Hyperbolicity and change of type in sink flow

Abstract
We consider the problem of steady fast flow of a family of Oldroyd fluids into a hole, and show that the field of flow is partitioned into elliptic (subcritical) and hyperbolic (supercritical) regions. We analyse the characteristics and show that the vorticity changes type as in the experiments of Metzner, Uebler & Fong (1969).