The Form Union/Assembly Node — Uniting the Geometry

To unite the geometry, COMSOL Multiphysics evaluates the geometry sequence from the top down. The final node in the geometry sequence (before any virtual operations, if present), the node, determines how to form the geometry that is used for meshing and analysis (possibly after also applying virtual operations). There are two methods to form the geometry, which also determine the name of the node: or (

). There are some differences and aspects to consider when choosing a method:

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The default method is to form a union. The software then forms a union from all geometry objects that the geometry sequence contains or creates. The union is divided into domains separated by boundaries according to the participating geometry objects. You can mesh the entire geometry and model the physics by assigning material properties, boundary conditions, and other data for the model. It is also possible but often not necessary to specify boundary conditions on interior boundaries between domains in the geometry. By default, COMSOL Multiphysics ensures continuity in the physics interface fields across interior boundaries. See also Removing Interior Boundaries.

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The alternative method is to form an assembly. The software then treats the geometry as a collection of the geometry objects instead of uniting them. This means that you must use pairs to connect boundaries where a field is continuous, but it also makes it possible to use special pair conditions for applications such as contact modeling, where you can add contact pairs to model contact between geometric parts (requires the Structural Mechanics Module or the MEMS Module). Forming an assembly is also required to model geometry domains that slide or move relative to each other. By default, identity pairs are created automatically when forming an assembly. An assembly can also be useful for meshing each geometry object independently in, for example, thin and slender geometries with high aspect ratios. Another case where you need to use an assembly is when the geometry is too complex for forming a union, which might be the case when importing an assembly geometry from CAD data.


	When forming a union or an assembly for axisymmetric models, COMSOL Multiphysics removes all parts of the geometry from the r<0 half plane.

Physics Considerations

The physical implication of the Form Union step is that the domains in the model cannot slide or move relative to each other. This is an appropriate default assumption for most modeling within COMSOL Multiphysics, but it is not valid when using any of the following physics interfaces:

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Solid Mechanics, when contact features are included

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Multibody Dynamics

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Rotating Machinery, Magnetic

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Rotating Machinery, Fluid Flow

When you use any of the above physics interfaces, or when you want to have a nonconforming mesh between adjacent domains, finalize the geometry with the Form Assembly setting instead.

The node (

) ends each geometry sequence in 1D. In 2D and 3D, it is possible to add virtual operation nodes, some partitioning nodes, and selection nodes after that node. In the Model Tree, its label is or depending on its settings. By default, it unites all geometry objects into a single geometry object (this is the variant). You cannot delete or disable the node. When you leave the geometry sequence to define materials or physics nodes, the window provides information about the method (forming a union or an assembly) and about the number of geometric entities (domain, boundaries, and so on) in the geometry.

Form Union/Assembly

The default method, , forms a union of all geometry objects. Select from the list if you do not want the geometry objects to be united. The program then forms the geometry by collecting the objects in an assembly object. If you form an assembly, select the check box to get imprints of the geometry objects that touch each other. An imprint of a usually smaller object’s boundary on an adjacent larger object’s boundary inserts points on the boundary in 2D and creates edges on the boundary in 3D. Creating imprints can be useful when you need identical matching meshes on both objects’ boundaries or when you want to split the larger boundary so that it contains a segment or area that matches the smaller boundary. Select the check box (selected by default) to generate pairs corresponding to the objects that touch each other. Select the check box to generate one pair for each connected set of boundaries. Clear the check box to generate one pair for each pair of objects that touch each other. From the list, select (the default) to generate identity pairs, which makes it possible to connect the physics fields across the objects’ boundaries, or to generate contact pairs. Contact pairs are only useful for contact modeling in structural mechanics and require a license for the Structural Mechanics Module or the MEMS Module.

You can change the settings for the list if you experience problems with the Form Union/Assembly operation.

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The default value in the list is , which for 3D objects represented using the CAD kernel determines the repair tolerance internally. For 3D objects represented using the COMSOL kernel, and for 2D and 1D objects, means a relative repair tolerance of 10−6.

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Choose to enter a value for the field (the default is determined by the main node’s setting). This value is relative to the largest absolute value of the coordinates of all input objects.

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Choose to enter a value for the field (the default is determined by the main node’s setting; SI unit: m). This value uses the same unit as the geometry sequence’s length unit.

Meshing Aspects

The options for creating meshes depend on whether you form a union or form an assembly, with or without imprints. The following figure shows a comparison of the meshes created with Form Union, Form Assembly with imprints, and Form Assembly without imprints.

The Form Union mesh is a conforming mesh across the mating face. That is, the mesh elements on either side of the boundaries share the same nodes and faces. With such a mesh, the continuity of the fields and fluxes is naturally satisfied via the finite element method. The Form Assembly mesh makes it possible to create nonconforming meshes. The element faces at that boundary will share nodes. With imprints enabled, the element boundaries line up. The elements on either side have nonconforming nodes, but the software adds constraint equations to approximately enforce continuity of the fields and fluxes. With imprints disabled, the element boundaries between objects do not line up. The constraint equations that approximately enforce continuity of the fields and fluxes between the domains will be more approximate. The Form Union mesh will be the most accurate but require the most memory to solve. The Form Assembly without imprints will be the least accurate and require relatively less memory to solve.


	In 2D and 3D, virtual operation nodes and selection nodes can appear after the Form Union/Assembly node.