  We study suspensions of oblate rigid particles in a viscous fluid for
different values of the particle volume fractions. Direct numerical simulations
have been performed using a direct-forcing immersed boundary method to account
for the dispersed phase, combined with a soft-sphere collision model and
lubrication corrections for short-range particle-particle and particle-wall
interactions. With respect to the single phase flow, we show that in flows
laden with oblate spheroids the drag is reduced and the turbulent fluctuations
attenuated. In particular, the turbulence activity decreases to lower values
than those obtained by only accounting for the effective suspension viscosity.
To explain the observed drag reduction we consider the particle dynamics and
the interactions of the particles with the turbulent velocity field and show
that the particle wall layer, previously observed and found to be responsible
for the increased dissipation in suspensions of spheres, disappears in the case
of oblate particles. These rotate significantly slower than spheres near the
wall and tend to stay with their major axes parallel to the wall, which leads
to a decrease of the Reynolds stresses and turbulence production and so to the
overall drag reduction.
