Sedimentation of finite-size particles in quiescent wall-bounded shear-thinning and Newtonian fluids

We study the sedimentation of finite-size particles in quiescent wall-bounded Newtonian and shear- thinning fluids by interface resolved numerical simulations. The suspended phase consists of Non- Brownian rigid spherical particles with particle to fluid density ratio 1.5 at three different solid volume fractions 1%, 5% and 20%. Firstly, to focus on the effect of shear-thinning on the particle dynamics and interactions, the Archimedes number is increased for a single particle to have the same settling speed in the Newtonian fluid as in the shear-thinning fluid. Secondly, we consider fixed Archimedes and vary the shear-thinning properties of the fluid. Overall, we report a twofold effect of shear thinning. First and more important, the substantial increase of the particle sedimentation velocity in the shear- thinning case due to the increase of the shear rate around the particles, which reduces the local viscosity leading to a reduced particle drag. Secondly, the shear-thinning fluid reduces the level of particle interac- tions, causing a reduction of velocity fluctuations and resulting in particles sedimenting at approximately the same speed. Moreover, the mean settling velocities decrease with the particle concentration as a con- sequence of the hindering effect. Particles tend to sediment in the middle of the channel, preferentially positioning in the wake of neighbouring particles or aside them, resulting in lower levels of fluid velocity fluctuations in the gravity direction in the shear-thinning fluid.