Settling of inertial nonspherical particles in wavy flow

Microplastics are an increasingly significant problem in the world's oceans. They are transported by various ocean phenomena, one of the most fundamental of which is surface gravity waves. Since microplastics are irregularly shaped and are not typically neutrally buoyant, understanding the settling of negatively buoyant, nonspherical plastic particles under surface gravity waves is important for accurately predicting the fate of microplastics in the ocean. Here we experimentally investigate the settling of plastic rods, disks, and spheres in wavy flows. We find that the average vertical velocities of the particles can both increase and decrease in waves, relative to the particle settling velocity in quiescent flow. This variation is a function of the flow inertia at the length scale of the particle, which we characterize with a particle Reynolds number, Rep, and is also a function of particle shape. We further examine the average vertical particle velocities by looking at two factors contributing to their behavior: The relative velocities between the particles and the flow and the manner in which the particles sample the flow. We find that the average relative velocities between the particles and the flow remain constant with Rep, even though the variation of the relative velocities of the rods with orientation increases with increasing Rep. The observed variation of the average vertical particle velocities with Rep can be explained instead by how the particles sample the flow, as each of the particle shapes nonuniformly sample the flow as a function of Rep. Accounting for the variation of particle settling velocities with shape and inertia in models is necessary to improve the accuracy of predictions of the transport of microplastics in the ocean.