Effect of Saltation Bombardment on the Entrainment of Dust by Wind

Saltation is the wind-driven, hopping motion of sand-sized particles across an erodible surface. This mode of motion not only transports sand and similar materials in its own right but can also initiate (through bombardment of the surface) the entrainment and subsequent transport by suspension of smaller dust particles. In this paper, we report a wind tunnel study of the effect of saltation bombardment on dust entrainment. The technique is to allow sand grains to saltate from an upwind sand source onto a bed of dust particles. The experiment confirms that the ejection of dust particles by saltation bombardment (as opposed to detachment of dust particles by aerodynamic forces) is the principal mechanism for the natural entrainment of dust by wind. The data are used to examine the dependence of the dust emission flux Fa (mass per unit ground area per unit time) upon the friction velocity u,; it is found that Fa is closely proportional to the streamwise flux of saltating sand grains, which in turn is approximately proportional to u, 3. At a given u,, Fa increases as the size of the bombarding sand grains increases. On the basis of the hypothesis that Fa is proportional to the kinetic energy flux of the saltating sand grains, we derive theoretically the result that Fa scales with the streamwise saltating sand grain flux and thence approximately with u, 3, as observed in this experiment.