Non-Brownian suspensions: simulation and linear response

We have developed and applied numerical techniques to study the dynamics of non-Brownian suspensions of spheres in viscous fluids. The numerical appro aches reproduce experimental results like the mean settling velocity and pr essure drops in particle arrays for solid volume fractions up to about 0.30 in 3D. In two dimensions we study the correlations between the velocity an d the density distribution at small particle Reynolds numbers Re approximat e to 1 under the influence of gravity both by full hydrodynamic simulations and by linear analysis of corresponding continuum equations. For the case of the stratification of a dense homogeneous fluids on top of a less dense one, classical Rayleigh-Taylor theory predicts exponential growth of the ar ising initial velocity fluctuations. We find, however, that the system is d riven by the initial density inhomogeneities that necessarily exist in a pa rticulate suspension. The corresponding velocity modes saturate exponential ly. The spatial correlation length of the emerging fingers grows in time un til it reaches a limit, which, in our simulations, depends on the system si ze. (C) 1999 Elsevier Science B.V. All rights reserved.