The Influence of Surface Stress Fluctuation on Saltation Sand Transport Around Threshold

Abstract

Multiscale wind velocity fluctuation and complex inertial response of sand saltation to wind variations give rise to intermittent and nonuniform sand transport, which greatly challenges the prediction of transport rate under wind conditions around saltation threshold. In this paper, three-dimensional sand saltation within simulation domain of wind tunnel scale is modeled based on large eddy simulation of turbulent wind. Special attention is paid to the prediction of surface shear stress and consequently aerodynamic entrainment. When the wall stress model considering the influence of large-scale turbulent structures was employed in case of relatively small resistance level for calculating particle aerodynamic entrainment, the model results predict sand transport below the impact threshold. The sand transport rate in this regime increases exponentially with mean wall stress, in contrast to the standard power law-like behavior above the impact threshold. Aerodynamic entrainment and strong transport events lag behind high-speed fluids sweep on a large scale even in wind tunnel, but the spatiotemporal variability of small-scale sand transport is still unclear and quite complex due to transport inertia.

Characterization of Wind‐Blown Sand With Near‐Wall Motions and Turbulence: From Grain‐Scale Distributions to Sediment Transport

“…The resultant velocity and lift‐off angle are 0.63 (Anderson & Hallet, 1986) and 36° (Namikas, 2003), respectively, which is consistent with Zheng et al. (2020). Aerodynamic entrainment is normally considered as an inception of saltation and can be negligible in the stationary state of wind‐blown sand (Bagnold, 1941; Kok et al., 2012).…”

Unified Model of Sediment Transport Threshold and Rate Across Weak and Intense Subaqueous Bedload, Windblown Sand, and Windblown Snow

“…There are two distinct kinds of transport intermittency. The first kind occurs when turbulence‐driven bed sediment entrainment events associated with energetic turbulent eddies (Cameron et al., 2020; Paterna et al., 2016; Valyrakis et al., 2011; Zheng et al., 2020) generate intermittent rolling events of entrained grains (Pähtz et al., 2020). This kind of intermittency is negligible for saltation, since the transport rate of rolling grains is much smaller than that of saltating grains.…”

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.…”