Orientation and tumbling of inertial rod and disk particles in a turbulent boundary layer*

The behavior of inertial disk and rod particles in an open channel flow are investigated experimentally in order to examine the effects of shape on their orientation and rotation in turbulence. The friction Reynolds number of the flow is Re_τ~600, and the particle Stokes number based on the viscous timescale is St⁺~O(10). Particle tracking velocimetry is used to obtain time-resolved particle trajectories. 3D orientations and tumbling rates of the rods and disks are reconstructed from their silhouettes projected onto the imaging plane. Rods tend to orient in the streamwise direction, while disks prefer to align their symmetry axis normal to the wall. This alignment is much more stable for disks. Rods undergo stronger tumbling in the near-wall region, and they tumble freely in response to the mean shear and turbulent fluid velocity fluctuations, whereas disks wobble about their preferential wall-normal orientation. In addition, ascending rods have higher tumbling rates than descending rods; this trend is not observed for disks. This implies a correlation between the turbulence events that resuspend rods and those that cause them to tumble.