We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersedin a turbulent channel flow. We performed several Direct Numerical Simulations using an Immersed Boundary Method forfinite-size particles changing the solid to fluid density ratio R, the mass fraction and the volume fraction. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statisticsas much as when varying the volume fraction at constant R and at constant mass fraction.
Interestingly, the increase in overall drag found when varying the volume fraction is considerablyhigher than that obtained for increasing density ratios at same volume fraction. The main effect atdensity ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When thedensity ratio R is further increased up to 100 the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas andthe fluid and solid phase statistics drastically change. In this regime, the collisionrate is high and dominated by the normal relative velocity among particles.