The use of evenly spaced hemispheres to evaluate aerodynamic forces on a soil surface

Abstract

Direct and indirect measurements showed a substantial lift force on the surface roughness elements (such as soil grains resting on the surface) in a windstream. Lift was approximately four‐fifths of drag on hemispherical roughness elements ranging from 0.16 to 5.08 cm in height. The ratio of lift to drag on the roughness elements of the surface remained essentially constant for any drag velocity of the fluid and any depth of the fluid boundary layer beyond a relatively shallow depth. The depth of the fluid boundary layer, on the other hand, had a profound influence on the magnitude of both lift and drag. This study throws new light on equilibrium conditions existing between soil grains and moving fluid at the threshold of movement of the grains.