### 28.1.3. Fluid Models and Fluid Specific Models Tabs for Domain Objects

The settings for Additional Variables on the Fluid Models tab are used to enable Additional Variables in the domain. For multiphase simulations, settings for unspecified and volumetric Additional Variables are available only on the Fluid Specific Models tab. For specific Additional Variables homogeneous transport equations can be set on the Fluid Models tab or on a per-fluid basis on the Fluid Specific Models tab if the Additional Variable has been set to `Fluid Dependent` on the Fluid Models tab.

#### 28.1.3.1. Additional Variable Details: List Box

This list box is used to select an Additional Variable in order to set the details of its application to the domain.

#### 28.1.3.2. Additional Variable Details: [Additional Variable Name] Check Box

This check box determines whether or not the Additional Variable is to be modeled in the domain.

##### 28.1.3.2.1. Option

###### 28.1.3.2.1.1. Transport Equation

The transport of the Additional Variable of type `Volumetric` is modeled by a transport equation. For details, see Additional Variables in the CFX-Solver Theory Guide.

###### 28.1.3.2.1.2. Diffusive Transport Equation

The transport of the Additional Variable is modeled by a transport equation. The advection term is dropped from the transport equation. For details, see Additional Variables in the CFX-Solver Theory Guide.

###### 28.1.3.2.1.3. Homogeneous Transport Equation

The transport of the Additional Variable is modeled by a transport equation. This option is available only on the Fluid Models tab and only for multiphase flows (that is, only for homogeneous applications). For details, see:

- Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Modeling Guide

- Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Theory Guide.

###### 28.1.3.2.1.4. Homogeneous Diffusive Transport Equation

The transport of the Additional Variable is modeled by a transport equation. The advection term is dropped from the transport equation. This option is available only on the Fluid Models tab and only for multiphase flows (that is, only for homogeneous applications). For details, see:

- Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Modeling Guide

- Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Theory Guide.

###### 28.1.3.2.1.5. Poisson Equation

The transport of the Additional Variable is modeled by a transport equation. The transient and advection terms are dropped from the transport equation. For details, see Additional Variables in the CFX-Solver Theory Guide.

###### 28.1.3.2.1.6. Homogeneous Poisson Equation

The transport of the Additional Variable is modeled by a transport equation. The transient and advection terms are dropped from the transport equation. This option is available only on the Fluid Models tab and only for multiphase flows (that is, only for homogeneous applications). For details, see:

- Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Modeling Guide

- Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Theory Guide.

###### 28.1.3.2.1.7. Fluid Dependent

When the `Fluid Dependent` option is selected, the Additional Variable model details can be set for each fluid on the Fluid Specific Models tab.

###### 28.1.3.2.1.8. Algebraic Equation

A given quantity or CEL expression specifies the value of the Additional Variable throughout the domain. Application of this option is, in the context of the fluids to which the Additional Variable is applied, effectively the same as setting the Additional Variable type to Unspecified.

###### 28.1.3.2.1.9. Vector Algebraic Equation

A total of three given quantities, CEL expressions, or both, specifies the vector value of the Additional Variable throughout the domain. Application of this option is, in the context of the fluids to which the Additional Variable is applied, effectively the same as setting the Additional Variable type to Unspecified.

##### 28.1.3.2.2. Value

(applies only when Additional Variable Details: [Additional Variable name] Check Box: Option is set to Algebraic Equation)

Enter a numerical quantity or CEL expression that specifies the value of the Additional Variable throughout the domain.

##### 28.1.3.2.3. Kinematic Diffusivity Check Box

(applies only when Additional Variable Details: [Additional Variable name] Check Box: Option is set to Transport Equation, Diffusive Transport Equation, or Poisson Equation)

When running a Transport Equation Additional Variable, this check box determines whether the molecular diffusion term is added to the transport equation for the Additional Variable. For turbulent flow, the turbulent diffusion term (which is a consequence of averaging the advection term) is automatically included. Setting the kinematic diffusivity to zero includes the turbulent diffusion term only.

You must select this check box when using the Poisson equation or diffusive transport equation. If you do not, a blue warning message will appear to remind you.

##### 28.1.3.2.4. Kinematic Diffusivity Check Box: Kinematic Diffusivity

(applies only when Additional Variable Details: [Additional Variable name] check box: Option is set to Transport Equation, Poisson Equation, Diffusive Transport Equation)

Enter a numerical quantity or CEL expression that specifies the value of the kinematic diffusivity throughout the domain.

##### 28.1.3.2.5. AV Properties for Fluid: Frame Overview

(applies only for homogeneous Additional Variables)

The settings contained in this frame are used to optionally specify the kinematic diffusivity of the selected Additional Variable. The kinematic diffusivity may differ for each fluid in the domain. The solver calculates a single effective kinematic diffusivity based on the kinematic diffusivity of the Additional Variable in each fluid. For details, see Homogeneous Additional Variables in Multiphase Flow in the CFX-Solver Theory Guide.

##### 28.1.3.2.6. AV Properties for Fluid: List Box

This list box is used to select a fluid in the domain in order to optionally specify the kinematic diffusivity of the selected Additional Variable in that fluid.

##### 28.1.3.2.7. AV Properties for Fluid: [Fluid Name] Check Box

This check box determines whether or not the kinematic diffusivity of the selected Additional Variable in the selected fluid is specified. Not specifying the kinematic diffusivity implies that the Additional Variable is non-diffusive.

##### 28.1.3.2.8. AV Properties for Fluid: [Fluid Name] Check Box: Kinematic Diffusivity

(applies only when Additional Variable Details: [Additional Variable name] Check Box: Option is set to Homogeneous Transport Equation, Homogeneous Diffusive Transport Equation, or Homogeneous Poisson Equation)

Enter a numerical quantity or CEL expression that specifies the value of the kinematic diffusivity, throughout the domain, of the selected Additional Variable in the selected fluid.

##### 28.1.3.2.9. Vector xValue, Vector yValue, and Vector zValue

(applies only when Additional Variable Details: [Additional Variable Name] Check Box: Option is set to Vector Algebraic Equation)

Enter a numerical quantity or CEL expression for each vector algebraic equation component.

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