Stokes Drag on Self-Similar Clusters of Spheres

Abstract

Aggregation phenomena in gas and liquid phases often produce agglomerates that appear to be fractals in the sense that the cluster mass varies with the radius of gyration to a fractional power. The creeping (Stokes) flow around the cluster may be represented by the superposition of k single-sphere velocity fields chosen to satisfy the boundary conditions at k points on the cluster. Computations can be performed for large self-similar clusters of k" monomers by decomposing the cluster n times into smaller clusters which may be replaced by hydrodynamically equivalent spheres. The method recovers the correct results for chains, disks, and compact clusters. Computations done on random, connected clusters similar to real aggregates show that both the primary particle diameter and the cluster radius influence the drag. The influence of the primary particle size and other small-scale structure features is lost very rapidly as the fractal dimension increases above 1.3 and the number of monomers in the cluster increases above 100.

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