# 4.8. Initial Conditions¶

Initial Conditions (ICs) are specified in the Initial conditions task pane.
ICs summarized in the *IC summary table* illustrated in Fig. 4.2.
The MFiX solver will apply ICs in the order that they are defined in this pane.
As a result, if two or more regions overlap, the IC with the larger index (ID)
is used in the intersecting region. IC IDs are shown in the table, and can be
changed using the up and down arrows.

## 4.8.1. Creating an Initial Condition Region¶

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

Initial conditions are specified over rectangular regions corresponding to the scalar grid. A new IC region is created by clicking the add button, , at the top of the IC region table, and selecting one or more valid regions (see Fig. 4.3).

**Valid IC Regions**

Box regions

XY-planes (2D simulations only)

**Invalid IC Regions**

Any region already used by another IC

XZ- and YZ-planes

Point regions

STL regions

## 4.8.2. Setting Fluid Phase Initial Values¶

When the fluid phase is enabled, initial values for fluid field variables must be specified for the entire computational domain. The models being solved dictate which variables require an initial value.

**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 settings are used:

The energy equation is solved

Fluid density is computed by the Ideal Gas Law

Fluid viscosity is computed by the Sutherland’s Law

**Pressure**
An initial pressure field may be specified. If none is provided, the fluid
solver will attempt to impose a hydrostatic pressure drop across the domain.

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

**Species Mass Fractions**
Species mass fractions must be defined for all species 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 using the mixing length turbulence model, a mixing length scale must
be defined for the whole domain.

If solving the k-ε turbulence model, initial values for 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.

## 4.8.3. Setting Solids Phase Initial Values¶

When one or more solids phases are enabled, initial values for solids field variables must be specified for the entire computational domain. The models being solved dictate which variables require an initial value.

**Volume Fraction**
The solids phase volume fraction has a default value of zero. The sum
of all solids phase volume fractions are used to calculate the volume
(void) fraction of the fluid phase. 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 volume fractions from one.

**Temperature**
The solids phase temperature has a default value of 293.15K. Specification
of solids temperature is required when solving energy equations.

**Pressure** *(TFM solids only)*
Solids pressure is an optional input for TFM solids models. There is only
one value for all solids phases.

**Granular Temperature**
The initial granular temperature has a default value of zero.

For TFM solids, granular temperature can only be specified when solving one of the non-algebraic viscous stress models.

For DEM solids models, a value may be specified when using the automatic particle generator to modify the initial particle velocity distribution.

**Species Mass Fractions**
Species mass fractions must be defined for all species 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.

**Fit particles to region** *(DEM solids only)*
This option is used with the automatic particle generator to expand the
initial particle lattice to span the region.

**Radiation Coefficient**
The 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.

## 4.8.4. Setting User-Defined Scalar Initial Values¶

When one or more user-defined scalars are enabled, initial values for the scalars must be specified for the entire computational domain.

**Scalar Value**
For each user-defined scalar, an initial field value must be provided. By
default, all scalars are set to zero.