these interfaces, you do not track each particle in detail but just 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 or the phase field method. It is in general not obvious which one of these to use, but it is easy to switch between the two models within the Two Phase Flow interfaces. You can also easily switch turbulence modeling on and off or modify compressibility assumptions.

The Multiphase Flow Interface Options

For all multiphase flow modeling, you can assume laminar or turbulent flow as the starting point. Every multiphase flow group has these options. This enables you to make the appropriate mathematical and model assumptions required to solve the flow. Turbulence is modeled using the standard k- model.

• The Turbulent Flow, k- Interface

See Also

The Relationship Between the Interfaces

Several of the interfaces vary only by one or two default settings (see Table 6-1, Table 6-2, and Table 6-3) in the Physical Model section, which are selected either from a check box or drop-down list. For the Multiphase Flow branch, the Bubbly Flow (bf) and Mixture Model Flow (mm) have two interfaces each and both have the same Interface Identifier. All the Two-Phase Flow interfaces also have the same Interface Identifier (tpf). The differences are based on the default settings required to model that type of flow as described in this section. Figure 6-1 shows the Turbulent Two Phase Flow, Phase Field Settings window, which has options to select whether to describe the Two-phase flow, phase field or level set method, whether flow is

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Compressible or Incompressible, whether it is best described as Laminar or Turbulent flow, and whether to use Stokes flow. Combinations are also possible.

Figure 6-1: The Settings window for the Turbulent Two Phase Flow, Phase Field interface. Select whether to model compressible or incompressible flow, laminar or turbulent flow, and Stokes flow. Combinations are also possible using either the phase field or level set methods.

B U B B L Y F L O W

TABLE 6-1: BUBBLY FLOW PHYSICAL MODEL DEFAULT SETTINGS

The Bubbly Flow branch () interfaces are used primarily to model two-phase flow where the fluids are a gas-liquid homogeneous mixture, and the content of the gas is not more than 10%. The Laminar Bubbly Flow Interface () and The Turbulent Bubbly Flow Interface () solve the flow equations, whether described by the

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Navier-Stokes equations or the k- turbulence model, and where the momentum equation is corrected by a term defined by the slip velocity. The slip velocity can be described by the Hadamard-Rybczynski method, a term for the surface tension coefficient, or by defining your own velocity components.

By default, the Bubbly Flow interface assumes that the volume fraction of the gas is less than 0.1. It is then valid to approximate the continuity equation by an incompressibility condition for the liquid velocity. This is significantly more easy to solve numerically. It is possible, though, to use the complete continuity equation.

This interface also allows for easy access to define your own relation for the density of both phases and the dynamic viscosity of the gas phase, although definitions of non-Newtonian fluid flow through the power law and Carreau models are not possible.

The Bubbly Flow interface also allows you to model the mass transfer between the two phases, using the two-film theory or your own expression for interfacial mass transfer.

M I X T U R E M O D E L F L O W

TABLE 6-2: MIXTURE MODEL PHYSICAL MODEL DEFAULT SETTINGS

The Mixture Model branch () interfaces are similar to the Bubbly Flow interfaces but assume that the dispersed phase consists of solid particles or liquid droplets, not gas bubbles. The continuous phase should be a liquid.

Like the Bubbly Flow interfaces, The Mixture Model, Laminar Flow Interface () and The Mixture Model, Turbulent Flow Interface () solve the flow equations, whether it be described by the Navier-Stokes equations or the k- turbulence model, and where the momentum equation is corrected by a term defined by the slip velocity. The slip velocity can be described by the Hadamard-Rybczynski or Schiller-Naumann methods or by defining your own velocity components.

This physics interface also allows for easy access to define your own relation for the dynamic viscosity and density of both phases, although definitions of non-Newtonian fluid flow through the power law and Carreau models are not possible. The properties of the two phases are linked through the Mixture Model, which can be either a Krieger

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type (which uses a maximum packing concentration), a volume-averaged (for gas-liquid, liquid-liquid systems), or a user-defined expression for the dynamic viscosity.

You can also describe other material properties such as density by entering equations that describe this term as a function of other parameters like material concentration, pressure, or temperature. The Mixture Model interface also makes it possible to model the mass transfer between the two phases, using the two-film theory or your own expression for interfacial mass transfer.

T W O - P H A S E F L O W I N T E R F A C E S

TABLE 6-3: TWO-PHASE FLOW PHYSICAL MODEL DEFAULT SETTINGS

Two-Phase Flow, Level Set

The Laminar Two-Phase Flow, Level Set Interface () and The Turbulent Flow, Two-Phase Flow, Level Set Interface (), found under the Two-Phase Flow, Level Set branch (), are used primarily to model two fluids separated by a fluid interface. The moving interface is tracked in detail using a level set method. Surface tension acting at the fluid interface is automatically included in the fluid flow equations.

Similar to other fluid flow interfaces, compressible flow is possible to model in this interface at speeds of less than 0.3 Mach in the Two-Phase Flow, Level Set interface. You can also choose to model incompressible flow, and simplify the equations to be solved. Stokes’ law is an option.

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