AuxKernel

An AuxKernel computes values that are stored in "Auxiliary Variables". There is an infinite number of ways to generate these values but some common ones are: based on other variables/data in the simulation (i.e. coupled data), read from an external file and interpolated from a Function. The Auxiliary system is _extremely_ flexible on purpose. The data values computed by AuxKernels and stored in Auxiliary Variables (AuxVariables) are often used for visualization (i.e. written to output files) but can also be coupled back into other calculations (including in Kernels) or provided as the input to other systems such as Postprocessors.

When an auxiliary kernel depends on user objects, including postprocessors, those user objects are put in a group of user objects named as pre_aux, so that those user objects can be evaluated before the auxiliary kernel. By default, all user objects are evaluated after auxiliary kernel evaluation, i.e. in post_aux group. If the depending user objects depend on some other user objects, these indirectly depending user objects for auxiliary kernels are automatically put in the pre_aux group.

For more information: see the documentation for the Auxiliary System