Rocking and rolling in a wind tunnel

Coarse particles, such as sand, can be moved by the wind, creating ripples and dunes in a process known as incipient aeolian entrainment. Zhao et al. carried out experimental studies of the phenomenon using a unique approach involving ping pong balls in a wind tunnel.

This technique allowed wind tunnel experiments to be carried out at much lower speeds, since the balls are much lighter than natural particles of sand or gravel. Even though they are light, they are 67 times denser than air, so their motions are still governed by the same aerodynamic forces that drive sand dune formation.

The group observed two types of entrainments in their experiments: incomplete entrainment, or rocking, in which the wind moves a particle back and forth over a depression in the surface; and full entrainment, or rolling, in which the particle breaks free and is blown across the rough surface.

The investigators observed two types of rocking motions: one in which the ball rocks forward and back only once, and another type involving multiple rocking movements. They also observed two types of rolling: prompt dislodgement, in which a particle breaks free immediately, and tardy dislodgement, in which a particle decelerates and hovers before accelerating again and breaking free.

“In our experimental study, the threshold level of angular displacements required to achieve full rolling entrainment was measured and found to be 0.41 pi,” said author Manousos Valyrakis. This constitutes the first measurement of threshold conditions for full entrainment of solid particles on aerodynamically rough surfaces.

Source: “Rock and roll: Incipient aeolian entrainment of coarse particles,” by Zhao Xiao-Hu, Manousos Valyrakis, and Li Zhen Shan, Physics of Fluids (2021) The article can be accessed at https://doi.org/10.1063/5.0047604 .