Mechanical APDL Structural Analysis Guide

Mode-Extraction Comments

Method

ments, Lagrange multiplier option on contact elements, mixed

u-P formulation elements, etc.).

This method works with the various Lev_Diff values on the

PCGOPT command. It also works with MSAVE to reduce

memory usage.

By default, this method does not perform a Sturm sequence

check; however, internal heuristics have been developed to

guard against missing modes. If a Sturm sequence check is

absolutely necessary, it can be activated via the PCGOPT

command.

This method is the only eigenvalue solver optimized to run in

a distributed manner in Distributed ANSYS.

Used to solve for many modes (up to 10,000) in one solution.

Typically, the reason for seeking many modes is to perform a

Supernode

subsequent mode-superposition or PSD analysis to solve for

the response in a higher frequency range.

This method typically offers faster solution times than Block

Lanczos if the number of modes requested is more than 200.

The accuracy of the solution can be controlled via the SNOP-

TION command.

Subspace Used for large symmetric eigenvalue problems.

This method uses the sparse matrix solver. The advantage of

the Subspace method over the Block Lanczos method is that

both [K] and [S] / [M] matrices can be indefinite at the same

time. Some options can be controlled via the SUBOPT com-

mand.

Used for problems with unsymmetric matrices, such as fluid-

structure interaction problems.

Unsymmetric

Used for problems where damping cannot be ignored, such

as bearing problems.

Damped

Faster and achieves better calculation efficiency than the

damped method. It uses the reduced modal damped matrix

QR damped

to calculate complex damped frequencies in modal coordin-

ates.

For more detailed information, see Comparing Mode-Extraction Methods (p. 61).

For most applications, you will use the Block Lanczos, PCG Lanczos, Subspace, or Supernode method.
The unsymmetric, damped, and QR damped methods are applicable in special applications. ( The damped,
unsymmetric, and QR damped methods may not be available, depending upon the Mechanical APDL
license in use at your site.)

When you specify a mode-extraction method, the program automatically chooses the appropriate
equation solver.

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Modal Analysis