Sedimentation of a bidispersed cluster of solid spherical particles

A new method for the experimental study of gravitational sedimentation of a polydisperse cluster of solid spherical particles in a viscous fluid is presented. The method is based on the preliminary ultrasonic mixing of the particles in a spherical container and assumes the introduction of a spherical cluster of particles at a given concentration and zero initial velocity into a fluid. This method is used to determine sedimentation characteristics of a bidisperse cluster of particles (steel balls, $2$ and $3$ mm in diameter) in silicone oil. A qualitative pattern of the cluster evolution, a sedimentation rate, and a drag coefficient are obtained. A comparative analysis of sedimentation characteristics of monodisperse and bidisperse particle clusters is carried out in the range of Reynolds numbers $\mathrm{Re}=(0.30 \div 0.66)\cdot 10^{-3}$. It is shown that, in contrast to a monodisperse cluster of particles, the drag coefficient of the bidisperse cluster of particles does not correspond to a correlation $C_D=24/\mathrm{Re}_c$ for the Stokes sedimentation.