  We study the effect of varying the mass and volume fraction of a suspension
of rigid spheres dispersed in a turbulent channel flow. We performed several
Direct Numerical Simulations using an Immersed Boundary Method for finite-size
particles changing the solid to fluid density ratio, the mass fraction and the
volume fraction. We find that varying the density ratio between 1 and 10 at
constant volume fraction does not alter the flow statistics as much as when
varying the volume fraction at constant and at constant mass fraction.
Interestingly, the increase in overall drag found when varying the volume
fraction is considerably higher than that obtained for increasing density
ratios at same volume fraction. The main effect at density ratios of the order
of 10 is a strong shear-induced migration towards the centerline of the
channel. When the density ratio is further increased up to 1000 the particle
dynamics decouple from that of the fluid. The solid phase behaves as a dense
gas and the fluid and solid phase statistics drastically change. In this
regime, the collision rate is high and dominated by the normal relative
velocity among particles.
