  We introduce a finite volume scheme for the two-dimensional incompressible
Navier-Stokes equations. We use a triangular mesh. The unknowns for the
velocity and pressure are both piecewise constant (colocated scheme). We use a
projection (fractional-step) method to deal with the incompressibility
constraint. We prove that the differential operators in the Navier-Stokes
equations and their discrete counterparts share similar properties. In
particular, we state an inf-sup (Babuska-Brezzi) condition. We infer from it
the stability of the scheme.
