Equilibrium of Soil Grains at the Threshold of Movement by Wind

Abstract This paper, based on wind tunnel studies, presents an analysis of the nature and magnitude of forces on soil grains at the threshold of their movement by wind. Forces of drag, lift, and gravity were analyzed in relation to each other. The equilibrium between these forces and the soil grains was found to be influenced by the diameter, shape, and immersed density of the grains, the angle of repose ø′ of the grains with respect to the mean drag level of the fluid, the closeness of packing η of top grains on the sediment bed, and the impulses of fluid turbulence T D and T L associated with drag and lift, respectively. All those factors were measured and related with actual and theoretical forces involved. New approaches to measurement of η, ø′, T D and T L are presented. Analyses indicate that the magnitude of pressure impulses of both T D and T L is statistically distributed according to the somewhat skewed normal error law. The ratio of mean pressure to standard deviation σ was constant for any size of grains or fluid velocity and could be expressed by equation σ = cP̄ in which P̄ is the mean pressure of lift or drag and c is a constant which was found to have a mean value of 0.49.