(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-χ 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).