1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Forces that keep a free vortex turning

  1. Aug 11, 2016 #1
    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. jcsd
  3. Aug 12, 2016 #2

    edguy99

    User Avatar
    Gold Member

    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?

     
  4. Aug 15, 2016 #3
    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.
     
  5. Aug 15, 2016 #4

    edguy99

    User Avatar
    Gold Member

    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.
     
  6. Aug 21, 2016 #5
    So could it be a kind of Ventury effect?
     
  7. Aug 22, 2016 #6

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    You mean a centripetal force component, perpendicular to their velocity? This usually comes from a pressure gradient.
     
  8. Aug 22, 2016 #7

    tech99

    User Avatar
    Gold Member

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted