Strange magnus force in whirlpool

[Proud Teslian]

 

1. Imagine a big river with water flowing constant velocity , assume there is a whirlpool at center as shown in the figure.

2. Because of the magnus force the whirlpool should move towards left or right towards river bank depending on the spin, but it does't happens it just stays fine in its position....so why?

3. I belive that magnus force will exists in whirlpool only when the velocity of water changes (acceleration), not just velocity , since the whirlpool is itself a part of the water flow.

4. apply the same to faraday's law of electromagnetic induction as anology ( magnetic feild as river and charge as whirlpool )

, is this correct?

 

 

Edited April 23, 2016 by Proud Teslian

[swansont]

My understanding is that the magnus effect is that of a fluid on a solid that is spinning. If you have a whirlpool in a river, then the flow of the river is causing it, which is a different situation.

 

You also have to consider that the river banks are a boundary condition not normally present for a spinning object in a fluid.

[Proud Teslian]

ok forget the whirlpool, imagine a tornado, suppose a strong wind current flows towards it will it experience magnus force?

[studiot]

I am going to disagree with swansont here and state that the Magnus effect does indeed apply to your whirlpool, and to your tornado.

 

 

Stanley: Mechanical Properties of Solids and Fluids

The Magnus Effect

A transverse force acting on a rotating body moving relative to a fluid medium. It is equally applicable for the movement relative to a liquid or a gas and whether the rotating body be moving through the liquid ir the liquid flowing past the rotating body.

 

A rotating body of fluid is called a vortex and Helmholtz first vortex theorem tells us that it can be considered a 'body'.

 

So to answer you questions about what is going on.

 

The generators of the whirlpool and the tornado are entirely different.

Natural flowing water, of the type you describe, is not subject to appreciable temperature difference.

Tornados, on the other hand, are thermally generated.

Either way a laminarly flowing fluid does not suddently start rotating, there is always a causative agent (Kelvin's Law).

In the case of open channel hydraulics this will be due to the shape and nature of the edges and bottom. Ther whirlpool will be caused by some static obstruction in the bottom (since you have placed it in the middle) or edges.

Now the 'strength' of the vortex is low but it is constantly being reinforced by new passing water being swung round the obstruction so it stabilises at that point in the flow.

 

This does indeed generate a transverse Magnus force.

 

So why doesn't it move?

 

Two reasons

 

The rotational and flow speeds are actually quite low so the Magnus force is low. Not high enough to overcome bottom friction.

Objects can be subject to a force but not move if another force, such as friction is acting.

 

As already noted the vortex strength is low so any water that does separate and move off will quickly mix with the main flow.

 

Note this vortex does not have (thermally induced) axial flow like the tornado.

 

I think that analogies to electromagnetic field patterns are best avoided (even though this was also Helmholtz).

Don't forget that EM fields have no material substance.

This is an interaction between material bodies.

[swansont]

I's d like to know where you observed the effect in the OP.

[studiot]

 

I's d like to know where you observed the effect in the OP.

 

 

Whilst the ocurrence of a single midstream whirlpool is unlikely and the streamlines as drawn indicative more than anything else, I'm responding to the OP's idea that an immersed spinning body of fluid is subject to the same mechanics as any other spinning body, whilst noting the effect to be very small in the case offered.

 

I also thought of tornados whilst I was reading it and then found he offered the same in his second post.

 

I think it is a reasonable question, reasonably put.

[John Cuthber]

 

 

1. Imagine a big river with water flowing constant velocity , assume there is a whirlpool at center as shown in the figure.

 

 

Which do you want me to do?

"a big river with water flowing constant velocity"

or

"assume there is a whirlpool "

 

I can't do both because the rotating bit clearly hasn't got a constant velocity.

 

In order for there to be a whirlpool something must be causing it and there's definitely a reaction force against whatever that cause might be.

But unless you can tell us what the whole story is, there's no way we can get very far.

[J.C.MacSwell]

 

 

1. Imagine a big river with water flowing constant velocity , assume there is a whirlpool at center as shown in the figure.

 

2. Because of the magnus force the whirlpool should move towards left or right towards river bank depending on the spin, but it does't happens it just stays fine in its position....so why?

 

3. I belive that magnus force will exists in whirlpool only when the velocity of water changes (acceleration), not just velocity , since the whirlpool is itself a part of the water flow.

 

4. apply the same to faraday's law of electromagnetic induction as anology ( magnetic feild as river and charge as whirlpool )

, is this correct?

whrilpool.png

 

 

 

The magnus force manifests due to a frictional effect of a spinning cylinder or sphere in a relatively moving fluid. (it effectively changes the shape of the object)

 

Where a standing vortex is maintained by the surrounding flow, the frictional effect would be opposite.

[Proud Teslian]

Which do you want me to do?

"a big river with water flowing constant velocity"

or

"assume there is a whirlpool "

 

I can't do both because the rotating bit clearly hasn't got a constant velocity.

 

In order for there to be a whirlpool something must be causing it and there's definitely a reaction force against whatever that cause might be.

But unless you can tell us what the whole story is, there's no way we can get very far.

It is possible to produce whirlpool in constantly moving fluid

1. immerse an Egg beater vertically and run it

2. make a hole at bottom and suck out fluid fastly

off-course the water in whirlpool is not in constant velocity, but the rest of the river is.

[John Cuthber]

OK, so the first thing you need to understand is that a whirlpool won't have a clearly defined edge.

 

The second is that the flow velocity in a river isn't constant across the stream.

It's fastest near the middle and the top, an it's slowest near the bottom and the sides.

[swansont]

Whilst the ocurrence of a single midstream whirlpool is unlikely and the streamlines as drawn indicative more than anything else, I'm responding to the OP's idea that an immersed spinning body of fluid is subject to the same mechanics as any other spinning body, whilst noting the effect to be very small in the case offered.

 

I also thought of tornados whilst I was reading it and then found he offered the same in his second post.

 

I think it is a reasonable question, reasonably put.

I was asking the one who posted it. Part of it sounds like a thought experiment, but part of it is phrased as an observation.

[Proud Teslian]

i'm not intrested in what happens to whirlpools at rivers, but i was actually trying to relate it to an universal phenomeon, which i feel difficult to reveal directly.

 

some people will understand what i'm trying to relate it to.

[studiot]

 

i'm not intrested in what happens to whirlpools at rivers, but i was actually trying to relate it to an universal phenomeon, which i feel difficult to reveal directly.

 

some people will understand what i'm trying to relate it to.

 

 

So this is a speculation then?

 

Not an application of Classical Physics and you have been wasting my time.