6.6 Exporting U-splines

Once constructed, a U-spline can be exported to a number of formats with the command

export uspline {<integer>uspline_id} <enum>type<string>file_name_prefix

6.6.1 Export formats

Remark: The type enum can take one of the following values.

• Officially supported:

  • bext: Exports the U-spline geometry to the ASCII Bézier extraction file format (.bext file).

  • vtk: Exports the Bézier elements of the U-spline to a VTK unstructured grid file (.vtu file) for visualization.

• Early development (these options are considered beta functionality and are not officially supported):

  • json: Exports to a JSON version of the Bézier extraction format.

  • exodus: Exports the Bézier extraction data in an Exodus file format.

  • exodus_bezier_mesh: Same as exodus, but converts the U-spline to a Bézier mesh first, and switches node ordering.

  • step: Exports the U-spline Bézier elements to a STEP file.

  • projection_error: Calculates the deviation between the original CAD and the U-spline at quadrature points and outputs these deviations to a VTK file format for viewing in Paraview. These exported files are useful in combination with files from the VTK export option.

Remark: The file name specified by the user with file_name_prefix should not include a file extension. The file extension associated with the chosen export format will be automatically appended.

6.6.2 Special options for BEXT U-spline export

When exporting U-splines as BEXT files, Coreform Cubit can utilize previously-defined block and sideset information.

Specifically, a U-spline can be exported as multiple, block-partitioned BEXT files, and sidesets created in Coreform Cubit can be exported as LS-DYNA node sets. If these node sets will be used in conjunction with other *NODE cards in LS-DYNA, a node offset can be specified to prevent node ID collision.

Details are provided below.

6.6.2.1 Exporting block-partitioned BEXT U-splines

When using the bext option, a single U-spline can be partitioned into multiple BEXT files using Cubit-defined block entities.

To export a partitioned U-spline to multiple, block-defined BEXT files, first generate a U-spline from a block-partitioned Cubit model (see Element Block Specification for information on creating blocks in Cubit).

Export the resulting U-spline with the bext option specified. The U-spline elements will be automatically segregated based on which block they belong to, and a separate BEXT file will be written for each block. The relevant _<block_id> will be appended to each resulting file.

Additionally, elements which are not contained in a block are grouped and written to a separate BEXT file.

6.6.2.2 Exporting U-splines with LS-DYNA node sets

When a U-spline is created from a mesh that includes Cubit sidesets, those sidesets can be exported as LS-DYNA node sets in conjunction with BEXT-type U-spline exports. This is accomplished by adding the the dyna_cards flag to an BEXT-type export command as follows (using for illustration an arbitrary U-spline ID of 31):

export uspline 31 bext ’my-uspline-export’ dyna_cards

The dyna_cards flag will cause the LS-DYNA input file <file_name_prefix>_node_sets.k to be written out alongside the BEXT output.

This file contains *SET_NODE_LIST cards that define sets of nodes that are contained in the Cubit sidesets. The sidesets must be defined before the build uspline command is executed to be exported as node sets. If there are no Cubit sidesets defined, this file will not be written.

6.6.2.3 Exporting block-partitioned U-splines with node sets

The dyna_cards flag will cause an additional LS-DYNA input file to be written when a U-spline export involves both node sets and block-partitioning. This additional file, denoted <file_name_prefix>_constrained_nodes.k, is necessary because when a U-spline is partitioned via Cubit blocks, nodes that have support across multiple blocks are duplicated across multiple BEXT files.

To handle these duplicate nodes, the dyna_cards flag writes the _constrained_nodes.k file, containing *CONSTRAINED_LINEAR_GLOBAL cards that tie the displacement and rotation of these duplicate nodes. The result is eqivalent to connecting adjacent parts via shared nodes on the interface.

6.6.2.4 Specifying node offsets to prevent node ID collision

The node offset keyword is to be used in conjunction with the BEXT export type and the dyna_cards flag. It specifies a node ID offset for exported sidesets. This allows for avoiding node ID collision if BEXT files are used in conjunction with other *NODE cards.

The syntax, following the example above and using for illustration an arbitrary offset of 1000, is as follows:

export uspline 31 bext ’my-uspline-export’ dyna_cards node offset 1000