Global Dependent Variables
Add a Global Dependent Variables node ()under Global Definitions > Equation Contributions to declare one or more global scalar degrees of freedom, or DOFs. Each DOF is given a unique name, which can be used anywhere in equations and variable definitions. Variables representing the first and second time derivatives of the DOFs are optionally generated by appending t and tt to the supplied DOF names.
The Settings window contains the following sections:
Degrees of Freedom
Enter a name for each degree of freedom in the DOF name column. For each DOF, specify an Initial value expression and optionally a Description which will be shown in menus, plot annotations, etc. When time derivatives are defined by a second-order time-dependent solver, an Initial time derivative expression must also be given.
Unit
By default, global dependent variables are dimensionless. You can assign a unit either by selecting a predefined quantity or by explicitly entering a unit expression.
Click the Select Dependent Variable Quantity button () to open the Physical Quantity dialog to browse to find a physical quantity to use. You can also type a search string in the text field at the top of the dialog and then click the Filter button () to filter the list of physical quantities. For example, type potential and click the Filter button to only list physical quantities that represent some kind of potential.
Alternatively, click the Define Dependent Variable Unit button () to edit the unit directly in the Unit column, typing a unit to define the dependent variable quantity. The Quantity column then shows Custom unit.
Only the dimensions of the specified custom unit matter. The global degrees of freedom will always refer to the unit system’s base unit or derived unit with the given dimensions. For example, if you set the custom unit to feet, ft, in a model using SI as unit system, then the unit of the global dependent variables will still be meters, which is the SI system’s unit of length.
Discretization
By default, Time derivatives are defined From solver. Time derivative variables will then be computed as the time derivative of a time-dependent solution or zero if the solution is not time-dependent. Choose Frequency domain to define time derivatives using a time-harmonic assumption with the specified Frequency expression. For example, the time derivative of a DOF named u will be ut = 2πifu where f is the specified frequency. Choose Off to turn time derivative variables off. The time derivative variable name, if it exists, will still be interpreted as a time derivative. This allows manually redefining the time derivatives of global DOFs using, for example, a global Variables node.
Set the Value type when using splitting of complex variables to decide how degrees of freedom should be declared when Split complex variables in real and imaginary parts has been enabled in a Compile Equations solver node. The default setting, Complex, will lead to the declaration of both real and imaginary degrees of freedom. The specified DOF names will therefore be complex variables. Select Real to only declare real DOFs, guaranteeing that the DOF names are real-valued variables.