Fluid Properties
The Fluid Properties node adds the momentum and continuity equations solved by the physics interface, except for volume forces, which are added by the Volume Force feature. The node also provides an interface for defining the material properties of the fluid.
For the Turbulent Flow interfaces, the Fluid Properties node also adds the equations for the turbulence transport equations.
Model Inputs
Fluid properties, such as density and 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 Common 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 choice of pressure variable depends on the system of equations being solved. For example, in a unidirectional incompressible flow problem, the pressure drop over the modeled domain is probably many orders of magnitude smaller than the atmospheric pressure, which, when included, may reduce the stability and convergence properties of the solver. In other cases, such as when the pressure is part of an expression for the gas volume or the diffusion coefficients, it may be more convenient to solve for the absolute 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
Fluid Properties
Density
The density can either be specified by a material, or by a User defined expression. The density in a material can depend on temperature and/or pressure and these dependencies are automatically replaced by pref for weakly compressible flows and pref and Tref for incompressible flows (as specified by the Compressibility setting at the physics interface level). If density variations with respect to pressure are to be included in the computations, Compressibility must be set to compressible. Any dependencies in the density on quantities other than temperature and pressure must be consistent with the Compressibility setting at the interface level.
Dynamic Viscosity
The Dynamic viscosity μ describes the relationship between the shear rate and the shear stresses in a fluid. Intuitively, water and air have low viscosities, and substances often described as thick (such as oil) have higher viscosities.
Using the built-in variable for the shear rate magnitude, spf.sr, makes it possible to define arbitrary expressions of the dynamic viscosity as a function of the shear rate.
For laminar flow, the Non-Newtonian power law may be used to model the viscosity of a non-Newtonian fluid.
The following model parameters are required for the Power law:
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Fluid consistency coefficient m
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Flow behavior index n
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Lower shear rate limit
For the Carreau model, the following parameters are required:
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Zero shear rate viscosity μ0
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Infinite shear rate viscosity μinf
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Model parameter λ and n
For the Bingham-Papanastasiou model, the following parameters are required:
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Plastic viscosity μp
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Yield stress τy
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Model parameter mp
For the Herschel-Bukley-Papanastasiou model, the following parameters are required:
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Fluid consistency coefficient m
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Flow behavior index n
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Yield stress τy
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Model parameter mp
For the Casson-Papanastasiou model, the following parameters are required:
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Plastic viscosity μp
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Yield stress τy
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Model parameter mp
The non-Newtonian fluids models have a shear-rate dependent viscosity. Examples of non-Newtonian fluids include yogurt, paper pulp, and polymer suspensions. See Non-Newtonian Flow in the CFD Module User’s Guide.
Mixing Length Limit
For the Turbulent Flow, k-ε, Turbulent Flow, k-ω, and Rotating Machinery, Turbulent Flow k-ε interfaces, an upper limit on the mixing length is required.
When the Mixing length limit lmix,lim is set to Automatic, it is evaluated to 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.
Distance Equation
For the Turbulent Flow, Low Reynolds number k-ε, Turbulent flow, Algebraic yPlus, Turbulent Flow, L-VEL, Turbulent flow, SST, the Turbulent Flow, Spalart-Allmaras, and the Turbulent Flow, v2-f interfaces, a Wall Distance interface is included.
When the Reference length scale lref is set to Automatic, it is evaluated to 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.