The Mixture Model, Algebraic yPlus Interface
The Mixture Model, Algebraic yPlus (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, Algebraic yPlus 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 included using and enhanced viscosity model based on the scaled wall distance, and the physics interface therefore includes a wall distance equation.
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 and The Mixture Model, k-ε Interface.
Turbulence
The default Turbulence model type is RANS. A different turbulence model can be selected under Turbulence model. The default turbulence model is Algebraic yPlus.
Wall Treatment
Algebraic yPlus employs per default an Automatic wall treatment, which switches between a low-Reynolds-number formulation and a wall function formulation depending on how well resolved the flow is close to the wall. The automatic wall treatment gives a robust formulation that makes the most out of the available resolution. Select the Low Re option in order to enforce resolution all the way down to the wall. This can be more accurate than the automatic wall treatment provided that the mesh is fine enough.
Edit Turbulence Model Parameters
Turbulence model parameters are optimized for turbulent pipe flow. For certain 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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Reciprocal wall distance G (SI unit: 1/m)
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Wall distance in viscous units yPlus (dimensionless)
The names can be changed but the names of fields and dependent variables must be unique within a component.
The Mixture Model, Algebraic yPlus interface requires a Wall Distance Initialization study step in the study previous to the Stationary or Time Dependent study step.
For study information, see Stationary with Initialization, Time Dependent with Initialization, and Wall Distance Initialization 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