Reduction of the finite element flexible body to modal space

Simcenter Nastran, ANSYS, and Abaqus use their respective flexible body solutions to generate a Component Mode Synthesis reduction of the flexible body's mass and stiffness to modal space.

In Pre/Post, when using the Flexible Body command to create a finite element flexible body, you define possible connection degrees of freedom in the finite element model that represents the flexible component. The connection degrees of freedom are degrees of freedom that will get connected by constraints (for example, joints, bushings, and generic forces) in the Motion mechanism. Use the Fixed Boundary Degrees of Freedom constraint in Pre/Post.

When you solve the flexible body solution in Pre/Post, the behavior varies based on your solver:

• For the RecurDyn solver, Simcenter Nastran performs a two-step modal solution of the component matrices. The first eigensolution computes:The constrained normal modes of the system, which are the modes with the fixed boundary degrees of freedom restrained.The constraint modes of all fixed boundary degrees of freedom, which are the static modes solved by applying unit displacements at individual boundary degrees of freedom while restraining all other boundary degrees of freedom.Simcenter Nastran then performs a second eigensolution on the reduced system. The reduced matrices from the modal solution results, which represent the component's dynamic behavior, are saved in the RecurDyn Rflex input file (.rfi). You can then import the .rfi file into Motion to represent the flexible body.

• For the Simcenter 3D Motion Solver, Simcenter Nastran generates an .op2 file, ANSYS generates an .rst file, and Abaqus an .odb, with the modal coordinates the solver requires. You can then import the file into Motion to represent the flexible body. You can view the animation of the results for the flexible body as a contour display in the context of the overall mechanism animation.

RecurDyn Rflex and Simcenter Nastran data recovery

When you solve the flexible body solution in Motion using the RecurDyn solver, the solve process imports the .rfi file into RecurDyn, along with any other .rfi files representing other flexible bodies.

RecurDyn uses the Rflex method to represent the flexible body as a linear superposition of the modes stored in the associated .rfi file. For each configuration of the mechanism, the resulting physical deformations at the flexible body's connection degrees of freedom are written to a modal deformation file (.mdf). Using the .mdf file and the original .dat input file used to generate the .rfi file, the solution process calls Simcenter Nastran to recover results (such as displacement and stress) on the original, unreduced flexible body. These transient results are then returned to Motion.

After the solve is complete, you can view the animation of the recovered results for the flexible body as a contour display in the context of the overall mechanism animation.

Note:

For more information about the Simcenter Nastran Component Mode Synthesis reduction and data recovery process, including a mathematical description, see Multi-body Dynamics and Control System Software Interfaces and Superelements in Dynamic Analysis.

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Flexible body analysis

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Reduction of the finite element flexible body to modal space, Simcenter 3D 2021.1 Series

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