Fluid Dynamic Lift on a Bed Particle

Direct measurement of lift forces acting on an exposed sphere at the boundary of a fluid flow shows that the lift force may be negative (i.e., directed toward the boundary) if the grain-size Reynolds number R* is less than about 5. The graphs presented strongly suggest that in a Newtonian fluid the dimensionless lift force is a function only of R*, in which case the results obtained can be expected to apply to exposed grains at the bed of a loose boundary flow if similarity of R* obtains. The onset of negative lift in such a situation as R* decreases through about 5 coincides with well-documented changes in bedform (from dunes to ripples) and in incipient motion conditions. It is suggested that the change of sign of lift force is a significant factor in these behavioral characteristics; physical mechanisms capable of effecting the observed changes are considered in qualitative terms.