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eigen Command

This command is used to perform a generalized eigenvalue problem to determine a specified number of eigenvalues and eigenvectors.

eigen <$type> $numEigenvalues

$type

eigen-value analysis type:

 

 

frequency

solve: K- ? M

 

 

generalized

solve: K- ? M (default)

 

 

standard

solve: K- ? I

$numEigenvalues

number of first eigenvalues (?? to be determined

The eigenvectors are stored at the nodes and can be printed out using Node Recorder or the Print command.

NOTE: The current eigensolver is able to solve only for N-1 eigenvalues, where N is the number of interial DOFs. When running into this limitation, the user should add negligible masses to additional unrestrained degrees of freedom.

For example, for a 3-dof/node problem, instead of:

mass 1 100 0 0

the user should input:

mass 1 100 1e-9 0

where dof-2 at node 1 is unrestrained.

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