From sweeps to sieving: a particle scale work-based criterion for intermittent aeolian entrainment of gravel and plastics under coherent turbulent structures

Coherent, energetic airflow structures control the incipient aeolian entrainment of coarse sediment and plastic debris, but their effect is poorly captured by classical, time-averaged shear-stress thresholds. This contribution showcases the results from recently published work [1, 2] that combines a particle-scale energy framework with wind-tunnel observations to quantify how individual sweeps and related structures trigger rocking, creep and full rolling, thereby regulating geomorphic work and debris mobility at Earth’s surface.

Wind-tunnel experiments were conducted in a 30 m environmental facility over a fixed rough bed of identical 40 mm celluloid spheres, representing idealized gravel and light plastic debris under fully turbulent, near-threshold flow (U ≈ 7.5–8.2 m s⁻¹). Synchronous 1 kHz measurements of near-bed airflow (2D hot-film) and particle displacement (0.1 mm laser distance sensor) resolve intermittent rocking and episodic rolling of a single exposed particle on a regular bed, under an atmospheric boundary layer with logarithmic mean profile and near-surface turbulence intensities up to ~20%.

A micromechanical model defines “energetic airflow events” as intervals where instantaneous drag exceeds an initial resistance level and persists for a finite duration, and relates their energy content Ef ∝ ∫u³dt to the minimum mechanical work F_g z_cr required to push a particle over its micro-topographic barrier. The resulting work-based criterion C_eff∫u³dt ≥ const introduces a normalized efficiency C_eff, estimated from the ratio of drag work ∫u²vdt to event energy, which partitions motion regimes from creep through rocking to incipient rolling and near-saltation. Quadrant analysis of uw shows that >85% of both rocking and rolling events are associated with Q4 sweeps; a simple peak-force condition u²_f,p ≥ u²_cr,0 is necessary for motion but insufficient for full entrainment, whereas the energy criterion correctly classifies ≈90–95% of observed rocking vs. rolling events. These results provide a transferable, event-based description of how coherent turbulent structures drive low-mobility aeolian transport, including mechanical sieving on gravel-mantled megaripples and the mobilisation of meso- to micro-plastic debris.

 

References

[1] Valyrakis, M., Zhao, X., Pähtz, T., & Li, Z. (2025). The role of energetic flow structures on the aeolian transport of sediment and plastic debris. Acta Mechanica Sinica, 41(1), 324467. https://doi.org/10.1007/s10409-024-24467-x.
[2] Zhao, X. H., Valyrakis, M., Pähtz, T., & Li, Z. S. (2024). The role of coherent airflow structures on the incipient aeolian entrainment of coarse particles. Journal of Geophysical Research: Earth Surface, 129(5), e2023JF007420. https://doi.org/10.1029/2023JF007420.