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[FAQ]

The default treatment for a rigidwall is to set a node’s velocity to zero when it reaches the wall. The kinetic energy of the node is lost and not recoverable. Most *CONTACT interfaces use a penalty method; this can be applied to the rigidwall by specifying RWPNAL in *CONTROL_CONTACT.  With a penalty method, the node’s motion is essentially resisted by a spring.  The node’s kinetic energy is converted to potential energy in the spring (RWPNAL=1.0 is a good starting point). *CONTACT and *RIGIDWALL generally give the same change in momentum of the body.  The penalty-based rigidwall formulation differs from the default formulation in that (1) slave nodes may belong to rigid bodies and (2) the rigidwall energy is recoverable.