Accessing _qp of a particular element

[M16R24]

How can the Gauss points associated with an element be accessed? How to get the elements that belong to a particular grain? Any source or documentation to understand the further mesh related functions can be highly useful.

[ngrilli]

@M16R24 if by "grain" you mean a region related to a single crystal in a polycrystal, then you can use neper to generate a mesh and MOOSE will be able to see each grain as a block. There is a related video here:
https://www.youtube.com/watch?v=qG7EoX34hZ8&t=1123s
Once you have a grain as a block, there are plenty of things you can do, for instance postprocessors
to process data on a single grain or assign a specific material to that grain.

[GiudGiud]

How can the Gauss points associated with an element be accessed?

most objects in moose (kernels, auxkernels) loop over element quadrature points, or are called during those loops (materials)

The inner routines of these objects, the ones that MOOSE users write (for example computeQpResidual for a kernel, computeQpProperties for a material), are called on every quadrature points. _current_elem is usually the name of the attribute for the element and _qp is the index of the current quadrature point. Then there are numerous class attributes (variables values, material properties, quadrature point locations as _q_point) that are indexed by quadrature points, and can be accessed with the index _qp within those inner routines