Selecting the Right Physics Interface
The Multiphase Flow branch () included with this module has a number of subbranches to describe momentum transport for multiphase flow. One or more physics interfaces can be added; either singularly or in combination with other physics interfaces for applications such as mass transfer and energy (heat) transfer.
Different types of flow require different equations to describe them. If you know the type of flow to model, then select it directly. However, when you are not certain of the flow type, or when it is difficult to obtain a solution, you can instead start with a simplified model and add complexity as you build the model. Then you can successively advance forward, comparing models and results.
The Bubbly Flow, Mixture Model, Euler-Euler Model, and Phase Transport Mixture Model interfaces are appropriate when you want to simulate a flow with many particles, droplets, or bubbles immersed in a liquid. With these physics interfaces, you do not track each particle in detail. Instead you solve for the averaged volume fraction. If you are interested in the exact motion of individual bubbles, including how the fluid interface deforms due to, for instance, surface tension, use any of the Two-Phase Flow interfaces.
To model the detailed dynamics of fluid interfaces, either use the level set method, the phase field method, the phase field thin-film flow method, or by a moving mesh, respectively. For problems involving topological changes (for example, jet breakup), use either the Level Set, Phase Field or Phase Field Thin-Film Flow interfaces.These techniques use an auxiliary function (the level set and phase field functions, respectively) to track the location of the interface, which is necessarily diffuse. The Level Set interface does not include the surface tension force per default, and is recommended for use in large scale problems dominated by inertia, or when the effects of the gradient of the surface tension coefficient are relevant. The phase field method is physically motivated and is usually more numerically stable than the level set method. It is can also be extended to more phases and is compatible with fluid-structure interactions (requires the MEMS Module or the Structural Mechanics Module). In general, it is not obvious which one of these to use when the flow is convection dominated. However, when the flow is diffusion dominated, for example, in the case of phase separation, the phase field method should be used. Phase Field Thin-Film Flow method may be used to track the location of the interface in narrow channels, represented by a surface or an edge within the geometry, which are thin enough for the Reynolds equation or the modified Reynolds equation to apply.
The flow of two immiscible fluids in narrow channels can be simulated using the Two-Phase Thin-Film Flow, Phase Field interfaces.
The moving mesh method represents the interface as a boundary condition along a line or surface in the geometry. Because the physical thickness of phase boundaries is usually very small, for most practical meshes The Laminar Two-Phase Flow, Moving Mesh Interface describes the two-phase boundary more accurately. However, it cannot accommodate topological changes in the boundary.
Three-phase flow can be simulated using the Laminar Three-Phase Flow, Phase Field interface.