Modeling Multiphase Flow in Equipment with Rotating Parts
Selecting the Right Physics Interface
The Multiphase Flow branch () has a number of subbranches to describe momentum transport for multiphase flow in equipment containing one or more rotating parts. 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 Rotating Machinery, Mixture Model interfaces are appropriate when you want to simulate a flow with many particles or droplets 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 Rotating Machinery, Two-Phase Flow interfaces.
To model the detailed dynamics of fluid interfaces, either use the level set method or the phase field method. The Level Set or Phase Field in Fluids interfaces 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 surface tension force per default, and is recommended for use in larger scale problems with larger velocities, 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, only phase field should be used.
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Modeling Multiphase Flow in the CFD Module User’s Guide