  Use of curvilinear coordinates is sometimes indicated by the inherent
geometry of a fluid dynamics problem, but this introduces fictitious forces
into the momentum equations that spoil strict conservative form. If one is
willing to work in three dimensions, these fictitious forces can be eliminated
by solving for rectangular (Cartesian) momentum components on a curvilinear
mesh. A thoroughly geometric approach to fluid dynamics on spacetime
demonstrates this transparently, while also giving insight into a greater unity
of the relativistic and nonrelativistic cases than is usually appreciated.
