Numerical simulations with a finite-difference method have revealed th
at a Lamb dipole when placed in a viscous fluid moves along a straight
line with decreasing velocity and increasing radius. The relationship
between vorticity and streamfunction, which initially is linear, beco
mes more and more sinh-like as the dipole decays. Some other initial d
ipolar vorticity distributions (like two oppositely signed monopolar v
ortices) were found to evolve to a dipolar structure with Lamb-like ch
aracteristics. (C) 1998 The Japan Society of Fluid Mechanics Incorpora
ted and Elsevier Science B.V. All rights reserved.