[klausb]

I need additional, temporary scalarFields of the same sizes and data types as diag() and lower() to store intermediate values during the creation of a custom preconditioner.

The sizes should be:

const label nCells = rD_.size(); // for the scalarFields like diag()
     const label nFaces = solver_.matrix().upper().size(); // for the scalarFields similar to lower()

Where and how should I create additional, temporary scalarFields considering, that they will be used in every timestep?