  The role of coherent airflow structures capable of setting gravel-size
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-size 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).
