COMSOL 6.3 - Identity Pairs

for an identity boundary pair) to specify two selections of boundaries that overlap but belong to different parts of an assembly. Then assign a boundary condition to connect the physics nodes in the two parts in a physics interface. Identity pairs connect overlapping boundaries in different connecting parts of an assembly.

The (

) is the most commonly used node. For 3D models, the is available (

, which can be useful for connecting two edges in a shell model, for example. You can also choose the (

).

	Go to Pair Name for information about the Settings window Pair name. Also see Settings and Properties Windows for Feature Nodes.

The default is , or select or . Identity, Contact, and Sector Symmetry pairs are available for boundary pairs, and Identity pairs are also available on edge and point levels (where there is no list).

If the pair was created automatically when forming an assembly, the checkbox is visible. Click to clear this checkbox to be able to make manual changes to the and selections. Pairs in manual mode do not have their selections updated when the geometry sequence is rebuilt.

The operator mapping an expression E on the source side to the destination side is denoted src2dst_pn(E), where _pn is the pair name.

For an , the variable src2dst_pn (defined on the destination) is 1 where there is a corresponding source point, and 0 otherwise. The corresponding operator and variable for use on the source side are denoted dst2src_pn.

Similarly, for a there is also an operator src2dst_pn_mph that is suited for use in multiphysics coupling. The variable geomgap_dst_pn is the geometric gap between the source and the destination in the deformed configuration, seen from the destination side (following the normal of the destination boundary). The corresponding operators and variables for use on the source side are denoted dst2src_pn, dst2src_pn_mph, and geomgap_src_pn.

For a , a function sector_pn, which gives the value of the input on the other side of the pair, and a variable sectorcontactvar_pn, which is equal to 1 when the sides of the pair are in contact and 0 otherwise, are available. If the number of sectors is even, a variable sectorsignvar_pn, which is −1^(the number of sector symmetry rotations), is available.

The destination boundaries should overlap the source boundaries. The condition that connects the physics nodes on the destination and source boundaries is specified in the physics interface. For example, it can be a constraint that constrains a dependent variable (temperature, for example) on the destination side to be equal to a dependent variable on the source side.

To define the source or destination boundaries, from the lists, choose , , or any other available selection for the boundaries on the source or destination side. If is selected, click in the window to add boundaries to the section. If required, click the button (

) to swap the source boundaries and the destination boundaries.

Click the button to toggle between turning ON

and OFF

selections directly in geometry, for example. Such selections also become active when you choose from the list.

	For Identity Edge Pairs and Identity Point Pairs, edges and points, respectively, replace boundaries in the selections of the pair’s source and destination.

Use and the settings to control the frames to use for the source and destination of the pair. For nodes generated by the node, the frame settings are controlled from the node by default. To manually control the frames for automatically generated nodes, select the checkbox. Source and destination points are connected if their coordinates in their respective frames are equal.

When using the pair in a boundary condition, the software couples a destination point to its nearest source point if they have a distance that is less than the search distance. By default, the search distance is determined automatically. To tune it manually, choose from the list and enter a value in the field (SI unit: m). A larger search distance can be needed when the geometry has a large gap between source and destination.

If the source and destination mesh have their node points in the same spatial positions they are considered as compatible. You can select which strategy to use for the src2dst_pn map by setting the list to any of , , or :

If you have selected or , a field appears, where you can specify the relative tolerance, compared to the mesh element size, for when the mesh elements are considered compatible (default: 0.001).

See also the Periodic Boundary Conditions section for a short discussion on when this setting is expected to be relevant.