In contact analysis, normal forces Fn follow from the condition that bodies cannot penetrate each other. Since we use a penalty formulation, the normal force is given by
where u is the penetration and
is called
the contact_penalty_velocity because its generates
forces on the velocity unknowns.
You can also impose groundflow_pressure and condif_temperature contact conditions
by respectively the penalty factors contact_penalty_pressure and
contact_penalty_temperature.