LEAPFROGGING VORTEX RINGS AS SCALING LIMIT OF EULER EQUATIONS

We consider an incompressible fluid with axial symmetry without swirl, assuming initial data such that the initial vorticity is very concentrated inside Ν small disjoint rings of thickness ε, each one of vorticity mass and main radius of order |logε|. When ε→0, we show that, at least for small but positive times, the motion of the rings converges to a dynamical system first introduced in [C. Marchioro and P. Negrini, NoDEA Nonlinear Differential Equations Appl., 6 (1999), pp. 473-499]. In the special case of two vortex rings with large enough main radius, the result is improved reaching longer times, in such a way to cover the case of several overtakings between the rings, thus providing a mathematical rigorous derivation of the leapfrogging dynamics.