Straight and curved cylindrical rods settling in quiescent fluid with application to atmospheric microplastics

In this study, an experimental analysis is conducted on the settling of straight and curved cylindrical rods, which are used to replicate a Reynolds number range applicable to atmospheric settling of microplastic fibres. The rods are dropped in a chamber filled with a quiescent water–glycerin mixture, and their settling velocity is determined from the images of two cameras arranged with perpendicular views. It is shown that a curved cylindrical rod settles faster than a straight cylindrical rod with the same diameter and length. As the rod radius of curvature decreases, the terminal velocity increases, and the corresponding drag coefficient decreases. The maximum difference in the terminal velocity between the straight and curved rods depends on the rod aspect ratio, curvature index, and Reynolds number. A new semi-empirical model is also developed to estimate the drag coefficient and terminal velocity of both straight and curved cylindrical rods studied in this research. The results of the new model are significantly more consistent with the experimental data compared to the previous models, with a low RMS error of 6.8%. This novel model has been utilized to predict the terminal velocity of realistic fibres in the atmosphere.