The Mixture Model, k-ε Interface
The Mixture Model, k-ε (mm) interface (), found under the Multiphase Flow>Mixture Model>Mixture Model, Turbulent Flow branch () when adding a physics interface, is used to model the flow at high Reynolds numbers of liquids containing a dispersed phase. The dispersed phase can be bubbles, liquid droplets, or solid particles, which are assumed to always travel with their terminal velocity.
The Mixture Model, k-ε interface solves one set of Navier–Stokes equations for the momentum of the mixture. The pressure distribution is calculated from a mixture averaged continuity equation and the velocity of the dispersed phase is described by a slip model. The volume fraction of the dispersed phase is tracked by solving a transport equation for the volume fraction. Turbulence effects are modeled using the standard two-equation k-ε model with realizability constraints. Flow close to walls is modeled using wall functions.
The physics interface can also model the distribution of the number density, which in turn can be used to calculate the interfacial area, which is useful when simulating chemical reactions in the mixture.
Except where indicated below, the settings for this physics interface are the same as for The Mixture Model, Laminar Flow Interface.
Turbulence
The default Turbulence model type is RANS. A different turbulence model can be selected under Turbulence model. The default turbulence model is k-ε.
Wall Treatment
Wall treatment for the k-ε model can only be set to Wall functions. More options become available by selecting another option under Turbulence model.
Edit Turbulence Model Parameters
Turbulence model parameters are optimized to fit as many flow types as possible, but for some special cases, better performance can be obtained by tuning the model parameters. For a description of the turbulence model and the included model parameters see Theory for the Turbulent Flow Interfaces.
Dependent Variables
Enter values for the dependent variables (field variables):
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Velocity field, mixture j (SI unit: m/s)
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Pressure p (SI unit: Pa)
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Volume fraction, dispersed phase phid (dimensionless)
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Squared slip velocity slipvel
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Number density, dispersed phase nd (SI unit: 1/m3)
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Turbulent kinetic energy k (SI unit: m2/s2)
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Turbulent dissipation rate ε (SI unit: m2/s3)
The names can be changed but the names of fields and dependent variables must be unique within a component.
Consistent and Inconsistent Stabilization
To display this section, click the Show More Options button () and select Stabilization. The settings for this section are the same as for The Mixture Model, Laminar Flow Interface with the addition of this section: stabilization for the turbulence variables in the Turbulence equations area.
When using a turbulence model, streamline and crosswind diffusion are by default applied to the turbulence equations.
Advanced Settings
To display this section, click the Show More Options button () and select Advanced Physics Options. The Turbulence variables scale parameters subsection is available when the Turbulence model type is set to RANS.
In addition to the settings described for The Mixture Model, Laminar Flow Interface, enter a value for Jscale and Lfact under the Turbulence variables scale parameters subsection.
The Jscale and Lfact parameters are used to calculate absolute tolerances for the turbulence variables. The scaling parameters must only contain numerical values, units or parameters defined under Global Definitions. The scaling parameters cannot contain variables. The parameters are used when a new default solver for a transient study step is generated. If you change the parameters, the new values take effect the next time you generate a new default solver.
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Pseudo Time Stepping for Laminar Flow Models in this guide and Pseudo Time Stepping in the COMSOL Multiphysics Reference Manual
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Domain and Boundary Nodes for the Mixture Model Interfaces
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Slip Velocity Models
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Theory for the Mixture Model Interfaces