Critique of camp and Stein’s RMS velocity gradient

The analysis of relative fluid motion by Camp and Stein, which led to their well-known formulation of particle collision rate in flocculation, is examined using standard tools from continuum mechanics. As a result of certain apparent conceptual errors, including the notion that a three-dimensional relative velocity field can be represented in general by a single velocity gradient, several recommendations are offered, including the abandonment of Camp and Stein’s terms “absolute velocity gradient” and “root mean square (rms) velocity gradient” Revisions in nomenclature and an alternate theory of particle collisions are discussed, as well as some implications of inhomogeneity in the turbulent flocculation problem.