As a service to our customers, we are cataloguing this known issue and its fix.
When running an NX Nastran Structural SOL 103 Flexible Body solution in the Pre/Post application, the modal content of the generated OP2 result file will contain an incorrect set of modes. This issue will not prevent the successful completion of any Motion flexible solutions, but results will be affected by an incorrect definition of local stiffness at any interface points between the flexible link and the mechanism. Customers please login for more details.
A fix for this issue will be provided in Simcenter 3D v12.0.2, scheduled for release by July 17, 2018.
A workaround is available until a fix is supplied: in the Case Control section the following parameters must be set:
- "Number of Generalized DOF": this parameter must be set as equal to the number of desired normal modes PLUS 6 rigid body modes.
- For example, if the user wants to represent the global flexibility by its first 10 normal modes, then the number of generalized DOFs must be set to 16.
- Lanczos Data – Real Eigenvalue – Lanczos: "Number of Desired Modes" must be set equal to the expected total number of modes, including normal and static modes, as well as 6 rigid body modes. In order to calculate the number of expected total number of modes, please consider the following notes:
- the number of static modes is univocally determined by the boundary conditions (DOF, which are constrained).
- Typically, 6 DOF are constrained at the interface nodes. This means the number of static modes is up to 6 times the number of interface FE-nodes.
- The final mode set will contain the generalized DOF (normal modes + 6 rigid body modes) plus the number of static modes.
- For example, if the flexible body will be connected to the mechanism through 4 connection elements, then constraint elements will be defined at 4 interface nodes, adding 4x6 = 24 static modes to the complete mode set. Considering 10 requested normal modes, the number of desired modes will have to be set equal to 40: 10 (normal modes) + 6 (rigid body modes) + 24 (static modes).