Simulation of Multiphase Flows in Complex Geometry Using a Hybrid Method Combining the Multi-Fluid and the Volume-of-Fluid (VOF) Approaches

A computational method combining the multi-fluid and the Volume-of-Fluid (VOF) approaches is presented to simulate industrial multiphase flows in complex geometry. This method is particularly applicable for flows where well-defined interfaces between different phases/fluids co-exist with small-scale multiphase structures. The interfaces in relatively large scales (that can be accurately resolved on a computational mesh with a practical size) are tracked by the VOF method, whereas the small scale multiphase flow structures (that are too computationally expensive to be explicitly tracked by the VOF method) are accounted for by using the multi-fluid approach. In order to provide more computational flexibility, any two of the phases tracked by the multi-fluid approach can either have different velocities (two-fluid model) or share the same velocities (equilibrium model). The hybrid method presented here enables efficient simulation of complex flows with multiple phases/fluids on arbitrary-shaped unstructured meshes. It is fully implemented in the commercial CFD software, AVL FIRE/SWIFT. The governing equations are discretized based on a finite volume method (FVM) and the pressure field is obtained using the SIMPLE algorithm. The effect of surface tension is also included for the phases tracked by the VOF method using a Continuum Surface Force (CSF) model. Application to a well-established example of multiphase flow—a Taylor bubble rising inside a stagnant liquid—is presented to demonstrate the capability of the method.