Fluid Properties
The Fluid Properties node contains the material properties for the liquid and the gas. It also contains settings for the slip model.
Model Inputs
Fluid properties, such as the gas density and liquid viscosity, 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 materials to use for the material properties of the liquid and the gas (when they are set to take their value from the material). The default is to use the Domain material for the Liquid and None for the Gas. Select another material to use that material’s properties for the liquid or gas as needed.
Liquid Properties
The default Density, liquid phase ρl (SI unit: kg/m3) uses values From material. For User defined enter another value or expression.
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, liquid phase μl (SI unit: Pa·s) uses values From material; the value is then defined for the material selected in the Materials section for the continuous phase. For User defined enter another value or expression.
The dynamic viscosity 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.
Gas Properties
The default Density, gas phase ρg (SI unit: kg/m3) uses values From material. For User defined enter another value or expression. Alternatively, select Calculate from ideal gas law and enter the Molecular weight M (SI unit: kg/mol) of the gas.
Enter the Bubble diameter db (SI unit: m). The default value is 10−3 m (1 mm).
Slip Model
Select a Slip model — Homogeneous flow (the default), Pressure-drag balance, or User defined.
Homogeneous flow assumes that the velocity of the two phases are equal; that is, uslip = 0. For User defined enter different values or expressions for the components of the Slip velocity uslip (SI unit: m/s).
For Pressure-drag balance it uses a model based on the assumption that the pressure forces on a bubble are balanced by the drag force:
Here db (SI unit: m) is the bubble diameter, and Cd (dimensionless) is the drag coefficient.
Select a Drag coefficient model:
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Small spherical bubbles (Hadamard-Rybczynski) for bubbles with a diameter smaller than 2 mm.
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Large bubbles for gas bubbles with a diameter larger than 2 mm. Then enter the Surface tension coefficient σ (SI unit N/m). The default is 0.07 N/m.
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Air bubbles in tap water (Schwarz-Turner) for air bubbles of 1–10 mm mean diameter in water.
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User defined to enter a different value or expression for the Drag coefficient Cd (dimensionless). The default value is 1.
See the Slip Model theory section.
Mixing Length Limit
This section is available for the Bubbly Flow, k-ε, Bubbly Flow, Realizable k-ε, and Bubbly Flow, k-ω interfaces, 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.
Distance Equation
This section is available for the Bubbly Flow, Low Re k-ε, Bubbly Flow, Algebraic yPlus, Bubbly Flow, L-VEL, Bubbly Flow, SST, Bubbly Flow, Spalart–Allmaras, and Bubbly Flow, v2-f interfaces.
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.