Flow-induced forces in sphere doublets

Motivated by applications in solids formation and handling processes we numerically investigate the force and torque required for maintaining a fixed contact between two equally sized solid spheres immersed in fluid flow. The direct numerical procedure applied is based on solving the Navier–Stokes equations by means of the lattice-Boltzmann method, with no-slip conditions at the surfaces of the moving spheres. It is validated by means of the analytical results due to Nir & Acrivos (1973) for sphere doublets in creeping flow. Subsequently, doublets are released in turbulent flows. In these cases, the contact force and torque strongly fluctuate, with peak levels much higher than would follow from simple dimensional analysis. The force fluctuation levels have been quantified by a linear relation in the ratio of sphere radius over Kolmogorov length scale.