  I explain a generalization of Bjorken flow where the medium has finite
transverse size and expands both radially and along the beam axis. If one
assumes that the equations of viscous hydrodynamics can be used, with
p=epsilon/3 and zero bulk viscosity, then the flow I describe can be developed
into an exact solution of the relativistic Navier-Stokes equations. The local
four-velocity in the flow is entirely determined by the assumption of symmetry
under a subgroup of the conformal group.
