Turbulence‐Based Model for Subthreshold Aeolian Saltation

Abstract

Sand transport initiation and cessation occurs when the surface shear velocity exceeds a fluid threshold and falls below an impact threshold, respectively. Even when average shear velocity is below fluid threshold, turbulent fluctuations can initiate saltation, leading to turbulence‐driven transport intermittency. We leveraged the dynamic properties of large‐eddy simulation to recover a shear velocity time series due to atmospheric turbulence and recover a probability density function for saltation based on the frequency of events where wind has previously exceeded fluid threshold but not yet dropped below impact threshold. By conditionally sampling, we can quantitatively predict the frequency of intermediate saltation. Results show that a compensated, subthreshold shear velocity exhibits linear dependence upon the actual shear velocity. This compensated shear velocity compares favorably against field data. Model performance under terrestrial and Mars conditions is also shown.

Hysteresis and Surface Shear Stresses During Snow-Particle Aeolian Transportation

“…The random-flight model used in our study reproduced the drifting snow at the uniformhorizontal airflow and is suitable for investigating the steady-state transport properties. Recently, numerical models considering the turbulent structure have been developed for the aeolian transport of snow and sand particles (Groot Zwaaftink et al 2014;Okaze et al 2018;Pähtz et al 2020;Rana et al 2020;Zheng et al 2020;Rana et al 2021). The spatio-temporal inhomogeneous structure and variability of aeolian transport have been investigated using these unsteady models.…”

“…Airborne grains are deposited on the surface by decreasing the friction velocity (or wind speed), and aeolian transport ceases at the impact threshold. Recent numerical models incorporating turbulent fluctuations have reproduced the intermittent saltation around thresholds (Okaze et al 2018;Pähtz et al 2020;Rana et al 2020Rana et al , 2021Zheng et al 2020); thus, aeolian transported particles at the non-steady state exhibit more complicated transport properties. Bagnold (1941) estimated that the relationship between the fluid and impact thresholds is approximately u i * /u f * ≈ 0.85, which implies that two stable states (transport and no transport of particles) exist at friction velocities between the impact and fluid thresholds.…”

Influence of Wind Velocity and Soil Size Distribution on Emitted Dust Size Distribution: A Wind Tunnel Study

“…It must be admitted that there are many differences in the wind and dust emission conditions between a wind tunnel and the field. On the one hand, the characteristics of turbulent eddies in a wind tunnel are very different to those in the field, which can influence u *t , aerodynamic lift, and ultimately the EDSD (Rana et al., 2020). On the other hand, although a wind tunnel is too short to obtain steady‐state saltation (Kok, 2011b), saltation flux in the field is also never in a steady state owing to the heterogeneous soil conditions (Andreotti et al., 2010; Pähtz et al., 2014).…”

Unsteady dynamics of turbulent flow in the wakes of barchan dunes modulated by overlying boundary-layer structure

“…mean bed shear stress) struggle to accurately predict transport, even for perfectly uniform sediment distributions (Barchyn et al 2014;Yager et al 2018). Recent modifications of transport laws have revealed the effect of hysteresis in this regard, wherein the fluid threshold for transport must be exceeded to initiate saltation, but transport does not cease until the fluid forcing falls below the lower impact threshold (Martin & Kok 2018;Comola et al 2019;Rana, Anderson & Day 2020).…”