A new methodology to study the free-settling motion of single particles in a fluid at ReP < 5000 within a three-dimensional reference frame is presented. After certain value of ReP the descend of any non-spherical solid is characterised by secondary motions which alter the particle orientation, therefore, their influence on quantities such as the velocity and drag coefficient have to be investigated within a context that ensures the interaction between the solid and the fluid is preserved undisturbed. To meet this requirement, this work is entirely based on high-speed imaging techniques, such as stereo vision and Schlieren photography, combined with digital image processing, vector algebra, and differential geometry operations. For spherical particles, the evolution of the structures in the surrounding fluid and their impact on the settling trajectories was observed. Additionally, a strong similarity between the calculated values of the coefficient of drag and those from literature correlations was achieved, which validates the proposed methodology. For non-spherical solids, it was found that after ReP > 200, secondary motions such as oscillation, gliding, and tumbling occurred and caused the formation of a turbulent wake structure in the fluid surrounding the particle. Their effect on the solids velocity and drag coefficient was also quantified, being more significant on disks than on cylinders, however a direct relation between the angular orientation changes and the drag coefficient could not be suggested. It was noticed too that for irregular particles the secondary motions were not evenly defined, and that the settling may be accompanied by continuous rotations, even at low ReP, which further alter the trajectory, orientation, and surrounding fluid structure, thus complicating the quantification of the motion parameters and imposing restrictions on the visualisation systems employed here.
