Wall effects for motion of spheres in power-law fluids

The steady motion of spheres representing particles inside tubes filled wit h different fluids has been investigated using both a finite-element and a finite-volume method. The rheology of the fluids has been modelled by the p ower-law able to describe the shear-thinning (pseudoplastic) behaviour of a series of polymer solutions. New results have been obtained for a series o f tube/sphere diameter ratios in order to investigate the wall effects on t he drag exerted by the fluid on the sphere. The results agree well with pre vious simulations for an unbounded medium (infinite diameter ratio). Experi mental investigations have also been carried out and simulated, and the res ults compare favourably with the experiments. The present simulations revea led the convergence of the drag coefficient to a constant value independent of tube-to-sphere diameter ratio when the power-law index approaches zero. (C) 2001 Elsevier Science B.V. All rights reserved.