Cyclic symmetric boundary coupling

Use the Cyclic Symmetry simulation object to define boundary coupling in thermal, coupled, and structural solutions by either manually defining region pairs or letting the software automatically determine them, requiring you to select only the source side faces. The meshes do not need to match.

The structural and thermal solvers use the boundary coupling to create the coupling conditions for the solutions. The coupling occurs between any source and target region nodes within a specified search distance using a weighted area method.

Cyclic symmetric model with boundary coupling defined and the global cyclic analysis coordinate system displayed

You define the coupling definition at the solution level, not at the solution step. Therefore, the multi-point constraint equations (MPCs) that represent the coupled DOF are applied in all solution steps you define (statics, modal, and cyclic normal modes).

The coupling definition you define is written to the CYCSET bulk entry in the Simcenter Nastran input file.

Global cyclic analysis coordinate system (CSYS)

When you create a cyclic symmetry model, you must define a global cyclic analysis coordinate system to specify the axial direction for the model. You define a cyclic analysis coordinate system when creating or editing the FEM. It is the default coordinate system in any operations that include coordinate system selections. For example, when you create loads and boundary conditions, you must define the Z-axis for the global cyclic analysis coordinate system in the axial direction, and you must orient it so the source rotates into the target in a positive angular direction.

When you define the cyclic symmetric boundary coupling, you can select the global cyclic analysis coordinate system or select another coordinate system. The coordinate system you select must be cylindrical and its Z-axis must be collinear with the global cyclic analysis coordinate system.

For more information about the global cyclic analysis coordinate system, see Simulation coordinate systems.

Methods for defining face pairs

You can use an automatic or manual method to define the face pairs for boundary coupling.

Automatic cyclic symmetry pairing

Select the faces on the source region and the software automatically finds the matching target faces. The grouping option controls how the software creates the regions and pairs. You can preview the matching faces and the calculated segment properties.

Automatic Cyclic Symmetry from Elements (SOL 401, 402, or 414)

Based on 3D cyclic symmetry elements that you select, the solver finds the source and target faces.

Manual cyclic symmetry

Manually create and pair the source and target regions. Regions can contain multiple disjoint faces.

The source region must be able to rotate into the target region in the positive theta direction. The Z-axis of the selected cyclic analysis coordinate system and the right-hand rule determine the positive theta direction.

After you use either method to create face pairs, you can modify the regions or create additional manual pairings to specify additional regions.

Solving cyclic symmetry for normal modes

You can use the Cyclic Modes step for both structural and coupled analyses. The Cyclic Modes step supports the cyclic symmetric solution for normal modes at higher harmonic indices. The software computes the modes about the nonlinear stress or displacement state at the end of a previous static step in the solution. You can specify the harmonics to solve or you can define a set of harmonics using the Harmonics Set modeling object.

Note:

Static analyses do not allow cyclic loading; therefore, they only support the 0th harmonic.

For more information, see Cyclic symmetric normal modes solutions.

Viewing cyclic symmetry results

In Post-processing, you can view:

• Results of a model that includes the Cyclic Symmetry simulation object.

• Wave animations for the results of Cyclic Modes solution steps.

For more information, see Cyclic symmetric post-processing options.

Considerations for Multiphysics thermal and coupled analyses

For thermal sector analyses, the Cyclic Symmetry simulation object applies a conductive rotational periodic condition to the selection. The thermal solver uses the axis of revolution in addition to the resolution and your specified overlap options. It scales convecting areas, thermal conductivity, and specific heat at appropriate boundary conditions where cyclic symmetry is defined, to account for the number of segments in each stage.

To consider radiative rotational periodicity for the entire model, the thermal solver uses the axis of revolution and the number of segments to compute the view factors for radiation.

Considerations for SOL 402 analyses

If your model contains components that are connected by contact or glue conditions, create multiple cyclic symmetry conditions instead of a single cyclic symmetry condition for the whole model. For example, the following figure shows coincident edges that lead to redundant conditions in the solver.

(1) Elements that have coincident edges leading to redundant conditions in the solver

To avoid the coincident edges, create two cyclic symmetry conditions, as shown in the following figure:

One cyclic symmetry condition (CYCSET) for the top components—(2) dark blue and (3) brown	One cyclic symmetry condition (CYCSET) for the bottom components—(4) light blue and (5) purple

When coincident nodes exist in the same cyclic symmetry condition (either in the source or the target region), the software displays a fatal error message:

FATAL ERROR with the card CYCSET xxx

To resolve this error, do one of the following depending on whether the redundant condition is glue or contact:

• To resolve redundant contact conditions, add stiffness by selecting a stiffness type from the Normal Stiffness Model Type list on the Normal Behavior page in the Contact Parameters - Multi-Step Nonlinear Kinematics Pair dialog box.

• To resolve redundant glue conditions, manually edit the region in the Region dialog box to remove a layer of the element faces that are common between the glue condition and the cyclic symmetry condition. Note: You must exclude element faces and not edges. Although you can exclude an edge in Pre/Post, Nastran can exclude only the faces.

Note:

If the redundancy does not affect your model or if you want to bypass the fatal error, use PARAM,BAILOUT to allow the solution to solve. In this case, the software displays the following message:

Warning with the card CYCSET xxx

Summary of cyclic boundary coupling

• To perform coupling, the solver automatically creates constraint equations using the target as the dependent DOF and the source as the independent DOF.

• The meshes on the source and target regions do not need to match.

• You must select the source side faces so they sweep into the sector volume when rotated positively about the Z-axis of the global cyclic analysis coordinate system.

• If you define single-point constraint (SPC) conditions on the target region DOF, the solver ignores the conditions and the solution continues.

• If you include a target region DOF on an RBE2, RBAR, or RBE3 element as a dependent DOF, the solver reports a warning message that it ignores the connections, and the solution continues.

• The displacement coordinate system of nodes defined on the Z-axis of the global cyclic analysis coordinate system must be a Cartesian coordinate system.

• Source side nodes are independent DOF of multipoint constraint set (MPC) relationships.

• Target side nodes are dependent DOF.

Where do I find it?

Application	Pre/Post
Prerequisites	A Simulation file as the work part and the displayed partSimcenter Nastran or Simcenter 3D Multiphysics as the specified solverA solution containing a Cyclic Symmetry simulation objectGlobal cyclic analysis coordinate system defined in the FEM(Optional) Cyclic Modes solution step in a Coupled or Structural analysis type
Command Finder	Cyclic Symmetry
Simulation Navigator	Right-click Simulation Object container node→New Simulation Object list→Cyclic Symmetry

How do I

Define boundary coupling using the Automatic method

Define boundary coupling using the Manual method

Learn more

Simcenter 3D Multiphysics workflow—cyclic symmetric boundary coupling

Cyclic symmetric post-processing options

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Cyclic symmetric boundary coupling, Simcenter 3D 2021.1 Series

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