  Light experiences a non-uniformly moving medium as an effective gravitational
field, endowed with an effective metric tensor $\tilde{g}^{\mu \nu}=\eta^{\mu
\nu}+(n^2-1)u^\mu u^\nu$, $n$ being the refractive index and $u^\mu$ the
four-velocity of the medium. Leonhardt and Piwnicki [Phys. Rev. A {\bf 60},
4301 (1999)] argued that a flowing dielectric fluid of this kind can be used to
generate an 'optical black hole'. In the Leonhardt-Piwnicki model, only a
vortex flow was considered. It was later pointed out by Visser [Phys. Rev.
Lett. {\bf 85}, 5252 (2000)] that in order to form a proper optical black hole
containing an event horizon, it becomes necessary to add an inward radial
velocity component to the vortex flow. In the present paper we undertake this
task: we consider a full spiral flow, consisting of a vortex component plus a
radially infalling component. Light propagates in such a dielectric medium in a
way similar to that occurring around a rotating black hole. We calculate, and
show graphically, the effective potential versus the radial distance from the
vortex singularity, and show that the spiral flow can always capture light in
both a positive, and a negative, inverse impact parameter interval. The
existence of a genuine event horizon is found to depend on the strength of the
radial flow, relative to the strength of the azimuthal flow. A limitation of
our fluid model is that it is nondispersive.
