Particle Entrainment by Non-Uniform Eolian Flow

We measured the spatially varying flow field and entrainment flux under an axisymmetric jet impinging on a particle bed. We found that the erosion of the bed is driven by excess turbulent surface shear stress, readily quantified by the turbulent kinetic energy of the flow. This observation differs from uniform flow parallel to the bed wherein the horizontal flux of particles depends linearly on the local surface shear stress computed from the time-averaged flow and in excess of a critical value. Our findings suggest a more general approach for relating the entrainment flux to the bead shear stress: use the time-dependent shear stress acting on the erodible soil bed. In the case of uniform flow the shear stress is for all intents and purposes invariant in time and space; for non-uniform flow this is not the case and the shear stress based on the turbulent fluctuations (second moments) of the flow should be used.