As described in Equations, you need a Weak Form Equation () (a differential equation using a weak formulation) for all features that contribute to the governing equations.

To add a Weak Form Equation, first add a Component node under Building Blocks>Components, then:

The Settings window has the following sections:

The content of the Expression field adds up as a weak form contribution to the system of equations that you later solve for in the Model Builder.

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.

Clear the Use volume factor in axial symmetry or for non-orthonormal systems check box if you want to skip any volume factors in the weak form equation. In axial symmetry this means that you do not get the factor 2πr. For more information on non-orthonormal systems. It is also possible to select the Assume constant volume factor check box in this section.

If you want to use a different base vector system than the parent feature or property, you can choose a different system in the Base vector system list. Choose your preferred order of integration in the Integration order list: Automatic (the default); From discretization, where you specify a discretization as its name in the Discretization field; or Custom shape order, where you specify a shape order as a positive integer in the Order field.