Fluid Properties

The Fluid Properties feature adds the momentum equations solved by the interface, except for volume forces which are added by a separate feature (Volume Force), and provides an interface for defining the material properties of the fluid.

For the Turbulent Flow interface, the Fluid Properties feature also adds the

equations for the turbulence transport equations.

Note

D O M A I N S E L E C T I O N

From the Selection list, choose the domains to apply the fluid properties.

M O D E L I N P U T S

Edit input variables to the fluid-flow equations if required. For fluid flow, these are typically introduced when a material requiring inputs has been applied.

To define the Absolute Pressure, see the settings for the Heat Transfer in

Fluids node as described in the COMSOL Multiphysics User’s Guide.

Tip

F L U I D P R O P E R T I E S

Density

The default Density (SI unit: kg/m3) uses the value From material. Select User defined to enter a different value or expression.

Dynamic Viscosity

The default Dynamic viscosity (SI unit: Pa·s) uses the value From material and describes the relationship between the shear rate and the shear stresses in a fluid. Intuitively, water and air have a low viscosity, and substances often described as thick, such as oil, have a higher viscosity. Select User defined to define a different value or expression. Using a built-in variable for the shear rate magnitude, spf.sr, makes it

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possible to define arbitrary expressions of the dynamics viscosity as a function of the shear rate.

The non-Newtonian fluids have a shear-rate dependent viscosity. Examples of non-Newtonian fluids include yoghurt, paper pulp, and polymer suspensions. See Non-Newtonian Flow—The Power Law and

Note

the Carreau Model for information.

•Select Non-Newtonian power law to use the power law as the viscosity model for a non-Newtonian fluid. Enter the Power law model parameter m (SI unit: kg/(m·s)) and Model parameter n (unitless).

•Select Non-Newtonian Carreau model to use the Carreau model as the viscosity model for a non-Newtonian fluid. Enter the Carreau model parameters.

-Zero shear rate viscosity 0 (SI unit: Pa·s).

-Infinite shear rate viscosity inf (SI unit: Pa·s).

-Model parameters (SI unit: s) and n (unitless).

D I S T A N C E E Q U A T I O N

	This section is only available for Turbulent Flow, Low Reynolds number k-
	and Turbulent Flow, Spalart-Allmaras since those models include a Wall
Important	Distance interface.

Select how the Reference length scale lref (SI unit: m) is defined—Automatic (default) or Manual:

•If Automatic (the default) is used, the wall distance is automatically evaluated as half the shortest side of the geometry bounding box. This is usually quite accurate but it can sometimes give too great a value if the geometry consists of several slim entities. In this case, it is recommended that it is defined manually.

•Select Manual to define a different value or expression for the wall distance.

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