Whirl pool action
In my small pool I often get a lot of leaves that settle to the bottom. I can make a "whirl pool" and the leaves all come to a small pile right in the middle of the bottom where I can scoop them out quit easily with a dip net.
The wet leaves are heavier than water because they sink. Then why don't they move outwards in the whirl pool? What scentific principals cause them to pile toward the middle?
The wet leaves are heavier than water because they sink. Then why don't they move outwards in the whirl pool? What scentific principals cause them to pile toward the middle?
posted by Ragknot at
9:17 PM
42 Comments:
i belive it is simply the water pushing back towards the middle as it bounces off the walls... prolly wrong, but what the heck, worth a shot
We have a situation where the rotation forces the water outwards, and gravity increases the water pressure. The water pressure pushes against the force of the rotation and forces the water back towards the center of the rotation, but proportional the depth. So the deeper the water is, the more pressure the water exerts - because of gravity - and the more the water is able to overcome the rotational force and move towards the center of the vortex. And of course, higher up, where there is less pressure, the water is more thrown out to the perimeter of the rotating mass of water, leaving a larger hole.
If the water is deep enough, there will always be a point, a depth, where the water pressure caused by gravity will just balance the - for each speed - the rotational force (centrifugal force), and that will be the bottom of the vortex. Below that, the pressure will always overcome the outward force caused by the rotation.
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Anonymous, you forgot to mention why the leaves collect at the centre.
I don't think Anon was quite right, but he have some insight to the problem.
Here's a clue, I put a 2x4 about 6 inches long into the pool. Then began a clockwise rotation of the water. Of course the piece of wood, also went around with the water, but it also slowly begin to spin clockwise as it went clockwise around the pool. This "sub-rotation" is a clue.
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Ragknot, I know what he said. I was being sarcastic. I read some similar gobbledygook myself, via Google. None of the authors actually answer the question in a satisfactory fashion. They just rattle on about centripetal/centrifugal forces, pressure and flow and anything else that sounds remotely to do with it. I find it hard to believe that they actually know the answer themselves. I speak as an expert - take a look at my copout answer to "beam me up".
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Ragknot. Good question by the way. And just why were you throwing beams into your pond, I wonder???
Maybe it does have a connection to the shape. I put a circular sauce pan in the pool and it went directly to the center of the pond... with no whirlpool!!!
Just kidding. the shape has nothing to do with it. Sizes as small as can be seen also move to the center of the whirl pool. You can even see a cloud of murkey particals to small seen clearly, rotating like a small, very slow tornado.
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I don't think the fastest rotation is in the center. Maybe if you mean RPM and not ft/sec. In the center, the radial distance is small, and the velocity low. Near the edge, the radial distance is large, and the velocity is high.
This may give it away, but picture the rotations of the small rectangular block of wood. The edge toward the outside is being pushed by water that is moving faster that the edge toward the center. This causes the sub rotatation.. the clockwise rotating in a stream moving clockwise also.
It does not matter if the object is on the surface or toward the bottom, the shape nor the size matters, the resultant force of faster water on one side, pushes the object toward the slower velocity. The "sub-rotation" shows what is happening, the outside edge is being pushed more than the inside edge.
After a short while, the leaves and the block of wood are all moving slowly in near the center.
Get behind your buddy, push his right shoulder hard, his left shoulder easy. his motion will be counter clockwise and his resulant dirction will be toward the left.
Keep this up and he will move in a circle... then he will turn and hit you back.
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I had meant rotational speed. Having realised my error about the size of your "beam". I'll delete that post. I haven't got out my trusty saucepan yet.
You've probably come across "curl", a vector quantity in maths. It is sometimes called "rot" or "rotor". (Roughly) curl v = 2w, where v is the velocity vector field, and w is the rotational (speed) vector. You're seeing it with your beam as a sub-rotation. Fluid dynamics engineers throw small marked, neutral density test spheres's into their test tanks so they can analyse and measure "rot" with the aid of a slow-mo camera.
My previous post was meant to be the answer. Despite all the confusion about other post, the answer is very simple.
The small block of wood, about 3.5 by 6 inches shows that the water pushes on the block unevenly.
If the whirlpool is rotating counter clockwise, the right side of the block is in faster moving water. This is the side of the block away from center of whirlpool. Here the water is moving faster and pushes the right faster than the left side. The resultant force causes the block to move to the center of the whirlpool.
All objects small and large get moved the same way, and eventually arrive in the center moving in small circles.
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I'll try my saucepan after this post. Your explanation is consistent with Bernoulli's principle: moving fluid have less pressure. So the pressure gradient (low to high) should be pointing radially inwards. So I would expect the reverse centrifuge effect (high density stuff moves inwards). But I'm just playing with pressures etc. myself now. I don't feel that I am clear about the relative importance of all the things that are going on.
I mentioned the "small" test spheres being used because they help to measure curl "at a point", rather than an average over an extended region.
I think the explanation is lacking in that it does explain why the whirlpool has the rotational pattern that it has. The angular speed of the whirlpool seems to be largest near the centre. And the speed seems to be all over the place, but is possibly almost constant (over a fairly large relative range). Perhaps viscosity has something to do with the evolving velocity/angular speed distribution. I was expecting to get angular speed varies approximately inversely with radial distance => "vortex" flow. Unfortunately, if that's the case, Bernoulli's principle doesn't kick in. My textbooks don't seem to give a decent description of a real pond type whirlpool's velocity distribution. I also note that turbulence seems quite significant even when the whirlpool has almost died out. But the turbulence also seems to help get the heavy leaves towards the cente.
I think this is a hard problem.
Chris,
I don't think you have accepted that the fact that the velocity is greater on the outside edge. Why not? I think it because you have not observed you sauce pan yet.
When you do, I think you will note you've overlook an important part.
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I should have said that the comments were largely based on observations. Hence I noticing the turbulence/sub-vortices/whatever. The flow pattern seemed to change quite a bit after spinning the system up. I could hardly make head nor tail of it. I will look again in the morning, when the light's better.
I haven't got good enough data to accept or reject your claim with.
if you read the Trick of Mind closely you will see "I make a whirlpool". The whirlpool does not make itself. I introduce the energy at the outer edge by wading around the 10 foot diameter, 4 foot deep pool. This is how I know the water is faster there. I suspected you would stir the sause pan at the outer edge. It would be hard to stir the center, but if you could, the water would move faster there, the pile might then go to the edge, I don't know. If there were a hole at the bottom like a tub, then the center would drive the whilpool.
The trouble is this is a 3D problem with non-simple behaviour involved. The obvious feature is that the velocity field is non-constant. When the leaves are experiencing these varying velocities fields they're be pushed about all over the place and cannot settle, except near the middle, where the speeds are low - there the leaves simple settle out. That the leaves get to the centre seems to be a random process. But once there they tend to stay put. They are selected because they are heavy. Pretty much the same idea as sand being swirled about more or less uniformly in a turbulent stream, but settling out where the stream slows down and loses it's turbulence.
I think randomness combined with a "haven" near the middle of the whirlpool is the correct way to resolve the problem. I'm flying in the face of received wisdom here. Sadly I cannot make good enough experimental observations to see a more conventional explanation in terms of forces directing the leaves to the centre.
Aside: Under (non-turbulent conditions) a liquid has 0 speed relative to the fixed (rigid) boundaries of the container. This is attributable to viscosity. Check out e.g. http://en.wikipedia.org/wiki/Viscosity for confirmation.
PS I'm not going to stick my head out and insist that my interpretation is right, nor will I deny the possibility that for some of the possible whirlpool velocity fields that there isn't a "conventional" pressure gradient type of solution.
I personally feel that my answer is at least understandable, unlike the gobbledygook that others have posted on the Internet.
But I remain fairly sure that turbulence is a major difficulty when trying to employ the standard "schoolboy" approach to this problem.
Ragknot, I liked your visualisation of turning a friend around. You could employ the same argument to convince me that your buddy would simply turn on the spot. The problem being that if he gains translational energy, he moves to another part of the whirlpool, where the velocities are different. We are now moving into an area of physics comparable to orbital mechanics. Common sense can be your enemy there as well.
PPS I forgot to say "your beam floats". So I expect it to move away from the centre.
Note that the dense materials move down the pressure gradient (low to high presure). So your beam should be going in opposite direction suggested by Bernoulli pressure. i.e. it should move to the higher velocity region (lower pressure) i.e. outwards. The beam being on the surface, is also on a slope (water lower in middle) so will also be trying to go downhill. Need some numbers to see which of the forces dominate.
Are you beginning to see why I think this problem is difficult?
I'll vote fo the first Anonymous's post. I can't in my heart disagree with it.
Did you stir the sauce pan? Did you stir by moving a paddle around the outer edge?
Do you suppose that there are two different classes of whirlpools, one made by force applied at the outer circumference and one from energy in the center?
Hi Ragknot. I expect at least two whirlpool types. We haven't considered ones with a plughole.
If you imagine the spinning bucket one (with lots of baffles to force a "rigid" flow pattern (constant angular speed everywhere), then the regular centrifuge behaviour would come about - the leaves would go to the outside.
I stirred then took the stirrer out, so inherently was dealing with a dying/passive whirlpool. As for a continuosly powered (motorised) one - search me - too hard.
I actually use a middle sized circularly symmetric glass bowl. I stirred about halfway between the centre and the edge. I expect that regardless of how I stirred it, the flow pattern would probably end up much the same after say 30 seconds.
Anyway, the whole thing makes my head "spin".
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Two main distinctions are rotational and irrotational flow. With irrotational flow a small test arow will always point North, say. The "nicest" case of rotational flow is when the arrow always points to the centre, say. e.g. the Moon always points the same face to the Earth. So it's rotational speed is the same as its's orbital angular speed.
Does this mean you have not found a reason for the objects coming to the middle of the pool?
It seems to make no difference which direction, CW, or CCW the results are the same.
How would this relate to a hurricane? Velocities increase toward the center, until you reach the eye. Does my whirl pool have a downward flow at the center that gathers everything and push it to the bottom?
Hi Ragknot, I do quite like my explanation (random process and a haven), but am aware tht the rest of the planet (and myself really) are expecting an explanation in terms of pressure, centrifugal forces etc. I may well glance at a couple of my old physics textbooks and see if I can begin to get to grips with liquid dynamics. It is most definitely a weak spot in my knowledge.
Of course the direction of rotation is likely to be of low importance. But you are right to mention it as it is well known that the coriolis effect determines which way the whirlpool spins when emptying a bath. I seem to recall that I used to be able to reverse the direction (using my hand). I saw a Michael Palin proggy where it was demonstrated that the coriolis effect works as little as mile away from the equator. If the CE is of importance in your pond, the problem will have got even harder.
I think it likely that when there are flows, that the depositing effect will be masked to some degree.
I simply don't know what the velocity distribution's are for any of the whirlpool/tornado situations, yet alone why they have those distributions.
In short, currently, I'm out of my depth here.
sorry, not to be rude, but no one is right. the answer is that the cirular motion of the water caused centripetal force acceleration on the leaves, causing the leaves to rotate to the center of the moving mass, the water, or also the fixed axis on which it is all moving. Its all in newtons laws of motions. Basic Physics.
And before you start, i do know what i am talking about, im not guessing or rattling on. i went to school for 8 years studying this kind of thing, so. it may not be right, but there is no one right answer. everyone can have a theory, and technically speaking, there are more than one explanations to why this happens, even though it hasnt been proven, it has been studied.
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Jacob, sorry. In a centrifuge the leaves would move away from the centre. That is why Ragknot posted the problem - the leaves have moved in the opposite direction to what a naive knowledge of physics would say.
Your explanation has shed no light on the problem whatsoever. It is waffle.
I doubt anyone here imagined that non-Newtonian physics was required to deal wih the problem.
I agree (based on experiment) that there does seem to be more than one process causing the leaves to go to the centre.
I find it hard to put my faith in someone who can trot out a phrase like "centripetal force acceleration" (or indeed most of your first comment). It doesn't give me the impression that you really understand the subject. I also am extra wary of someone who feels it necessary to state that he is an expert, no matter how hard he tries to dumb that down.
Jacob, I'm sorry, my last post was rather harsh. But I don't feel that it is right to delete or modify it. I feel that I can only apologise for the bluntness, but not for the principal content of it.
If there's anything (other than the bluntness) that you'd like to take me up on, please do so. I don't always bite.
ur so smart guys
Gosh! Thanks!
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