The Role of Coherent Airflow Structures on the Incipient Aeolian Entrainment of Coarse Particles

• Published in: Journal of geophysical research. Earth surface 2024-05, Vol.129 (5), p.n/a
• Main Authors: Xiao‐Hu, Zhao, Valyrakis, Manousos, Pähtz, Thomas, Zhen‐Shan, Li
• Format: Article
• Language: English
• Subjects: Aerodynamic drag; Aerodynamics; Air flow; Anthropogenic factors; Coherence; Criteria; Drag; Eddy kinetic energy; Energy conservation; Energy transfer; Entrainment; Flow structures; Fluid dynamics; Gravel; Incipient motion; incipient rolling; Movement; particle rocking; quadrant event; Sediment transport; Soil mechanics; Soil surfaces; threshold energetic event; turbulent airflow; Turbulent energy; Turbulent flow; Wind tunnel testing; Wind tunnels
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2023JF007420

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). Plain Language Summary 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. Key Points 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