Particle wake effects on the drag force of an interactive particle

Direct measurements of the drag force on two interacting particles arranged in the longitudinal direction for particle Reynolds numbers (Re) varying from 20 to 130 are conducted using a microforce measurement system. The system is capable of measuring the force up to 50 g with an accuracy of 1 mg. The effects of the interparticle distance and Re on the drag forces of both the trailing and leading particles are examined. An empirical equation is obtained to describe the drag force variation of a single particle trailing in the wake of a leading particle. A mechanistic model is also developed to account for the motion of the trailing particle in the wake region of the leading particle as well as for their contact velocity. Moreover, a simultaneous flow-field visualization is conducted to characterize the wake flow phenomena. The results indicate that Re affects not only the magnitude of the drag force of an interacting particle but also its variation with the interparticle distance. At contact, the drag force of a trailing particle can be less than one-fifth of the drag force of a single particle without interparticle effects. Both tbe drag force measurements and the flow-field visualization reflect that the wake-influenced region of the leading particle can be longer at a lower Re. The mechanistic model suggests that both the drag force and the Basset force should be included in the description of the motion of the trailing particle in the wake region of the leading particle. The model predicting the contact velocity of the two particles is reasonably validated with the experimental data.