  We propose a ballistic coalescence model (punctuated-Hamiltonian approach)
mimicking the fusion of vortices in freely decaying two-dimensional turbulence.
A temporal scaling behaviour is reached where the vortex density evolves like
$t^{-\xi}$. A mean-field analytical argument yielding the approximation
$\xi=4/5$ is shown to slightly overestimate the decay exponent $\xi$ whereas
Molecular Dynamics simulations give $\xi =0.71\pm 0.01$, in agreement with
recent laboratory experiments and simulations of Navier-Stokes equation.
