Self-adaptation and viscous selection in concentrated two-dimensional vortex dipoles

In this Letter we deal with 2D direct numerical simulations of concentrated vortex dipoles. We show that various initial dipolar vorticity distributio ns evolve towards a specific family of dipoles parametrized by the dipole a spect ratio a/b, where a is the radius of the vortices based on the vortici ty polar moment in half a plane and b is the separation between the vortex centroids. This convergence is achieved through viscous effects. The consid ered Reynolds numbers Re=Gamma/nu are Re=3000 and Re=15000. Moreover, all t he dipoles of this family are quasi-steady solutions of the Euler equations . Their scatter plots and drift velocities are given for a/b < 0.3. (C) 200 0 American Institute of Physics. [S1070-6631(00)02602-7].