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−ξ. A mean-field analytical argument yielding the approximation
ξ=4/5 is shown to slightly overestimate the decay exponent ξ whereas
Molecular Dynamics simulations give ξ=0.71±0.01, in agreement with
recent laboratory experiments and simulations of Navier-Stokes equation.Comment: 6 pages, 1 figure, to appear in Europhysics Letter