
PERMAS-DEVX Extended Mode Analysis
This module provides additional methods for dynamic eigenvalue analysis:
- Dynamic condensation
- Complex mode analysis
- Eigenfrequencies over rotational speed for rotating structures
Dynamic Condensation
Two methods are available for dynamic condensation:
- Craig-Bampton method (CB condensation),
- Mixed-Boundary Craig Bampton method(MBCB condensation).
Craig Bampton method
The functionality may be summarized as follows:
- Structural dynamics
- Acoustics
- Coupled fluid-structure acoustics
Two condensation options are available for coupled fluid-structure acoustics:
- Dry Interface
- Solution of a coupled eigenvalue problem on subcomponent level, i.e. isolation of the acoustic component. External modes are coupled modes.
- Global solution may be a mechanical vibration analysis.
- Wet Interface
- Separate computation of mechanical and acoustic modes on subcomponent level.
- Global solution is a coupled vibration analysis.
- Condensation of the fluid-structure interface can also be made.
Mixed-Boundary Craig Bampton method
The above mentioned dry condensation of parts with enclosed fluids also works with the Mixed-Boundary Craig-Bampton method.
Complex Mode Analysis
This includes the calculation of complex eigenvalues and eigenvectors in modal coordinates. This method is based
on a previous solution of the real eigenvalue task.
The results of this analysis are as follows:
- Frequency
- Complex eigenvalues
- Complex eigenfrequencies (damping coefficient and circular frequency)
- Equivalent viscous damping ratio
- Complex mode shapes with physical and modal representation. The modal displacements of the complex modes represent the modal participation of the underlying real modes.
A suitable post-processor (like MEDINA) can be used to visualize and animate complex mode shapes.
Eigenfrequencies of Rotating Systems
For rotational systems it is often required to generate a so-called Campbell diagram, which relates the eigenfrequencies to the rotational speed. The values of such a diagram can be generated automatically in one single run. From these values all frequencies of interest can be selected for a subsequent frequency response analysis.
For rotating structures, any number of rotational speeds is defined in a separate input. A reference rotational velocity is used in the static pre-run. From this pre-run, additional matrices are built for the reference rotational velocity. The specified rotational velocities are used to scale the additional matrices during dynamic response analysis. This procedure makes the generation of Campbell diagrams very efficient and the response analysis of rotating structures is facilitated.