(6)
where f0 is a single-particle drag force given by Eq. (7) and ε is the porosity of the fluid element where the particle resides. The ε term is an empirical factor that accounts for local porosity. This correction term makes the force applicable to both high-porosity and low-porosity systems and for a large range of Reynolds numbers (Difelice et al., 1994; Xu et al., 1997).