# I Forces that keep a free vortex turning

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1. Aug 11, 2016

### bubal

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!

2. Aug 12, 2016

### edguy99

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?

3. Aug 15, 2016

### bubal

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.

4. Aug 15, 2016

### edguy99

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.

5. Aug 21, 2016

### bubal

So could it be a kind of Ventury effect?

6. Aug 22, 2016

### A.T.

You mean a centripetal force component, perpendicular to their velocity? This usually comes from a pressure gradient.

7. Aug 22, 2016

### tech99

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.