Nodesets and Sidesets

• Creating Nodesets and Sidesets
• Grouping Faces on a Surface into a Sideset
• Deleting Nodesets and Sidesets
• Displaying Nodesets and Sidesets
• Nodeset Associativity

Boundary conditions such as constraints and loads are applied to the finite element model using nodesets or sidesets, also known as Genesis entities. Rather than attempting to maintain specific boundary condition information, such as load, temperature, constraint, etc., Genesis entities are the generic vehicle for the user to set up boundary conditions on the model. Nodes, elements and element faces are instead grouped together and assigned unique IDs. Node, element and face IDs aassigned to Genesis entities can then be written to the Exodus II mesh file. Once imported to the intended analysis application, the nodeset and sideset IDs can be appropriately interpreted as specific physical boundary conditions.

The preferred method for creating Genesis entities is to assign vertices, curves, surfaces or volumes to a specific nodeset or sideset ID. Any mesh entity owned by the geometric entity in a nodeset or sideset is automatically assigned to the same nodeset or sideset. This allows greatest flexibility in generating and updating the finite element mesh. For example, if a surface belongs to a specific sideset, remeshing the surface will automatically delete any old faces from the sideset and add the faces of the new mesh.

In some cases, the geometric model does not provide enough resolution to define the desired boundary conditions. In this case, the model may be partitioned using CUBIT's virtual geometry features. Where this may not be feasible, mesh entities can also be added directly to the desired nodeset or sideset. Where individual mesh entities have been added to nodesets or sidesets, deleting the mesh will also remove these elements from the Genesis entity. If the geometry is remeshed, the new mesh entities must also be added once again to the nodesets or sidesets.

Nodesets can be created from groups of nodes categorized by their owning volumes, surfaces, curves or vertex. Individual nodes may also be added to a nodeset. Nodes can belong to more than one nodeset.

Sidesets can be created from groups of element sides or faces categorized by their owning surfaces or curves or by their individual face IDs. Element sides and faces can also belong to more than one sideset.

Creating Nodesets and Sidesets

Nodesets and Sidesets are created in CUBIT by assigning the appropriate geometry or mesh entities in the model to a nodeset or sideset ID. The following commands can be used:

Like element blocks, Nodesets and Sidesets are given arbitrary, user-defined ID numbers. If there are no user-defined Nodesets or Sidesets, none are written to the Exodus II file.

With Sidesets, direction is often important. For surfaces, the direction may be specified using the Forward, Reverse, or Both options. The Forward option will write a sideset in relation to hexes in the surface's forward volume, which is the volume that the surface's normal points away from. The Reverse option will write a sideset in relation to hexes in the surface's reverse volume, which is the volume that the surface's normal points into. The Both option will allow sidesets to be written in relation to the hexes that lie in volumes on both sides of the surface. The default is Forward. The user can additionally specifiy the volume from which the hexes should be taken in relation to by using the wrt Volume option.

Direction is equally important for curves in Sidesets. The wrt Surface option allows the user to indicate which surface's faces will be included in the Sideset. The wrt All option will include all faces attached to the curve. The default is wrt All.

Grouping Faces on a Surface into a Sideset

A sideset can be created by grouping a portion of the faces on a given surface by using the following command.

SideSet <sideset_id> Surface <surf_id> Patch { Maximum <x> <y> <z> Minimum <x> <y> <z> | Center <x> <y> <z> outer_radius <value> [inner_radius <value>]} [partition]

This command places only the faces meeting the specified criteria into the sideset. Specifying the Maximum and Minimum loactions of a bounding box will place all faces on the surface whose centroid fall within the box defined by the Maximum and Minumum vectors. Using the Center and outer_radius option will place into the sideset, all faces on the surface whose centroids fall within the circle defined by Center and outer_radius. An optional inner_radius may also be specified, where faces within the annulus defined by the inner_radius and outer_radius are placed in the sideset. The partition option will split the surface based on the sideset definitions, creating new surfaces.

Important: Unlike the other Sideset commands the use geometric entities, this command does not assign the geometric surface to the sideset. Instead only the mesh entity faces are added. If the mesh is deleted, the sideset will become invalid.

Deleting Nodesets and Sidesets

All Nodesets, Sidesets and Blocks may be deleted from the model using the following command:

Reset Genesis

To remove only nodesets or sidesets, the following may be used:

Reset Nodeset

Reset Sideset

To remove a specific nodeset or sideset, use:

Delete Nodeset <nodeset_id_range>

Delete Sideset <sideset_id_range>

Displaying Nodesets and Sidesets

Nodesets and Sidesets can be viewed individually through CUBIT by employing the following commands:

Draw NodeSet <nodeset_id_range> [Color <color_spec>] [add]

Draw SideSet <sideset_id_range> [Color <color_spec>] [add]

Nodeset and Sideset colors can also be changed using the following commands:

Color NodeSet <nodeset_id_range> {color|Default}

Color SideSet <sideset_id_range> {color|Default}

Nodeset Associativity Data

Nodesets can be used to store geometry associativity data in the Exodus II file. This data can be used to associate the corresponding mesh to an existing geometry in a subsequent CUBIT session. This functionality can be used either to associate a previously-generated mesh with a geometry (See Importing an Exodus II File), or to associate a field function with a geometry for adaptive surface meshing (See Adaptive Meshing).

The commands to control and list whether associativity data is written or read from an Exodus II files are the following:

Associativity data is stored in the Exodus II file in two locations. First, a nodeset is written for each piece of geometry (vertices, curves, etc) containing the nodes owned for that geometry. Then, the name of each geometry entity is associated with the corresponding nodeset by writing a property name and designating the corresponding nodeset as having that property. Nodeset numbers used for associativity nodesets are determined by adding a fixed base number (depending on the order of the geometric entity) to the geometric entity id number. The base numbers for various orders of geometric entities are shown in the following table. For example, nodes owned by curve number 26 would be stored in associativity nodeset 40026.

Nodeset id base numbers for geometric entities

Geometric Entity

Base Nodeset Id

Vertex

50000

Curve

40000

Surface

30000

Volume

20000

Instead of storing just the nodes owned by a particular entity, nodes for lower order entities are also stored. For example, the associativity nodeset for a surface would contain all nodes owned by that surface as well as the nodes on the bounding curves and vertices.