Influence of surface tension models on the hydrodynamics of wavy laminar falling films in Volume of Fluid-simulations

Three surface tension models are investigated within the Volume-of-Fluid code Free Surface 3D: unbalanced Continuum Surface Force, Continuum Surface Stress, and balanced Continuum Surface Force with height functions, where the latter was adapted to the simulation of laminar falling films. Unbalanced Continuum Surface Force exhibits large distortions when the grid is refined and is discarded for producing unphysical results. The remaining two models are thoroughly evaluated by comparison to recent experimental data of Dietze, and to an analytical solution for an oscillating free surface. Furthermore, grid dependence is examined. The most apparent hydrodynamical difference between Continuum Surface Stress and balanced Continuum Surface Force is the number of vortices inside the solitary wave in the reference frame of the moving wave. The first model shows three vortices, whereas the latter model displays only one vortex. It is demonstrated that Continuum Surface Stress gives inadequate results, whereas balanced Continuum Surface Force yields a considerable improvement. Thus it is shown that the choice of the surface tension model and its discretization heavily influence the resulting hydrodynamics in simulations of falling films. Although all models work in three dimensions, the numerical experiments are performed in 2D.

► Thorough validation of falling film simulations against experimental data.
► With CSS, parasitic currents can distort vortex structure in the roll waves.
► Balanced CSF with height functions can handle large curvatures near capillary waves.
► Surface tension model affects feasibility of grid refinement.