  A methodology is proposed for the calculation of the truncation error of
finite volume discretisations of the incompressible Navier-Stokes equations on
colocated grids. The truncation error is estimated by restricting the solution
obtained on a given grid to a coarser grid and calculating the image of the
discrete Navier-Stokes operator of the coarse grid on the restricted velocity
and pressure field. The proposed methodology is not a new concept but its
application to colocated finite volume discretisations of the incompressible
Navier-Stokes equations is made possible by the introduction of a variant of
the momentum interpolation technique for mass fluxes where the pressure-part of
the mass fluxes is not dependent on the coefficients of the linearised momentum
equations. The theory presented is supported by a number of numerical
experiments. The methodology is developed for two-dimensional flows, but
extension to three-dimensional cases should not pose problems.
