EIGRA

Bulk Data Entry

EIGRA – Real Eigenvalue Extraction Data using Automated Multi-Level Sub-structuring

Description

Defines the data required to perform real eigenvalue analysis with the Automated Multi-Level Sub-structuring technique.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
EIGRA	SID	V1	V2	ND		AMPFFACT		NORM

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
EIGRA		0.1	3.2	10

Field	Contents
SID	Unique set identification number. No default (Integer > 0)
V1,V2	Frequency range of interest in cycles per unit time. V2 must be present. Default = 0.0 for V1 (V1 < V2, Real, or blank for V1)
ND	Number of roots desired. See comment 3. No default (Integer > 0 or blank)
AMPFFACT	Amplification Factor. The substructure modes are solved up to the frequency of AMPFFACT*V2. Higher values of AMPFFACT will lead to more accurate results and longer running times. See comments 6 and 9. Default = 5.0 (Real or blank)
NORM	Method used for eigenvector normalization. If MASS, the eigenvectors are normalized to the unit value of the generalized mass. If MAX, the eigenvectors are normalized to the unit value of the largest displacement in the analysis set. Default = MASS for normal modes analysis.

Comments

1.	The units of V1 and V2 are cycles per unit time.

2.	The eigenvectors are normalized with respect to the mass matrix by default.

3.	The roots are found in order of increasing magnitude; those closest to zero are found first. The number and type of roots to be found can be determined from the following table.

V1	V2	ND	Number and Type of Roots Found
V1 V1 blank blank	V2 V2 V2 V2	ND blank ND blank	Lowest ND or all in range, whichever is smaller. All in range. Lowest ND roots below V2. All below V2.

Number and Type of Roots Found

blank

Lowest ND or all in range, whichever is smaller.

All in range.

Lowest ND roots below V2.

All below V2.

4.	Eigenvalues are sorted in the order of magnitude for output.

5.	In vibration analysis, small negative roots are usually computational zeros, indicating rigid body modes. Finite negative roots are an indication of modeling problems. If V1 is set to zero explicitly, V1 is ignored.

6.	AMPFFACT is used to increase the accuracy of the eigenvalue and eigenvectors at the expense of slightly longer run times. It is recommended to use higher values of AMPFFACT, between [5.0, 15.0], for solid structures like engine blocks and suspension components.

7.	EIGRA data can be referenced by multiple normal modes subcases with different MPC set SID’s in each subcase. However, only one can be used for modal frequency response or modal transient analysis.

EIGRA data can be referenced by multiple modal dynamic subcases, if the MPC set SID is the same in each SUBCASE and the set of SPCD DOF is the same in each SUBCASE. Additionally, multiple eigenvalue analysis subcases can be used with AMSES, however, only one can be used for modal frequency response or modal transient analysis.

9.	If AMPFFACT is not specified by you, and the model contains a large number of solid elements, the value of AMPFFACT is automatically reset to 10.

10.	This card is represented as a loadcollector in HyperMesh.