Recently, significant progress has been made in conceptually describing the dynamic aspects of coarse particle entrainment, which has been explored experimentally for open channel flows. The aim of this study is to extend the application of energy criterion to the low mobility aeolian transport of solids (including both natural sediment and anthropogenic debris such as plastics), ranging from incomplete (rocking) to full (rolling) entrainments. This is achieved by linking particle movements to energetic flow events, which are defined as flow structures with the ability to work on particles, setting them into motion. It is hypothesized that such events should impart sufficient energy to the particles, above a certain threshold value. The concept's validity is demonstrated experimentally, using a wind tunnel and laser distance sensor to capture the dynamics of an individual target particle, exposed on a rough bed surface. Measurements are acquired at a high spatiotemporal resolution, and synchronously with the instantaneous air velocity at an appropriate distance upwind of the target particle, using a hot film anemometer. This enables the association of flow events with rocking and rolling entrainments. Furthermore, it is shown that rocking and rolling may have distinct energy thresholds. Estimates of the energy transfer efficiency, normalized by the drag coefficient, range over an order of magnitude (from about 0.001 to 0.0048 for rocking, up to about 0.01, for incipient rolling). The proposed event-based theoretical framework is a novel approach to characterizing the energy imparted from the wind to the soil surface and could have potential implications for modelling intermittent creep transport of coarse particles and related aeolian bedforms.

The role of coherent airflow structures capable of setting gravel-sized particles in motion is studied theoretically and experimentally. Specifically, a micromechanical model based on energy conservation is proposed to describe the incipient motion of large-particles ranging from rocking (incomplete entrainment) to incipient rolling (full entrainment). Wind tunnel experiments were conducted on an aerodynamically rough bed surface under near-threshold airflow conditions. Synchronous signals of airflow velocities upwind of the test particles and particle displacement are measured using a hot film anemometer and a laser distance sensor, respectively, from which coherent airflow structures (extracted via quadrant analysis) and particle movements are interlinked. It is suggested that the incipient motion of gravel-sized particles (rocking and rolling) may result from sufficiently energetic sweep events corresponding to aerodynamic drag forces in excess of the local micro-topography resistance. However, full entrainment in rolling mode should satisfy the presented work-based criterion. Furthermore, using an appropriate probabilistic frame, the proposed criterion may be suitable for describing processes of energy transfer from the wind to the granular soil surface, ranging from the creep transport of gravels to the mechanical sieving of mega-ripples, as well as the transport of light anthropogenic debris (such as plastics). Entrainment of particles from a particle bed is a key problem in wind-driven sediment transport. Most existing models define critical entrainment conditions as those at which a bed particle begins to move. Such conditions are typically met during the passing of turbulent flow structures. However, the energy transferred from the structure to the particle may not be sufficient to leapfrog over neighboring bed grains. In this case, the particle will eventually fall back into its bed pocket. Here, we experimentally evaluate an alternative work-based criterion for particle entrainment during the passing of turbulent flow structures. Our results indicate that sweeps of sufficient turbulent energy are predominantly responsible for particle entrainment. Coherent airflow sweep structures exerting sufficient drag force can set coarse particles in motion A work-based criterion has been established to define the full aerodynamic entrainment of coarse particles The proposed criterion agrees with wind tunnel experimental observations