Forces that keep a free vortex turning

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Discussion Overview

The discussion revolves around the forces that maintain a free vortex, specifically vortex rings, and seeks to understand the qualitative aspects of these forces. Participants explore concepts related to fluid dynamics, including vorticity, pressure gradients, and the role of various forces such as viscosity and surface tension.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants inquire about the forces that keep particles in a vortex ring, likening it to gravitational forces that prevent particles from dispersing.
  • There is a suggestion that surface tension may play a role in maintaining vortex structures in water, particularly in the context of whales creating vortices.
  • Others clarify that in air vortices, such as smoke rings, surface tension is not a factor, and they propose that friction among fluid layers in turbulent regimes might be relevant.
  • A participant introduces the idea that low pressure in the core of a vortex could draw in molecules, while the spinning motion disperses them, leading to a balance of forces.
  • Another participant suggests that the centripetal force necessary to keep the particles in circular motion typically arises from a pressure gradient.
  • There is a mention of the Venturi effect as a possible explanation for the dynamics within the vortex.
  • One participant draws a parallel between the equilibrium in a vortex and the equilibrium seen in waves, suggesting that energy may be trapped in a vortex similar to a zero frequency wave.

Areas of Agreement / Disagreement

Participants express various hypotheses regarding the forces involved in maintaining a vortex, with no clear consensus on the primary mechanisms at play. Multiple competing views remain regarding the roles of pressure gradients, friction, and other forces.

Contextual Notes

Participants note the complexity of the topic, with references to turbulent regimes and the need for a deeper understanding of the underlying forces. There are also mentions of specific examples, such as vortex rings and hurricanes, which may influence the discussion.

bubal
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I am trying to understand what forces keep a free vortex, (i.e. a vortex ring). Its rotating particles must have a force to avoid them going out (like gravity on planets). I have read about vortices and vorticity, but get lost in equations.
Is it possible to explain the force that keep the particles in a vortex ring in a qualitative way? Who is the responsible? (viscosity?, friction?, etc.)

Thank you!
 
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bubal said:
I am trying to understand what forces keep a free vortex, (i.e. a vortex ring). Its rotating particles must have a force to avoid them going out (like gravity on planets). I have read about vortices and vorticity, but get lost in equations.
Is it possible to explain the force that keep the particles in a vortex ring in a qualitative way? Who is the responsible? (viscosity?, friction?, etc.)

Thank you!

In something like this, whales making vortexes in the ocean, I suspect the surface tension of the water is keeping the vortex together, but not sure.

Do you have a sample vortex in mind?

 
edguy99 said:
In something like this, whales making vortexes in the ocean, I suspect the surface tension of the water is keeping the vortex together, but not sure.

Do you have a sample vortex in mind?



Yes. A vortex smoke ring or an air vortex, like the ones from air vortex cannons. It is in the middle of the air so you don't have surfce tension. The particles in the vortex are rotating around a circular line and they are free. (Other vortices like a whirpool in a sink are not free because the water is falling). I think it is related to friction among layers of fluids in turbulent regimes, but don't understand where are the forces.
 
bubal said:
Yes. A vortex smoke ring or an air vortex, like the ones from air vortex cannons. It is in the middle of the air so you don't have surfce tension. The particles in the vortex are rotating around a circular line and they are free. (Other vortices like a whirpool in a sink are not free because the water is falling). I think it is related to friction among layers of fluids in turbulent regimes, but don't understand where are the forces.

The smoke ring or air vortex, its like a hurricane. I think the low pressure is what tends to draw in the molecules, the spinning is what tends to disperse the vortex.
 
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edguy99 said:
The smoke ring or air vortex, its like a hurricane. I think the low pressure is what tends to draw in the molecules, the spinning is what tends to disperse the vortex.
So could it be a kind of Ventury effect?
 
bubal said:
Its rotating particles must have a force to avoid them going out (like gravity on planets).
You mean a centripetal force component, perpendicular to their velocity? This usually comes from a pressure gradient.
 
A.T. said:
You mean a centripetal force component, perpendicular to their velocity? This usually comes from a pressure gradient.
This is very interesting. No wonder it fascinated Prof. Boys, who invented the "bazooka" cannon. If we assume there is a low pressure in the core and a velocity around the outside, giving centripetal force, the two are in equilibrium. It is the same equilibrium we see in a wave, when
PE = KE. The vortex seems to look like a zero frequency wave, where energy is trapped.
 

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