EIGRA

Bulk Data Entry 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 MSGLVL AMPFFACT   NORM  

Example

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

Definitions

Field Contents SI Unit Example
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. 3

No default (Integer > 0 or blank)

  
MSGLVL Switch to print singularity messages within the AMSES module.
0 (Default)
No messages are printed to the .out file if singularity is encountered.
1
Information listing the grids at which singularities are detected is printed to the .out file.
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. 6 9

Default = 5.0 (Real or blank)

  
NORM Method used for eigenvector normalization.
MASS
The eigenvectors are normalized to the unit value of the generalized mass.
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 Table 1.
    Table 1.
    V1 V2 ND Number and Type of Roots Found
    V1 V2 ND Lowest ND or all in range, whichever is smaller.
    V1 V2 blank All in range.
    blank V2 ND Lowest ND roots below V2.
    blank V2 blank 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.
  8. 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 load collector in HyperMesh.