  A new formulation is derived for the commutator-errors in large-eddy
simulation of incompressible flow. These commutator-errors arise from the
application of non-uniform filters to the Navier-Stokes equations. As a
consequence, the filtered velocity field is no longer solenoidal. The order of
magnitude of the commutator-errors is compared with the divergence of the
turbulent stresses. This shows that one can not reduce the size of the
commutator-errors independently of the turbulent stress terms by some judicious
construction of the filter operator. Similarity modeling for the
commutator-errors is presented, including an extension of Bardina's approach
and the application of Leray regularization. The performance of the
commutator-error parameterization is illustrated with the one-dimensional
Burgers equation. For large filter-width variations the Leray approach is shown
to capture the filtered flow with better accuracy than is possible with
Bardina's approach.
