Screening mechanisms in sedimentation

This paper considers a mixture of sedimenting particles at low Reynolds num bers and volume fractions. Simple theoretical arguments have long suggested that for a random suspension of particles in an infinite system, the fluct uations in the velocity about the mean should diverge with system size. On the other hand, experiments have shown no such divergence. The primary goal of this paper is to examine the effect of side walls on the predicted dive rgence in fluctuations, through theory, scaling arguments, and numerical si mulations. Side walls lead to important modifications of the standard argum ents. A scaling argument (based on wall effects) is presented to rationaliz e recent experiments by Segre et al. [Phys. Rev. Lett, 79, 2574 (1997)]. Th e paper also briefly discusses the role of inertia in screening fluctuation s in infinite systems when the particle Reynolds number is very low, and al so the coupling between the velocity fluctuations and the mean sedimenting velocity. A physical argument suggests that in some circumstances the fluct uations give a leading order correction to the mean sedimenting velocity as a function of volume fraction. (C) 1999 American Institute of Physics. [S1 070-6631(99)02104-2].