  The equations for LES are formally derived by low-pass filtering the NS
equations with the effect of the small scales on the larger ones captured by a
SGS model. However, it is known that the LES equations usually employed in
practical applications are inconsistent with the filter operator when no
explicit filter is used. Moreover, even for explicitly filtered LES, some
inconsistencies remain in the wall-normal direction due to the constraining
effect of the wall. A typically undesirable effect stemming from the
inconsistency between LES and the filtered NS (fNS) equations is that DNS data
can not be used to aid SGS modeling. Using the proposed form of the
incompressible fNS equations for flows over flat walls in which the continuity
equation, the filter operator, and the boundary condition at the wall are
consistently formulated, we compute a set of plane turbulent channel flow
explicitly filtered LES at different grid resolutions with the SGS term
computed from a parallel DNS computation starting with an equivalent initial
condition. The results show that the SGS term computed from DNS provide a
truthful SGS model for short times until the numerical errors compound and the
two parallel computations diverge. The effective time where the statistics
remain similar is longer for lower-order statistics such as mean velocity
profile, as expected. This demonstrates that with the consistent fNS equations,
the abundant DNS data can be used to inform SGS model development for
explicitly filtered LES. Moreover, the diverging of statitics at longer times
shows that the SGS contribution is not necessarily homogeous as most SGS models
assume, and a more precise correlation between the instantaneous LES velocity
field and SGS model is required to capture the dynamics of turbulence.
