A procedure is described for limiting the void fraction in fluid particle systems, computed by means of numerical multiphase flow simulation codes, to values which do not fall below those realisable in practice. It is based on a computation of the particle-particle contact forces which come into play only when computed void fractions fall to values below those corresponding to random packing of the particles. The general method is illustrated with reference to the process of sedimentation using a specific fluid-dynamic formulation of the equations of change for fluidization. Without the particle contact force algorithm, the particles compact to the physically meaningless void-fraction of 0.17; with the algorithm the random packing value of 0.4 is achieved.