Often it is claimed by CPR-AMG evangelists that this will make the pressure
system more elliptic. That may be the case. But even more important it also
decouples the pressure from the saturations.
Without this approach the pressure correction influences the smoothing of the
full system too much and e.g. for Norne CPR is worse than simple block ILU0.
The approach is inspired by Geiger's system-amg but we use dune-istl
aggregation AMG for it. On the fine level all unknowns attached to a cell
form a matrix block and are treated fully coupled. To form the first
coarse level system we use only the pressure component to guide the aggregation
and neglect all other unknowns on the fine level. All other level are formed
in the usual way by scalar aggregation.
Currently,it has to be requested for flow_ebos manually by passing
"linear_solver_use_amg=true amg_blackoil_system=true" to it.
