$ \boldsymbol{D}_{pq}=\frac{1}{2}\rho_qC_DA_p(\boldsymbol{U}_p-\boldsymbol{U}_q)|\boldsymbol{U}_p-\boldsymbol{U}_q| $ Per unit volume: $ \boldsymbol{D}_{pq}=\frac{1}{2}\rho_qC_D\frac{A_p}{V}(\boldsymbol{U}_p-\boldsymbol{U}_q)|\boldsymbol{U}_p-\boldsymbol{U}_q| $ Unknowns are drag coefficient and interfacial area per unit volume Use [[Schiller-Naumann drag model]] to determine drag coefficient [[Interfacial Area]]