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

  • Planes (along the boundaries of the MFiX box, or along the boundaries of a Box region)

  • STL regions

Invalid BC Regions

  • Any region already used by another BC

  • Point 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.