The location of surrounding particl

The location of surrounding particles can severely affect the drag coefficients of the particles involved [13,14]. Liang, Hong and Fan [15] proved that the alteration of the position of surrounding particles can change the drag coefficient significantly. A configuration of three coaligned particles led to a reduction of drag experienced by the centre particle compared with that of the leading particle at a separation distance of 2–3 dp. The study looked at several particle orientations and separation distances and also considered the effect of wall location on perceived drag force. Cheng and Papanicolaou [16] calculated the analytical force on an array of particles at low Reynolds numbers and volume fraction. The analytical results showed good comparison with the phenomenological results available at the time of this work. Kim, Elghobashi and Sirignano [17] solved the full Navier-Stokes equations for spherical particle motion at a range of Reynolds numbers and particle-to-fluid density ratios. The full Navier-Stokes solution showed considerable differences to some of the more commonly used particle motion formulas, and resulted in the authors proposing their own new particle motion formula. The present study provides experimental data which can provide a basis for the development of drag coefficient formula accounting for surrounding particles.