Drag on two coaxial, nonuniformly structured flocs in a uniform flow field.

The drag on two coaxial flocs of nonuniform structure in a Newtonian fluid is evaluated for Reynolds number ranges from 0.1 to 40. A two-layer model is adopted to simulate various possible structures of a floc. The influences of the key parameters of the problem under consideration, including the separation distance between two flocs, the Reynolds number, and the ratios (inner radius/outer radius) and (permeability of outer layer/permeability of inner layer), on the drag acting on two flocs are investigated. We show that if Reynolds number is small, the drag on the leading floc is about the same as that on the rear floc. However, if Reynolds number is sufficiently large, because wakes are formed in the rear region of the leading floc, the drag on it is greater than that on the rear floc. For a fixed mean permeability, the more nonuniform the floc structure is, the more important is its influence on the drag, and the more appreciable the deviation of the drag coefficient-Reynolds number curve from a Stokes'-law-like relation.