and the effects of external body forces such as gravity are not included.
In the case of an ideal fluid (i.e. viscous effects negligible) the only forces acting on the sprinkler are pressure forces, which may be obtained from the flow field by use of the Bernoulli equation. With ideal flow in two dimensions the surface of the sprinkler is traced by streamlines, and from the Bernoulli equation pressure is lowest where velocity is highest- such as where the fluid exits the nozzle. After sketching streamlines for sprinklers of a couple of shapes, I am confident that with ideal flow the shape can be chosen to make the sprinkler rotate either toward or away from an exiting flow . However, the shape that most resembles the common sprinkler rotates in a direction opposite from what most people probably expect (i.e. opposite the way the sprinkler turns when it ejects water when surrounded by air). And being that this is ideal flow, the direction of rotation for a sprinkler of a given shape is the same whether fluid is being forrced out or drawn in.
Of course, real fluid is not the same as an ideal fluid. An important effect not included in the above simplfication is flow separation. For outward flow from the nozzles there is flow separation from the nozzle edges that results in a jet or column of fluid being fired into the surrounding fluid. Separation occurs down stream of the flow direction, and when flow is drawn into the sprinkler any flow separation occurs inside the sprinkler- there is not a column of fluid sucked out of the surrounding fluid. The flow outside a sprinkler during suction resembles the flow field during ideal flow. It is not like the inverse of when flow goes outward, and this is a big differecne between the two situations. During outward flow, the turbulence in the jet of fluid causes energy losses- Bernoulli's equation can't be used to find pressure on the entire surface of the sprinkler, With these losses the outward flowing case could have higher pressure near nozzle outlet, and rotate in the opposite direction from when fluid is drawn into the sprinkler.
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Pyotr
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Re: Sprinkler
« Reply #51 on: Jan 25th, 2012, 10:14am » |
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Just on a side note, Port-a-Loo operators seem to be quite calm about just sticking in a hose to suck out toilet filth, while fire-fighters are usually quite concerned about a rampant nozzle.
From this, I surmised that however the inverse sprinkler behaved, it would depend on pressure differentials – sucking only gives one bar, while pumping can go a lot higher.
And yes, I did watch the video linked to above demonstrating the immersed sprinkler.
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Grimbal
wu::riddles Moderator
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Re: Sprinkler
« Reply #52 on: Jan 27th, 2012, 10:24am » |
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on Jul 30th, 2002, 3:16pm, Franklinstein wrote:| Feinman didn't reveal the answer in the book but he delighted in giving a convincing argument about what would happen until his colleagues agreed with him, then he would give them a different argument until they agreed that was the correct answer.Then he would remind them of his first argument, etc. |
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And yet, isn't it obvious what result he observed?
The result was that the device exploded under the pressure. This means that he didn't see any clear motion in either direction and decided to increase the pressure until he saw something happen.
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