We study the time evolution of an incompressible fluid with axial symmetry without swirl, assuming initial data such that the initial vorticity is very concentrated inside N small disjoint rings of thickness $$\varepsilon $$
ε
and vorticity mass of the order of $$|\log \varepsilon |^{ -1}$$
|
log
ε
|
-
1
. When $$\varepsilon \rightarrow 0$$
ε
→
0
, we show that the motion of each vortex ring converges to a simple translation with constant speed (depending on the single ring) along the symmetry axis. We obtain a sharp localization of the vorticity support at time t in the radial direction, whereas we state only a concentration property in the axial direction. This is obtained for arbitrary (but fixed) intervals of time. This study is the completion of a previous paper [5], where a sharp localization of the vorticity support was obtained both along the radial and axial directions, but the convergence for $$\varepsilon \rightarrow 0$$
ε
→
0
worked only for short times.