As described in Equations, as an alternative to the weak form you can enter the equation using the General Form Equation node ().

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

The Settings window has the following sections:

There are four coefficient in a general form equation. You define each coefficient by defining a tensor expression that requires a certain dimension. The dimensions are defined by the dimension of the dependent variable, N, and the number of spatial dimension (always 3). The table below lists the dimensions for all four coefficients.

If the dependent variable is a scalar, you can simplify the tensor dimensions by skipping all singleton dimensions. This simplifies the γ coefficient to be a spatial vector (length 3). The dependent variable can only be a tensor up to rank 1 (a vector), so use the weak form equation for dependent variables of higher ranks.

You also specify these coefficients for boundary conditions, which differs slightly from the General Form PDE interface in the Model Builder. It is straightforward to convert a Flux/Source boundary condition from this interface to an equivalent representation using only the f coefficient

where g and q are the coefficients in the Flux/Source boundary condition, and u is the dependent variable name.

In the Variable name text field, write the name of the dependent variable you enter the general form equation for. The variable name can also be a variable that you have defined as a shape function.

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 integration. 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.