Multiphysics Contact Analysis
Two different classes of multiphysics contact problems will be described in this section.
Fluxes Through Contact Surfaces
In some contact problems, there is some kind of flux from one domain to another through the contact zone. This can for example be a heat flux, an electric current, or moisture transport. The common property here is that the other physics fields than the displacements are present in domains where the solid mechanics problem is solved. Typically, there will be a more or less perfect insulation as long as there is no contact, but as soon as contact is established there will be a flux through the contact area.
This class of problems often exhibit a high degree of nonlinearity, which may lead to convergence problems in the nonlinear solver. As an example, consider heat transfer through the contact area, where initially only a small spot is in contact. The solution for the temperature field is then extremely sensitive to the size of the contact area. If, at the same time, the solid deforms due to thermal expansion, there may be large changes in the contact area between iterations,
It is important to resolve the size of the contact area accurately, that is, to use a very fine mesh in the contact area when modeling fully coupled multiphysics problems.
If the contact area is larger, a fine mesh is not required because then the temperature solution is not that sensitive to the size. If possible, start with an initial configuration where the contact area is not very small.
You can use the contact variables (gap and contact pressure) in expressions for quantities in other physics interfaces. As an example, a thermal resistance in the contact region can depend on the contact pressure.
In many cases, the penalty method is preferred in multiphysics contact problems because of its better stability and less restrictive requirements on solver selections. If the contact conditions depend strongly on the contact pressure, use the augmented Lagrangian method because if its higher accuracy.
Fields Exist in the Gap
In this class of problems, a field exists between the domains controlled by solid mechanics.
This is the case in, for example, fluid-structure interaction (FSI) problems. Here, the equations in the fluid are solved on a domain with a moving mesh, so that the shape of the fluid domain is controlled by the displacements of the solid. Another case of the same type is when there is an electric field in an air gap.
If contact is established, the mesh in the original gap between the source and destination boundaries will collapse. This must be avoided. The remedy is to add an offset in the contact settings to either the source boundary, the destination boundary, or both. If you do this, contact forces will be transmitted without the geometrical gap being fully closed.
If, for example, this technique is used when modeling a valve, there will still be some small flux even though the valve is closed, since there is a geometrical gap with the width of the artificial offset. By choosing a suitably small offset, you can however make that flux negligible.
