A Variable Declaration () specifies a variety of properties for a variable. You can also right-click to add and define Variable Definition, Component Settings, and Disable in Solvers subnodes.

To add a Variable Declaration first add a Component, Feature, Property, Physics Interface, or Multiphysics Interface node 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 and a Description to include text for the variable when shown in analysis and variable listings. You can pass the component’s arguments to the description. The argument name must be placed between two # characters. For example, if you have an argument arg.param = material, the string #Plot_in the _#arg.param#_frame will be displayed as Plot in the material frame.

Enter a LaTeX-encoded string in the Symbol (LaTeX encoded) field to define a symbol (\mu, for example, to display the Greek letter μ).

Select a Dimension: Scalar, Vector (3x1), Matrix (3x3), or Custom. For Custom, you can specify a nonstandard dimension as an MxN[xKxL] array, where M, N, K, and L are integers, and K and L are optional (for example, 3x3x3 if you need a tensor of rank 3 with indices of dimension 3).

For Custom you can enter two special formats for dynamically setting the size:

The Physical quantity list defines the unit of the variable and is the same as a Dependent Variable Declaration.

If there is no physical quantity defined, enter an SI unit for the dependent variable in the SI unit field. It is possible, in some contexts, to use arguments and values of user inputs to define the unit; this way you can enable dynamic units from arguments or other user inputs. There is also an operator, evalUnit, that you can use to parse units of known variables, typically dependent variables (example, evalUnit(dep.u)).

Select an option from the Operation between multiple definitions list: Replace, Add, Multiply, Logical or, Logical and, or Inner product.

Any variable definition for this variable uses the selected operator to combine multiple variable definitions of the same variable. You typically use the option Add when several contributing features add contributions to the same variable (heat sources, for example).

Select the Interpret as right-hand side check box if you use the variable in any right-hand side of an expression. Such variables get an extra factor

that enables plotting and animations of the entire harmonic cycle in frequency-domain simulations. It is also used to identify right-hand sides for harmonic perturbation features; see Harmonic Perturbation.

Select the Include in load groups check box if the variable should be affected by load grouping. To enable load groups for a feature add an Auxiliary Settings node and select the setting Enable load groups.

If the matrix is square (for example, if you select Matrix (3x3) from the Dimension list), you can force a matrix symmetry with the options in the Matrix symmetry for square matrix list. The choices are Diagonal, Symmetric, and Anisotropic (the default), and controls the cells that the user can edit. This option also adds a check to the data field, so the user cannot enter a matrix structure that is more complex than the selected option. For example, choosing Symmetric does not allow an anisotropic matrix but it allows both isotropic and diagonal matrices.

Select the Show in plot menu check box (the default) if you want the variable to show up when a user clicks the Insert expression () and Replace expression () buttons in any of the Results nodes during a Model Builder session. Edit the Menu field to group the variable into a submenu. The default setting is to place the variable directly under the menu of the physics interface. From the Show Add all list, select For more 2 components, Yes, or No to control if the All expressions in this group option should appear for a plot group of related variables.

Select the Announce variable to feature inputs check box so the variable notifies its existence to all physics interfaces. The variable uses the selected physical quantity as an identifier. Any feature input parameter with the same physical quantity can pick up an announced variable, so that the variable can be used by the physics interface that the feature input belongs to.

An example of a feature input is any of the model input parameters in the Model Inputs section of a physics feature instance in the Model Builder. It is also possible to supply an extra Match tag to the announced variable. The predefined options here are None (the default), and all physical quantity field names. None is equivalent to an empty tag and should be used unless it is necessary to limit the matching in some sense. In such cases, enter an arbitrary but relevant tag — for example, relative or absolute to distinguish between pressure fluctuations and absolute pressure levels. You typically need physical quantity options for synchronized matching.

Click the Select Physical Quantity as Tag button () to pick a physical quantity from the list in the Physical Quantity dialog box that appears. Click the Custom Tag button ()to use a custom tag that you type in for the announce tag. Click the Reset to None button () to reset the filter to None.

Select the Announce variable to common inputs check box to notify this variable to the common model input variable list.

This section contains advanced options that you do not have to change in most cases. In the Base vector system list you can override the base vector system specified by the parent (for example, a feature or property) by choosing something other than the option Same as parent. The Declare on all frame systems option results in three variable declarations, one for each of the frames (geometry, material, and spatial). The naming convention for components is done by appending the coordinate names of the frames. A consequence of this strict convention is that the Component Settings node cannot be allowed, so you cannot add it with this setting active. Any existing Component Settings node will automatically be disabled. In addition, the Base vector system column for individual tensor indices is also disabled and locked to the Use common setting option.

If the Variable Declaration node has a Variable Definition subnode, there will also be one definition added per frame system. The context’s frame will also change, so the parsing may behave differently depending on the current frame system used.

For 3-by-1 (vector) and 3-by-3 (matrix) tensors, the Frame information rule list contains the following options:

For tensors, choose a type from the Tensor type list: Normal tensor, Tensor density, or Tensor capacity. A tensor density is a concentration, for example, where it is multiplied with the volume factor. A tensor capacity is the inverse.