  One issue associated with the use of Large-Eddy Simulation (LES) to
investigate the dispersion of small inertial particles in turbulent flows is
the accuracy with which particle statistics and concentration can be
reproduced. The motion of particles in LES fields may differ significantly from
that observed in experiments or direct numerical simulation (DNS) because the
force acting on the particles is not accurately estimated, due to the
availability of the only filtered fluid velocity, and because errors accumulate
in time leading to a progressive divergence of the trajectories. This may lead
to different degrees of inaccuracy in the prediction of statistics and
concentration. We identify herein an ideal subgrid correction of the a-priori
LES fluid velocity seen by the particles in turbulent channel flow. This
correction is computed by imposing that the trajectories of individual
particles moving in filtered DNS fields exactly coincide with the particle
trajectories in a DNS. In this way the errors introduced by filtering into the
particle motion equations can be singled out and analyzed separately from those
due to the progressive divergence of the trajectories. The subgrid correction
term, and therefore the filtering error, is characterized in the present paper
in terms of statistical moments. The effects of the particle inertia and of the
filter type and width on the properties of the correction term are
investigated.
