Penalty formulation

In contact analysis, normal forces $F_n$ follow from the condition that bodies cannot penetrate each other. Since we use a penalty formulation, the normal force is given by

$$F_n = \lambda u_n$$

where $u$ is the penetration and $\lambda$ 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 specifying the penalty factors contact_penalty_pressure and contact_penalty_temperature.