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!