Moderately strong vorticity in a bathtub-type flow

Theoretical and numerical analysis is performed for an inviscid axisymmetri c vortical bathtub-type flow. The level of vorticity is kept high so that t he image of the flow on the radial-axial plane (r-z plane) is not potential . The most significant findings are: (1) the region of validity of the stro ng vortex approximation is separated from the drain by a buffer region, (2) the power-law asymptote of the stream function, specified by Delta Psi sim ilar to r(4/3) Deltaz, appears near the axis when vorticity in the flow is sufficiently strong and (3) the local Rossby number in the region of the 4/ 3 power-law the initial vorticity level in the flow and the global Rossby n umber.