Dynamics of dual-particles settling under gravity

As a first step towards understanding particle-particle interaction in flui d flows, the motion of two spherical particles settling in close proximity under gravity in Newtonian fluids was investigated experimentally for parti cle Reynolds numbers ranging from 0.01 to 2000. It was observed that partic les repel each other for Re > 0.1 and that the separation distance of settl ing particles is Reynolds number dependent. At lower Reynolds numbers, i.e. for Re < 0.1, particles settling under gravity do not separate. The orientation preference of two spherical particles was found to be Reyno lds number dependent. At higher Reynolds numbers, the line connecting the c entres of the two particles is always horizontal, regardless of the way the two particles are launched. At lower Reynolds numbers, however, the partic le centreline tends to tilt to an arbitrary angle, even of the two particle s are launched in the horizontal plane. Because of the tilt, a side migrati on of the two particles was found to exist. A linear theory was developed t o estimate the side migration velocity. It was found that the maximum side migration velocity is approximately 6% of the vertical settling velocity, i n good agreement with the experimental results. Counter-rotating spinning of the two particles was observed and measured in the range of Re = 0-10. Using the linear model, it is possible to estimate the influence of the tilt angle on the rate of rotation at low Reynolds nu mbers. Dual particles settle faster than a single particle at small Reynold s numbers but not at higher Reynolds numbers, because of particle separatio n. The variation of particle settling velocity with Reynolds number is pres ented. An equation which can be used to estimate the influence of tilt angl e on particle settling velocity at low Reynolds number is also derived. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.