Drag coefficients of irregularly shaped particles, constructed from spheres, were measured in lattice-Boltzmann simulations and compared to literature data. The agreement is very well. The distance between the spheres was increased to study the influence of inter-particle distance on the drag force in clusters. The drag coefficient of the clusters was found to increase with inter-particle distance. The drag force on an individual particle in a cluster is lower when that particle is shielded from the flow by other particles.
Beetstra, R., van der Hoef, M. A., & Kuipers, J. A. M. (2006). A Lattice-Boltzmann simulation study of the drag coefficient of clusters of spheres. Computers and fluids, 35(8-9), 966-970. https://doi.org/10.1016/j.compfluid.2005.03.009