  It is shown that laminar vortex rings can be generated by impulsive body
forces having particular spatial and temporal characteristics. The method
produces vortex rings in a fluid initially at rest, and once generated, the
flow field automatically satisfies the boundary conditions and is
divergence-free. Numerical simulations and analytical models show that the
strength of these rings can be accurately predicted by considering diffusion
alone, despite the nonlinear nature of the generation process. A particularly
simple model, which approximates the source of vorticity within vertical slabs,
is proposed. This model predicts the ring circulation almost as accurately as a
model which uses the exact geometry of the source of vorticity. It is found
that when the duration of the force is less than a time scale based on the
force radius and fluid viscosity, the ring circulation can be predicted
accurately using an inviscid model.
