In ParaGeo you can specify any pair of contact surfaces to be tied (i.e. their mechanical freedoms will be coupled following a master-slave approach to approximate a continuum geometry at the coupled contact surface). It should be noted that the master-slave coupling is internally performed via the coupled freedoms code through the definition of either Couple_freedoms or via Tied_contact_sets in Contact_global and Contact_set  data structures depending on the type of coupling. As this is an additional usage of coupled freedoms, for the sake of clarification we summarize all coupled freedoms functionalities available in ParaGeo in the following table:

 

 

 

 

 

Standard coupled freedoms

 

Standard coupled freedoms refers to a functionality to couple the specified freedoms (any of the specified displacements in x, y and z) of all the nodes in a given geometry entity or geometry set. One node/point in such geometry entity will be the master node and the remaining ones will be slaves (this may be user-defined). Consequently any displacement that the master node experiences will be transferred to the slaves so that they have the same displacement in the coupled directions. This is often used to maintain a given boundary to be planar/"flat" during the simulation (e.g. to maintain the top surface "flat" in a 3D problem a standard coupled freedoms with displacements coupled in the Z direction may be defined). Note therefore that this type of coupled freedoms is not relevant to the tied contact surfaces.

 

 

 

 

Geometry (node to facet) coupled freedoms

 

The geometry (node to facet) coupled freedoms is a master-slave coupling approach which enables contact surfaces to be tied in a sequential analysis. It is more computationally expensive than the node to node coupled freedoms but may be used for contact surfaces with non-matching meshes. It requires definition of the coupled freedoms in all displacement directions and the corresponding master and slave geometry sets.

 

 

 

 

Geometry (node to node) coupled freedoms

 

The geometry (node to node) coupled freedoms is a master-slave coupling approach which enables contact surfaces to be tied in both a sequential and parallel analysis with explicit mechanics. It can only be used for contact surfaces with matching meshes and the coupling may be defined either via Couple_freedoms or via Tied_contact_sets in Contact_global and Contact_set data structures.

 

Both options for master-slave coupling of the contact geometry surfaces are equivalent in the mechanical field when contact is not active.  However, there are differences in the mechanical and porous/thermal flow fields when using Couple_freedoms (node to node) with contact data :

•Definition using Tied_contact_sets in Contact_global data - master-slave mechanical node to node coupling in all directions take place, however, contact is not solved for the tied contact sets and the associated contact data disregarded.  This means that each pair of tied contact geometry surface will move together but because contact is not solved, there is no influence from the mechanical contact and the contact link enabling flow across and along the contact surfaces does not exist, so effectively a no-flow condition is imposed on each pair of tied contact set geometry.  The effect of this definition is equivalent to using Couple_freedoms (node to node) for the contact geometry surfaces but without definition of contact data.

 

•Definition using Couple_freedoms (node to node) with contact data - master-slave mechanical node to node coupling in all directions take place, however, contact data is active and porous/thermal flow can occur across and along the contact surfaces. Contact also has an effect on the mechanical field eventhough the contact pairs are coupled to move together.  To minimize the effect from the mechanical contact, it is recommended to set the normal and tangential penalty values to a very small value (e.g. 1E-4 of the material Young's modulus).  Thus this definition has the advantage of enabling flow transfer without the mechanical influence.

 

 

 

 

Tied Contact

 

An example of tying contact surfaces via Tied_contact_sets in Contact_global is provided below. It should be noted that all contact sets defined in the Tied_contact_sets keyword will have their contact data disregarded, i.e. no contact is solved and the contact data blocks for these sets will not be available in ParaView Extract Block filter.  This is equivalent to using Couple_freedoms (node to node) without any contact data defined.  Each pair of tied contact surfaces MUST be defined in their own separate contact set. For the sake of clarity the example below considers 5 faults in which the hanging wall of Fault01 and Fault02 are tied with their corresponding footwall counterparts whereas Fault03, Fault04 and Fault05 will be governed by a contact relationship.