Recent Advances Towards a Flow Pattern Adaptive Hybrid Multifield Two-Fluid Model

Physical phenomena in industrial gas-liquid flows typically span a wide range of length and time scales. Individual flow regimes are usually described using a tailored approach, prohibiting simulation of transitions and interactions. In order to address these challenges, a hybrid multiphase model is established by combining the Euler-Euler model with the Volume-of-Fluid (VOF) model. Interactions and transitions between different morphologies and scales require special attention, which is accounted for with dedicated models. This work gives an overview over recent advances towards a fully scalable hybrid multiphase model.
In particular, large interfaces might be represented on coarse numerical grids. With a usual VOF model this typically leads to over-prediction of interfacial shear stress, resulting in a deteriorated prediction of interface dynamics. By accounting for the resolved part of the flow in the vicinity of an interface, the interfacial drag between both phases is controlled. In that way the phases may slip along each other in the direction parallel to the interface surface, improving the prediction, i.e., of interface shape or of bubble rising velocity.
Furthermore, each disperse structure needs to be resolved as soon as it becomes large enough in relation to the local cell sizes. Unresolved bubbles may coalesce, grow, or enter highly-refined mesh regions, so that they should no longer be treated as disperse. Therefore, a transition to a continuous representation is realised, to make optimal use of the available numerical degrees of freedom. A necessary condition for such a transition is a stable behaviour of the disperse model on unusual fine meshes.
Another important aspect of the model is to track the number and size of dispersed phase particles. A class-method based solution approach was implemented, providing complete information about the size distribution, a necessity for modelling the number-conservative transition between dispersed and resolved structures. However, the associated computational cost is significant. Fortunately, the adopted solution procedure could be parallelised by outsourcing it to graphics processing units, which leads to a significant improvement in performance.
The hybrid model is implemented in OpenFOAM with strong focus on sustainable research, including a state-of-the-art IT approach. Both the source code and a comprehensive suite of simulation cases are publicly available.