The point vortex model predicts that a certain configuration of three
point vortices leads to a collapse of these vortices to one point. Num
erical simulations have been performed to investigate the effect of a
finite vortex size on this two-dimensional collapse interaction. The p
aper presents results obtained with contour dynamics simulations of pa
tches of uniform vorticity, and results obtained with finite differenc
e simulations of vortices with continuous properties. In addition, the
effect of viscosity and the presence of impermeable domain boundaries
are investigated. The results show that the motion of finite-sized vo
rtices is quite similar to the motion of point vortices as long as the
mutual distance between the vortices is larger than their size. When
the vortices are closer together their shapes start to deform and the
subsequent evolution is different from that of the point vortices, and
an actual collapse to one vortex does not take place. (C) 1997 Americ
an Institute of Physics.