Hydrodynamic drag force exerted on a moving floc and its implication to free-settling tests

This paper numerically evaluates the hydrodynamic drag force exerted on an individual floc moving steadily through an quiescent Newtonian fluid over a wide range of Reynolds numbers. The floc is modelled as a highly porous sphere, whose interior flow field is governed by the combined Darcy-Brinkman model; while the bulk fluid field is governed by Navier-Stokes equations. We used a computational fluid dynamics software, FIDAP 7.5, to solve the fluid field within and around the moving floc, from which the corresponding hydrodynamic drag force exerted on the floc is subsequently obtained. When the floc permeability is rather high, which is usually the case for activated sludge flocs, the drag force was found to be inversely proportional to the floc Reynolds number. The floc density versus size relationship obtained on the basis of the Stokes law-like correlation is thereby applicable to estimate the flocs' fractal dimension, however, is erroneous for estimating the floc density.