  We study turbulent channel flows of monodisperse and polydisperse suspensions
of finite-size spheres by means of Direct Numerical Simulations using an
immersed boundary method to account for the dispersed phase. Suspensions with 3
different Gaussian distributions of particle radii are considered (i.e. 3
different standard deviations). The distributions are centered on the reference
particle radius of the monodisperse suspension. In the most extreme case, the
radius of the largest particles is 4 times that of the smaller particles. We
consider two different solid volume fractions, 2% and 10%. We find that for all
polydisperse cases, both fluid and particles statistics are not substantially
altered with respect to those of the monodisperse case. Mean streamwise fluid
and particle velocity profiles are almost perfectly overlapping. Slightly
larger differences are found for particle velocity fluctuations. These increase
close to the wall and decrease towards the centerline as the standard deviation
of the distribution is increased. Hence, the behavior of the suspension is
mostly governed by excluded volume effects regardless of particle size
distribution (at least for the radii here studied). Due to turbulent mixing,
particles are uniformly distributed across the channel. However, smaller
particles can penetrate more into the viscous and buffer layer and velocity
fluctuations are therein altered. Non trivial results are presented for
particle-pair statistics.
