  We computationally determine the force on single spherical intruder particles
in sheared granular flows as a function of particle size, particle density,
shear rate, overburden pressure, and gravitational acceleration. The force
scales similarly to, but deviates from, the buoyancy force predicted by
Archimedes' principle. The deviation depends only on the intruder to bed
particle size ratio, but not the density ratio or flow conditions. We propose a
simple force model that successfully predicts whether intruders rise or sink,
knowing only the size and density ratios, for a variety of flow configurations
in physical experiments.
