Effect of mass distribution on falling cylindrical particles at intermediate Reynolds numbers

In natural sedimentation, many particles of interest are both large and nonspherical. Some common particle types (e.g. naturally occurring aggregates) do not have a uniform mass distribution. As a result, the centers of mass and buoyancy are not co-located, leading to more complex settling dynamics. Here we investigated the orientation and terminal velocity of freely falling cylinders, in which the mass distribution was either constant (uniform-density, UD) or bipartite, undergoing a step function halfway along the length (compound-density, CD). Cylinders had relatively low aspect ratios (1<AR<4), and fell at intermediate Reynolds numbers (of order 100). The cylinders, initially horizontal, were released at the top of a tall hexagonal still-water tank, and imaged by a high-speed camera. Two low-speed cameras simultaneously captured 1) full cylinder trajectory and 2) landing position. We recorded the terminal velocity, fall orientation, and landing site of each cylinder. Results showed significant differences in the settling characteristics of uniform- vs. compound-density cylinders. UD cylinders of AR=1 fell broadside initially, whereas AR=1 CD cylinders fell vertically. However, both cases showed oscillation in cylinder orientation upon descent. UD cylinders with AR=2 and AR=4 consistently fell broadside, with minimal cylinder axis oscillation. CD cylinders with AR=2 fell with two different modes. In mode 1, cylinders rotated 90° from their initial orientation before beginning to oscillate about the vertical axis. In mode 2, cylinder orientation remained constant at a slight angle from the horizontal. This mode was also observed in the CD AR=4 cylinders, which fell at a constant (tilted) orientation angle and moved horizontally as they fell. The landing sites for all CD cylinders were biased toward the side of the target where the denser end of the cylinder was initially oriented, whereas UD cylinders landed in a uniform distribution around the tank center. In general, cylinders with the smallest vertical projected area fell with the greatest terminal velocity; however, the mechanisms controlling orientation remain unclear. Our results have important implications for predicting the settling behavior of naturally-occurring particles, and lay the groundwork for further study of particles settling in complex flows such as turbulence. Given our results in still water, the interplay between the buoyant torques created by the offset between the center of mass and center of volume are likely to strongly impact particle motion in turbulence.