ON THE INTERACTION BETWEEN 2 OPPOSITELY SIGNED, SHIELDED, MONOPOLAR VORTICES

The formation two-dimensional dipolar vortices by the interaction betw een two shielded monopolar vortices with opposite vorticity, as shown in a numerical study by Couder and Basdevant,(1) is investigated in de tail, both experimentally, in a nonrotating stratified fluid and numer ically by direct solutions of the two-dimensional Navier-Stokes equati ons. A comparative study between the laboratory experiments and numeri cal simulations is performed. The vorticity distribution measured in t he early stage of the evolution in the laboratory is used as initial d ata for the simulations, and an additional damping term in the Navier- Stokes equations, that accounts for the vertical diffusion in the labo ratory experiments, is used. The results show that, depending on the i nitial separation between the vortices, the shields of the monopoles a re peeled off and indeed a compact dipole with a linear (omega,psi)-re lationship is formed, or when the monopoles are further apart the shie lds of the monopoles are perturbed and two tripoles are formed. The ch aracteristics of the emerged dipole are analyzed and a dye visualizati on of the dipole formation is performed. A second, more general numeri cal study yields a relationship between the formation time of the dipo le and the initial separation distance between the monopoles and it sh ows that the deshielding process can be explained by the domination of strain over vorticity. (C) 1998 American Institute of Physics. [S1070 -6631(98)02212-0].