Experimental Validation of the Near‐Bed Particle‐Borne Stress Profile in Aeolian Transport Systems

Abstract

Self‐regulation of sediment transport by wind is widely assumed to derive from the partitioning of momentum from the fluid flow to the particle cloud. Consequently, the fluid stress on the bed surface is suggested by some to drop below that required to entrain particles, while the cloud is sustained by particle ricochet and splash that derive from the impacts of particles moving ballistically along the bed surface. While these theoretical constructs underpin present‐day numerical models of aeolian saltation, the particle‐borne stress has never been measured directly in either a laboratory or field setting. Such measurements are required for model validation and calibration of numerical simulations and were undertaken in vertical profile in the Trent Environmental Wind Tunnel using particle tracking velocimetry. Test sand was normally distributed with a median diameter of 590 μm. Particle momentum was calculated from the diameter and velocity of each image pair captured using high‐speed photography and then summed within each elevation band. The median diameter of the air‐borne particles was found to increase with friction velocity (u*), suggestive of sorting of the bed surface, and to decrease with elevation. Within a two‐dimensional framework, the vertical and windward components of the particle velocity were found to increase with elevation, with u* having little to no influence. Confirming the results of previous numerical simulations, these experiments show that the normalized particle‐borne stress increases exponentially toward the bed surface; however, within the lowest few millimeters where particle splash dominates, no consistent change is observed with elevation.

Modeling the effect of saltation on surface layer turbulence

“…To better explain this problem, we plot the dimensionless Reynolds stress of sand-laden flow in Figure 10. It can be seen from the figure that the characteristics of the dimensionless Reynolds stress profiles are generally consistent with those of O'Brien and Neuman (2019). The difference is that we also have a focus feature between the constant stress region and the near-wall region.…”

“…We believe that since the rules of dimensionless Reynolds stress with the friction wind speed in the outer wake flow region and near wall region are opposite, there must be an intersection point or intersection zone in the middle. As for whether the focus is formed or closer to the intersection zone of O'Brien and Neuman (2019), it requires more in-depth studies.…”

“…This seems to support Owen's hypothesis again. Recently, O'Brien and Neuman (2016Neuman ( , 2017Neuman ( , 2019 used an improved particle tracking technique (PTV-EPAS) to make a more precise measurement of the particle trajectories, kinetic energy, flux density, and particle-borne stress in aeolian transport systems. Their results showed that the impact velocity of particles has no dependency on the frictional wind speed.…”

Barchan Slipface Grainflows: Characteristics and Kinematics

“…If grain speeds increase, then increased transport means that the zone of grainfall on the slipface would increase as well; thus, the size of the bulge would increase as suggested by others (Nickling et al., 2002; Pelletier et al., 2015). Some studies indicate that grain speeds remain approximately constant (Martin & Kok, 2017; O’Brien & McKenna Neuman, 2019; Pähtz & Tholen, 2021), although other work contradicts this (Rasmussen & Sørensen, 2008). Suppose, instead, increased transport is caused by increased grain numbers (e.g., Namikas, 2003).…”

Development of ornamentation on ventifacts: An examination of flow and saltation kinematic mechanisms

“…Similarly, Bridges et al (2005) investigate how the angle of the rock face with respect to surface normal, as well as the height above the surface, affects the abrasion rate. The presence of faster moving particles with increasing distance from the surface is predicted by numerical modeling (e.g., Durán et al, 2011) and observed by O'Brien and McKenna Neuman (2019); however, they are few in number as compared to the entire population of saltating grains. Nevertheless, the cumulative number count of particle impacts upon rock surfaces over time at distances well-above the bed could become quite substantial and result in more abrasion with increasing height due to increasing levels of kinetic energy in the saltation cloud (Anderson, 1986), to a maximum (e.g., Sharp, 1964;Wilshire et al, 1981).…”