The drag force on a particle in a fluid-multiparticle interaction syst
em may be expressed as the product of the drag force on an unhindered
particle, subject to the same volumetric flux of fluid, and a voidage
function. It is demonstrated that for a wide variety of both fixed-bed
and suspended-particle systems, the voidage function may be expressed
as epsilon(-beta), where the exponent beta is dependent on the partic
le Reynolds number but independent of other system variables.