Fluid Properties

The 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.

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.

By default, the model input is set to , and the temperature is controlled from Common Model Inputs under or by a locally defined Model Input. If a Heat Transfer interface is included in the component, it controls the temperature . Alternatively, the temperature field can be selected from another physics interface. All physics interfaces have their own tags (). For example, if a Heat Transfer in Fluids interface is included in the component, the option is available for T.

You can also select from the model input in order to manually prescribe T.

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 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 field can be edited by clicking (

) and entering the desired value in the input field.

	Model Inputs and Multiphysics Couplings in the COMSOL Multiphysics Reference Manual

The density can either be specified by a material, or by a 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 setting at the physics interface level). If density variations with respect to pressure are to be included in the computations, must be set to compressible. Any dependencies in the density on quantities other than temperature and pressure must be consistent with the setting at the interface level.

The 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.