  We consider quasiparticle propagation in constant-speed-of-sound (iso-tachic)
and almost incompressible (iso-pycnal) hydrodynamic flows, using the technical
machinery of general relativity to investigate the ``effective space-time
geometry'' that is probed by the quasiparticles. This effective geometry,
described for the quasiparticles of condensed matter systems by the
Painleve-Gullstrand metric, generally exhibits curvature (in the sense of
Riemann), and many features of quasiparticle propagation can be re-phrased in
terms of null geodesics, Killing vectors, and Jacobi fields. As particular
examples of hydrodynamic flow we consider shear flow, a constant-circulation
vortex, flow past an impenetrable cylinder, and rigid rotation.
