An Equation Variable Definition () specifies the expression or shape of an expression variable and where the definition is valid. An equation variable definition is suitable for overwriting variables that define weak equations and constraints. Doing so can be necessary for multiphysics coupling features that must alter the governing equation in one of the coupled physics. When clearing the solve-for in the study step flag for the coupling feature, the equation variable definitions will be ignored, and the governing equation is again defined as the physics would if it were alone.

To add an Equation Variable Definition, first add a Component node under Building Blocks>Components, then:

To find the definitions of the variable, click the Find Declarations of this Variable button () on the Settings window, or click the node and press F7, or right-click the node and choose Search>Find Definitions.

The Settings window has the following sections:

Enter a Variable name. The Variable name follows the rules described in Entering Names and Expressions and must match the name of a variable declaration somewhere in the same physics interface.

Also enter an Expression using the rules in Entering Names and Expressions.

The options in the Selection list and Output entities list define the selection where this variable definition is valid. See Specifying Selections for more information.

Select the Zero out components check box to enforce a symmetry to spatial vectors and matrices.

If you choose Zero out-of-plane, the out-of-plane component is set to zero in the space dimensions 1D, 2D, 1D axial symmetry, and 2D axial symmetry. The out-of-plane component in 2D axial symmetry is the second component (the φ-component).

The Zero in-plane setting does the opposite and has no effect in 3D, for scalars, or nonspatial tensors (length other than 3).

If you choose Property dependent, you can choose to zero out some components for specific values of some property values.

For each row in the table of property values, choose Zero out-of-plane, Zero in-plane, Keep all components or Explicit from the list under Components to zero out for the corresponding property value under Property value.

From the Setting for other property values list, choose Zero out-of-plane, Zero in-plane, Keep all components or Explicit to control how other property values are zeroed out.

Select the Use the setting above for undefined references check box to use the default value even when the property does not exist.

The Explicit setting to defined which components to zero out (default: 0, 1, 2 as zero-based indexes to elements in a vector). The explicit setting is useful when the number of components is not three.

Set preferences that enable protection of entered expressions. Select the Hide expression in equation view check box to remove the definition to display in the Equation View node, which is a subnode to a physics feature in the Model Builder. This disables any possibility to alter the expression; it also makes it harder to read the expression.

To further complicate reading of the expression, you can select the Encrypt expression check box. This turns on an encryption of the expression in the saved model file and when accessing the expression in a model file for Java® code. It also encrypts the tensor expression when you compile the archive (see Compiling an Archive), so the expression in a distributed builder file (*.mphphb) cannot be read.

The Base vector system setting controls the evaluation context of the definition. The Base vector system setting also controls the preferred variable to add the contribution to. If there are multiple declarations for a variable named A, for example, the evaluation context will first try to find a declaration in the same context. In case no matching context can be found, it will automatically transform the evaluated expression to the context of the first declaration.