The standard formulation for contact in COMSOL Multiphysics as described in Normal Contact and
Tangential Contact with Friction can be viewed as a biased (or unsymmetric) formulation in the sense the contact forces are added on one side of a contact pair only, the destination boundary. Often this is a preferred formulation since it involves the fewest possible evaluation points where equations and contact mappings have to be evaluated, making it attractive from a performance point of view. However, it puts requirements on, for example, the relative mesh density between source and destination to be accurate. In some situations it might also be difficult to
a priori split a set of boundaries in a source and destination as, for example, when objects are self-intersecting. This situation is typically referred to as self-contact.
For such cases, it is also possible to set up an unbiased (or symmetric) formulation for contact by using the same selection as source and destination in a Contact Pair. That is,
Γsrc =
Γdst in the schematic problem description in
Figure 3-34 and one can instead refer to a single contact boundary
Γcnt. Since contact forces will then be added on both sides of a pair of mapped points, the added equations needs to be modified in order to be consistent. To accomplish this,
Equation 3-176 is instead written as
