Mixture Properties
The Mixture Properties node contains the material properties for the continuous phase and the dispersed phase. It also contains settings for the viscosity model. For the Mixture Model, Turbulent Flow interfaces, the Mixture Properties node also adds the equations for the turbulence transport equations.
Model Inputs
The viscosity of each phase can be defined through user inputs, variables, or by selecting a material. For the latter option, additional inputs, for example temperature or pressure, may be required to define these properties.
Temperature
By default, the Temperature model input is set to Common model input, and the temperature is controlled from Default Model Inputs under Global Definitions or by a locally defined Model Input. If a Heat Transfer interface is included in the component, it controls the temperature Common model input. Alternatively, the temperature field can be selected from another physics interface. All physics interfaces have their own tags (Name). For example, if a Heat Transfer in Fluids interface is included in the component, the Temperature (ht) option is available for T.
You can also select User defined from the Temperature model input in order to manually prescribe T.
Absolute Pressure
This input appears when a material requires the absolute pressure as a model input. The absolute pressure is used to evaluate material properties, but it also relates to the value of the calculated pressure field. There are generally two ways to calculate the pressure when describing fluid flow: either to solve for the absolute pressure or for a pressure (often denoted gauge pressure) that relates to the absolute pressure through a reference pressure.
The default Absolute pressure pA is p + pref, where p is the dependent pressure variable from the Navier–Stokes or RANS equations, and pref is from the user input defined at the physics interface level. When pref is nonzero, the physics interface solves for a gauge pressure. If the pressure field instead is an absolute pressure field, pref should be set to 0.
The Absolute pressure field can be edited by clicking Make All Model Inputs Editable () and entering the desired value in the input field.
Model Inputs and Multiphysics Couplings in the COMSOL Multiphysics Reference Manual
Materials
Select the fluid materials to use for the material properties. The default material used for the Continuous phase is the Domain material. This corresponds to the material currently applied to the domain in question. The Dispersed phase uses None per default. A valid material must be selected instead.
Continuous Phase Properties
The default Density, continuous phase ρc (SI unit: kg/m3) uses values From material (as selected in the Materials section). For User defined enter another value or expression. In this case the default is 0 kg/m3.
The density in a material can depend on temperature and/or pressure and these dependencies are automatically replaced by pref and Tref, which are specified at the physics interface level.
The default Dynamic viscosity, continuous phase μc (SI unit: Pa·s), uses values From material. It describes the relationship between the shear stresses and the shear rate in a fluid. Intuitively, water and air have a low viscosity, and substances often described as thick, such as oil, have a higher viscosity. For User defined enter another value or expression. In this case, the default is 0 Pa·s.
Dispersed Phase Properties
The default Density, dispersed phase ρd (SI unit: kg/m3) uses values From material (as selected in the Materials section). For User defined enter another value or expression. In this case, the default is 0 kg/m3.
The density in a material can depend on temperature and/or pressure and these dependencies are automatically replaced by pref and Tref, which are specified at the physics interface level.
Enter the Diameter of particles/droplets dd (SI unit: m). The default is 10−3 m (1 mm). If Haider-Levenspiel is selected for the Slip model under Physical Model, enter a value between 0 and 1 for the Sphericity (dimensionless). The default is 1.
If Liquid droplets/bubbles is selected from the Dispersed phase list in the interface, then Dynamic viscosity, dispersed phase μd (SI unit: Pa·s) is also available. The default uses values From material (as selected in the Materials section) or select User defined to enter another value or expression. In this case, the default is 0 Pa·s.
Mixture Model
The options in this section are based on the selection made from the Dispersed phase list for the Mixture Model interfaces.
Slip Velocity Field
When a User defined Slip model is selected for the physics interface, specify an arbitrary expression for the relative velocity. For example, give a constant velocity based on experimental data.
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When Slip velocity is set to Specify slip velocity field, enter the slip velocity between the two phases, uslip (SI unit: m/s).
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When Slip velocity is set to Specify slip flux, enter the slip flux jslip (SI unit: m/s).
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The Mixture Model Equations in Theory for the Mixture Model Interfaces
Mixture Viscosity Model
Select the Mixture viscosity model.
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When Solid particles is the Dispersed phase, select either Krieger type (the default) or User defined.
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When Liquid droplets/bubbles is the Dispersed phase, select Krieger type (the default), User defined, or Volume averaged.
For User defined enter a value or expression for the Dynamic viscosity μ (SI unit: Pa⋅s). The default is 0 Pa⋅s. When using this option, make sure to limit the viscosity to bounded positive values.
When Krieger type is selected, enter a value or expression for the Maximum packing concentration (dimensionless). The default is 0.62.
Select Krieger type to model the most generally valid expression for the mixture viscosity:
where is the maximum packing concentration, which for solid particles is approximately 0.62. The dimensionless parameter μ* = 1 for solid particles and
for droplets and bubbles. When applying the Krieger type viscosity model, φd is replaced by min(, 0.999) for better robustness.
Select Volume averaged to model the mixture viscosity of liquid-liquid mixtures, which uses the following equation for the viscosity:
The Mixture Model interfaces always employ the mixture viscosity in the particle Reynolds number expression used to calculate the slip velocity, thereby accounting for the increase in viscous drag due to particle-particle interactions.
Mixing Length Limit
This section is available for The Mixture Model, k-ε Interface, The Mixture Model, Realizable k-ε Interface, and The Mixture Model, k-ω Interface, where an upper limit on the mixing length is required.
When the Mixing length limit lmix, lim is set to Automatic, the mixing length limit is evaluated as the shortest side of the geometry bounding box. If the geometry is, for example, a complicated system of slim entities, this measure can be too high. In such cases, it is recommended that the mixing length limit is defined manually. Select Manual to enter a different value or expression. The default is 1 (that is, one unit length of the model unit system).
Distance Equation
This section is available for The Mixture Model, Algebraic yPlus Interface, The Mixture Model, L-VEL Interface, The Mixture Model, SST Interface, The Mixture Model, Low Re k-ε Interface, The Mixture Model, Spalart-Allmaras Interface, and The Mixture Model, v2-f Interface.
When the Reference length scale lref is set to Automatic, it is evaluated one tenth of the shortest side of the geometry bounding box. The solution to the wall distance equation is controlled by the parameter lref. The distance to objects larger than lref is represented accurately, while objects smaller than lref are effectively diminished by appearing to be farther away than they actually are. This is a desirable feature in turbulence modeling because small objects would have too large an impact on the solution if the wall distance were measured exactly. The automatic value is usually a good choice but the value can become too high if the geometry consists of several slim entities. In such cases, it is recommended that the reference length scale is defined manually. Select Manual to enter a different value or expression.