The solver you select depends on the analysis type, as follows:
Both the DAMP and QRDAMP eigensolvers are applicable to a rotordynamic analysis. Before selecting an eigensolver, consider the following:
If you intend to perform a subsequent modal superposition, harmonic or transient analysis, use the QRDAMP eigensolver. The DAMP eigensolver is not supported for mode-superposition methods.
The DAMP eigensolver solves the full system of equations, whereas the QRDAMP eigensolver solves a reduced system of equations. Although the QRDAMP eigensolver is computationally more efficient than the DAMP eigensolver, it is restricted to cases where damping (viscous, material, etc.) is not critical. In particular, QRDAMP is not recommended when structural damping is present.
Note: When using the QRDAMP eigensolver in a multiple load step modal analysis (a
Campbell analysis for example), you can activate the
ReuseKey on the QRDOPT command
to reuse the eigenvectors from the symmetric eigensolution of
the first step. This will result in better performances. Note, however, that if
variable bearings are present (COMBI214 with tabular
characteristics), this option may lead to incorrect results given the change in
stiffness in each load step.
When rotating damping is included in the analysis
(RotDamp=ON in the CORIOLIS
command) and solid elements are used for the rotating parts of the structure, DAMP
eigensolver is recommended.
After a complex modal analysis using the QRDAMP method, complex frequencies are listed in the following way:
***** DAMPED FREQUENCIES FROM REDUCED DAMPED EIGENSOLVER *****
MODE COMPLEX FREQUENCY (HERTZ) MODAL DAMPING RATIO
1 -0.78052954E-01 49.844724 j 0.15659202E-02
(a) (b) (c)
where
| (a) is the real part of the complex frequency. It shows the damping of this particular frequency as well as its stability. A negative real part reflects a stable mode while a positive one reflects an unstable mode. More information on instability can be found earlier in this guide under Stability. |
| (b) is the complex part of the complex frequency. It represents the damped frequency. |
| (c) is the modal damping ratio. It is the ratio between the real part and the complex frequency modulus (also called norm of the complex frequency). |
Although the gyroscopic effect creates a "damping" matrix, it does not dissipate energy; therefore, if there is no damping in a rotating structure, all the real parts of its complex frequencies are zero.
The complex part is zero if the complex frequency corresponds to a rigid body mode, or if it corresponds to an overdamped frequency where the damping is so high that it suppresses the vibration.
For more information, see Complex Eigensolutions in the Mechanical APDL Theory Reference
The full method and the mode-superposition (based on QRDAMP modal analysis) method are supported for rotordynamic analyses.
If the SYNCHRO command is used (as in an unbalanced response calculation), the mode-superposition method is not supported. In this case, the gyroscopic matrix must be recalculated at each frequency step. Only the FULL method is applicable.
In a small-deflection transient analysis, the Coriolis effect is activated using the CORIOLIS command. Full method and mode-superposition based on QRDAMP modal analysis method are supported for rotordynamics.
For the full method, use the Newton-Raphson with unsymmetric matrices option (NROPT, UNSYM).
Support for a start-up or stop simulation is available. Issue the KBC command to ramp the rotational velocity using linear interpolation. This command also supports quadratic interpolation. If the rotational velocity is varying, the gyroscopic matrix needs to be recalculated at each time step; the mode-superposition method is not supported and only the FULL method is applicable.
In a large-deflection transient analysis, (NLGEOM,ON), the Coriolis effect and all other nonlinear inertial effects are automatically included in the analysis and the CORIOLIS command should not be used. This is the case for rotor-bearing simulation using COMBI214 or FLUID218, for example.