An extra average stress contribution on each of ,
and
is
where
is the group_materi_elasti_compressibility, which should not be
.
This pressure term can be used to model
nearly incompressible fluids or nearly incompressible solids (e.g. rubbers).
It typically is combined with a contribution for the deviatoric stresses
(see group_materi_viscosity and group_materi_hyperelasticity).