# 4.5. Fluid¶

The fluid pane is used to select the models and parameters defining the fluid phase. The fluid pane is only accessible if the fluid phase is being solved.

**Name**
The name used to refer to the continuous phase in the GUI. By default,
the continuous phase is termed “fluid”, However, it may be renamed for
convenience.

## 4.5.1. Fluid Model Options¶

**Solve momentum equation**
By default, all momentum equations are solved. Individual momentum
equations may be disabled by toggling the check box.

**Solve species equations**
By default, species transport equations are not solved for the fluid
phase. If species equations are enabled, species will need to be
added to the fluid phase using the fluid species tool.

**Enable scalar equations**
By default, no additional scalar transport equations are solved with
the fluid phase. Additional scalars may be added by toggling the
scalar checkbox and specifying the number of additional scalars to
track.

**Energy equations** Energy equations cannot be enabled or disabled on
a per-phase basis. As such, they are enabled or disabled for all
phases in the Model Setup pane.

## 4.5.2. Fluid Properties¶

**Density** has units of \(({kg}/{m^3})\) and may be specified
using one of the following approaches.

*Constant:*A positive (non-zero) number must be provided.

*Ideal gas law*Requires fluid temperature be specified for the whole domain and all flow boundary conditions.

Requires a fluid molecular weight.

*User-Defined Function (UDF)*A custom equation of state must be provided in the usrproperties.f

A custom solver must be built.

**Viscosity** has units of \((Pa \cdot s)\) and may be specified
using one of the following approaches.

*Constant:*A positive (non-zero) number must be provided.

*Sutherland’s law*Requires fluid temperature be specified for the whole domain and all flow boundary conditions.

*User-Defined Function (UDF)*A custom equation of state must be provided in the usrproperties.f

A custom solver must be built.

**Molecular Weight** has units of \(({kg}/{kmol})\) and may be specified
using one of the following approaches.

*Constant:*A positive (non-zero) number must be provided.

*Mixture*Requires fluid species definition.

Requires fluid species mass fractions specification for the whole domain and all flow boundary conditions

**Specific Heat** has units of \(({J}/{kg \cdot K})\) and may be specified
using one of the following approaches.

*Constant:*A positive (non-zero) number must be provided.

*Mixture*Requires fluid species definition.

Requires fluid species mass fractions specification for the whole domain and all flow boundary conditions

*User-Defined Function (UDF)*A custom equation of state must be provided in the usrproperties.f

A custom solver must be built.

**Thermal conductivity** has units of \(({W}/{m \cdot K})\) may be specified
using one of the following approaches.

*Constant:*A non-negative number must be provided.

*Dilute mixture approximation*Requires fluid temperature be specified for the whole domain and all flow boundary conditions.

*User-Defined Function (UDF)*A custom equation of state must be provided in the usrproperties.f

A custom solver must be built.

**Reference pressure** has units of \((Pa)\) and is zero by default. A
constant value may be specified to shift the simulation pressure prior
to scaling.

**Pressure scale factor** is dimensionless and is one by default. A
constant value may be specified to scale the simulation pressure.