- The number of requested modes is greater than 200.
- The beginning frequency input on the MODOPT command is zero (or near zero).
For models that are dominated by solid elements or have a bulkier geometry, the Supernode eigensolver
can still be more efficient than other eigensolvers; however, it may require higher numbers of modes
to become the best choice in terms of performance. Other factors, such as the hardware being used,
can also affect the decision of which eigensolver to use. For example, on machines with slow I/O per-
formance the Supernode eigensolver may be the faster eigensolver (compared to Block Lanczos).
3.8.4. Subspace Method
The Subspace method is an iterative algorithm that is appropriate for problems having symmetric
stiffness and mass matrices ([K] and [M]). It internally uses the sparse solver or the distributed sparse
solver (depending on whether or not the solution uses Distributed ANSYS) for the shift-invert logic. The
Subspace solver performs well when the goal is to obtain a moderate to medium number of eigenvalues
on large models run in distributed parallel mode.
Sturm sequence checking is available for this method (it is off by default) and can be controlled via the
SUBOPT command. The memory usage of the algorithm can also be configured via the SUBOPT com-
mand.
3.8.5. Unsymmetric Method
The unsymmetric method, which also uses the full [K] and [M] matrices, is meant for problems where
the stiffness and mass matrices are unsymmetric (for example, acoustic fluid-structure interaction
problems). The real part of the eigenvalue represents the natural frequency and the imaginary part is
a measure of the stability of the system - a negative value means the system is stable, whereas a positive
value means the system is unstable. Sturm sequence checking is not available for this method. Therefore,
missed modes are a possibility at the higher end of the frequencies extracted.
3.8.6. Damped Method
The damped method (MODOPT,DAMP) is meant for problems where damping cannot be ignored, such
as rotor dynamics applications. It uses full matrices ([K], [M], and the damping matrix [C]). Sturm sequence
checking is not available for this method. Therefore, missed modes are a possibility at the higher end
of the frequencies extracted.
3.8.6.1. Damped Method--Real and Imaginary Parts of the Eigenvalue
The imaginary part of the eigenvalue, Ω, represents the steady-state circular frequency of the system.
The real part of the eigenvalue, σ, represents the stability of the system. If σ is less than zero, then the
displacement amplitude will decay exponentially, in accordance with EXP(σ). If σ is greater than zero,
then the amplitude will increase exponentially. (Or, in other words, negative σ gives an exponentially
decreasing, or stable, response; and positive σ gives an exponentially increasing, or unstable, response.)
If there is no damping, the real component of the eigenvalue will be zero.
The eigenvalue results reported are actually divided by (2* π), giving the frequency in Hz (cycles/second).
In other words:
Imaginary part of eigenvalue, as reported = Ω/(2* π)
Real part of eigenvalue, as reported = σ/(2* π)Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationModal Analysis