Simulations of Sphere Sedimentation in Viscoelastic Fluids using openFOAM: Notes on Boundary Conditions

Sphere sedimentation is an important benchmark problem in fluid dynamics and (for certain viscoelastic fluids) an active topic of ongoing research. In particular, sphere sedimentation in Wormlike micelles (WLM) often shows instabilities with dramatic and irregular fluctuations in the sedimentation velocity. These experiments have motivated several computational fluid dynamics (CFD) studies in the past few years, but in this talk we will argue that the selection of boundary conditions should be taken more seriously. Paradoxically, it has been common to study unsteady sphere sedimentation by enforcing a constant sedimentation velocity - this is partly a choice of convenience, as the alternative (enforcing momentum balance on the sphere) is more difficult to set up in a general purpose CFD solver like openFOAM or rheoTool. In this talk, we discuss a workaround in the openFOAM framework to create boundary conditions with more general control schemes. Time permitting, we will also compare predictions for slip vs no slip along the bounding cylinder (though which the sphere is falling), where the no-slip boundary condition provides a means of mitigating confinement effects.