Abstract
With the increasing use of Computational Fluid Dynamics to investigate multiphase flow scenarios, modelling surface tension effects has been a topic of active research. A well known associated problem is the generation of spurious velocities (or currents), arising due to inaccuracies in calculations of the surface tension force. These spurious currents cause nonphysical flows which can adversely affect the predictive capability of these simulations. In this paper, we implement the Continuum Surface Force (CSF), Smoothed CSF and Sharp Surface Force (SSF) models in OpenFOAM. The models were validated for various multiphase flow scenarios for Capillary numbers of 10 − 3 –10. All the surface tension models provide reasonable agreement with benchmarking data for rising bubble simulations. Both CSF and SSF models successfully predicted the capillary rise between two parallel plates, but Smoothed CSF could not provide reliable results. The evolution of spurious current were studied for millimetre-sized stationary bubbles. The results shows that SSF and CSF models generate the least and most spurious currents, respectively. We also show that maximum time step, mesh resolution and the under-relaxation factor used in the simulations affect the magnitude of spurious currents.
Highlights
For a comprehensive understanding of flow physics in multiphase systems, which is ubiquitous in both nature and technological processes, consideration of surface tension is important
Using the time step dictated by Deshpande et al [63], the spurious currents generated by the Continuum Surface Force (CSF) model are reduced by less than half in comparison to when time step constraint proposed by Brackbill et al [66]
When using a lower under-relaxation factor, Smoothed CSF and Sharp Surface Force (SSF) models reduce the spurious currents by approximately one-fourth of the CSF model
Summary
For a comprehensive understanding of flow physics in multiphase systems, which is ubiquitous in both nature and technological processes, consideration of surface tension is important. Some examples of flow phenomena used to compare surface tension models are rising bubbles whose diameters are in the order of few millimetres [33,34,35], translating and rotating bubbles [64], oscillating droplets or bubbles [34], stagnant bubbles or droplets [34,35,39,64], Rayleigh–Taylor instability [37,38], Taylor bubbles [64], falling films [41], droplet splashing [38,39], capillary rise [42] and bubble evolution [37,40] These typically compare the CSF model with height functions [33,34,64], PROST [37], PLIC [42], CLSVOF and its variants [37,38,39,40,64], FSF and SSF [42], and CSS [35,41] models. In the interest of knowledge dissemination, the solvers and the test cases (implemented in OpenFOAM 6) discussed in the paper are available in the Supplementary Materials
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