Rock and roll: Incipient aeolian entrainment of coarse particles

Abstract

Aeolian transport of coarse grains is an important topic, finding applications in nature (for infrastructure exposed to wind scour) as well as industry (e.g., considering pneumatic transport). Incipient particle entrainment due to turbulent winds refers to the wind conditions where aeolian transport initiates, and as such, it is at the core of such studies. The research presented herein focuses on identifying and quantifying the dynamical processes responsible for coarse particle entrainment. Specifically designed wind tunnel experiments are conducted for a range of wind conditions near the aeolian transport thresholds. A high-resolution laser distance sensor is employed to provide information for the displacement of an exposed particle ranging from small simple rocking motions to complete entrainments (rolling). Measurements of the exposed particle's angular displacements are acquired, which allow the probabilistic study of incipient motion. The variation of statistical parameters, such as the frequency of entrainments, duration of dislodgements, magnitude of displacements, and time between displacements, is studied for a range of increasing airflow rates. The main findings from these experiments suggest that rocking can be observed only up to a limit angular displacement (equal to 0.41π for the conditions tested herein), which defines the position beyond which the resistance force can be overcome by just the mean aerodynamic forcing. Following this experimental framework to establish aeolian thresholds for a wider range of environments may be useful for the identification of the wind conditions under which aeolian transport may start occurring.