# 4.9. Boundary Conditions¶

Boundary Conditions (BCs) are specified in the Boundary conditions task pane.

## 4.9.1. Creating a Boundary Condition Region¶

To define a boundary condition, there must be a region already defined in the Regions Pane.

Boundary conditions are specified over rectangular regions.

**Valid BC Regions**

Box regions

XY-planes (2D simulations only)

**Invalid BC Regions**

Any region already used by another IC

Point regions

STL regions

## 4.9.2. Setting Fluid Phase Boundary Values¶

**Volume Fraction**
The fluid phase volume fraction has a default value of one. If solids are
defined and given volume fractions greater than zero, the fluid phase
volume fraction is automatically calculated by subtracting the sum of the
solids phase volume fractions from one.

**Temperature**
The fluid phase temperature has a default value of 293.15K. Specification
of the fluid temperature is required when the following models are used:

Solving energy equations

Fluid density mode is the Ideal Gas Law

Fluid viscosity model is Sutherland’s Law

**Pressure**
An initial pressure field may be specified. If none is provided, the fluid
solver imposes a hydrostatic pressure drop across the domain if one or
more pressure outflow boundary conditions is defined.

**Velocity**
The fluid phase velocity field has a default value of zero.

**Species Mass Fractions**
Species mass fractions must be defined for all components when solving
the species transport equations. By default the species mass fraction
of the last defined fluid species is set to one and all others are set
to zero. The total of all species mass fractions must equal one.

**Turbulence**
If solving the mixing length turbulence model, a mixing length scales must
be defined for the whole domain.

If solving the k-ε turbulence model, initial values for the turbulent kinetic energy and turbulent dissipation must be provided.

**Radiation Coefficient**
A radiation coefficient has a default value of zero, and can only be
specified when solving the energy equations. The default value is
strongly recommended.

**Radiation Temperature**
The radiation temperature has a default value of 293.15K, and can only
be specified when solving the energy equations. The default value is
strongly recommended.