Relationship between aerodynamic entrainment threshold and hydrodynamic settling velocity of particles

Abstract

Data on the critical shear stress (τc) at incipient grain motion under air flow have been carefully selected from the literature. It is shown that the non-dimensional Shields entrainment threshold (βc) in air is an exponential function of the dimensionless settling velocity (Wds) in water, of a sphere with a diameter equal to the sieve size. As Wd5 can be related to the measured dimensionless settling velocity (Wdm) of natural grains, Wdm can be used directly to calculate βc. For quartz grains in air, the equations are valid for sizes between 0.01 and at least 0.166 cm, i.e. silt to very coarse sand. The method is applicable to well sorted grains with a relatively high sphericity entrained on flat surfaces.