Objective: When a powder suspension in a flow inhomogeneously disperses, the particles form a cluster. Cluster is defined as the state where the fluid and particles move together through mutual fluid-dynamic interference. This represents a middle state between the lean scatter state, which is considered to be similar to single-particle sedimentation, and the agglomerate state. This study investigates the critical void ratio of the change from the cluster state to a lean scatter state. Design: Direct numerical simulations of a spherical particle cluster subjected to a uniform flow are performed, and the influence of the flow on the cluster under various void ratios is analyzed based on the calculation results. Methods: The relationship between the cluster void ratio (or particle volume fraction) and relative velocity between the flow and cluster, flow structure observed in visualization maps, velocity distribution of air flow passing through a cluster, and the fluid-dynamic energy budget were analyzed. Results: (i) When the void ratio of a particle clusters shifts from 0.96 to a higher value of 0.98, the mutual interference effect is lost and the powder suddenly shifts to a lean scatter state. (ii) The kinetic energy of the cluster and particle volume fraction phi(= 1 - epsilon) are inversely proportional. (iii) With regard to the fluid-dynamic energy budget around the cluster immediately after being subjected to air flow, a very strong mutual interference is confirmed (i.e., the cluster is similar to a rigid body) when epsilon = 0.850. At epsilon = 0.998 there is only a weak mutual interference, and so the powder behaves as a single particle, but the effect on the cluster remains. Conclusions: The panicle volume fraction and kinetic energy of the cluster retain an inverse proportional relationship while the cluster state continuously shifts to a lean scatter state. (C) 2015 Elsevier B.V. All rights reserved.