Periodic boundary conditions in OptiStruct
Altair EmployeeThis document described how to setup a specific periodic conditioning on an OptiStruct, meaning we want displacements along different faces of out model to match perfectly.
Use case examples: cyclic repetition (left picture), lattice structure (right picture).
A PERiodic Boundary Condition (or PERBC) will internally be managed through a direct link between DOFs of nodes on each face of the association (for example the two opposed faces of a segment in a part with cyclic repetition) though MPCs.
In case of a cyclical repetition, it will then be mandatory that the DOF for displacement of the linked nodes are correctly defined in a cylindrical coordinate system, centered on the rotation axis.
On the contrary, their definition system (in which nodes position are provided) must be the global system.
As PERBC is a boundary condition, it must not be set on the same DOFs as other types of boundary conditions (SPC, MPC) or dependent ends of rigid bodies (RBE2, RBODY…).
The setup of a periodic boundary condition requires:
- A periodic mesh on linked faces, meaning the mesh must be identical between the 2 faces, with only translation or rotation required to go from the first to the second one.
- A set of nodes, defining all nodes on one face to be linked (the “first face”)
- A RELOC card, used to define the geometrical transformation (translation / rotation) required to transform the first face into the second face.
- A PERBC card, defining the periodic boundary condition itself.
- A load step calling this boundary condition (all solutions are supported)
Periodic mesh
As a PERBC is a conditioning that will directly link DOFs together 2 by 2, it is mandatory to have similar meshes on both linked faces.
On a 3D mesh, the tool “Periodic Mesh” is available to reproduce an existing 2D mesh on another free face.
This tool can be found using the search tool (Crtl+F or magnifying glass icon)
Here is the global process:
1 – Mesh one of the 2 faces to be linked by the PERBC.
2 – Use the periodic mesh tool to map the mesh from face 1 (Source surfaces) on face 2 (target surfaces).
3 – Finalize the 2D mesh of the remaining faces.
4 – Generate the volumic 3d mesh (Mesh > Tetra) based on the 2D elements.
To finalize the mesh in case of a cyclic periodic boundary condition, you then need to assign a cylindrical system to all nodes in faces 1 & 2 (Model > Assign system) but only for displacement.
Generate a set of grids from face 1
The second step is to generate a set, type “SET_GRID”, containing all the nodes on the face 1 (and to be linked to their equivalent nodes on face 2).
No need to generate a second set for nodes on face 2 : OptiStruct will perform itself the nodes identification based on the RELOC card (next step).
Create the RELOC card
The RELOC card will contain the information describing the geometrical transformation (translation or rotation) that we would need to apply to face 1 to match face 2.
The RELOC card can be used in several process and define different geometrical operations, but in case of a PERBC, we only need those ones :
- Translation (Type = MOVE)
- Rotation (Type = ROTATE)
The MOVE type will be used in case of linear repetition. The translation vector will ne defined either by 2 nodes, or by the components of the vector in global system.
The ROTATE type will be used in case of cyclic repetition. The rotation will be defined by:
- Either a single node, and the components of the rotation vector in global system
- Or 2 nodes (defining the rotation axis) and an angle
Angle are defined in degrees.
In current version of HyperWorks (v2021), the RELOC card does not currently exist as its own entity.
It needs to be defined as a BULK UNSUPPORTED CARD. (Use search again for easy access).
Create the PERBC card
The PERBC card will be used to define the periodic boundary condition itself.
It belongs to the same family than SPC and SPC1, and are used the same way.
The PERBC card makes a link between:
- The load ID (ID), that will be called by the load step (SPC = load_ID)
- The set of nodes from face 1 (GSID)
- The RELOC card to define the geometrical transformation from face 1 to face 2 (RLID)
- A tolerance used to perform the pairing of nodes between faces 1 & 2 (TOL).
As RELOC card, the PERBC card does not currently exist as its own entity.
It needs to be defined as a BULK UNSUPPORTED CARD. (Use search again for easy access)
Call the PERBC in the load step definition
As other boundary conditions like SPC, PERBC must be called in load steps definition in order to be activated (or as a global case control to activate it for all load steps)
Periodic boundary conditions are barely never alone, and are usually to be combined together with more classical SPC. You then only need to give your PERBC the same ID as a load collector containing your remaining boundary conditions, then call this load collector in the load step definition.
Verify the PERBC is working as expected
When launching an OptiStruct run with PERBC, look for the following message in the out file :
PERBC,1 found 435 matching GRIDs.
Final grid equivalencing map has 435 GRID pairs.
The provided number of grid pairs should match the number of nodes on each face (face 1 and face 2).
If not, some node pairs have not been identified, meaning there is an error in the PERBC definition.
Post-processing tip
The tool “Symmetry” from HyperWorks post-processing interface can be used to visually rebuild the complete model based on one single segment.
