Tagged: ls-dyna, LS-DYNA Suite, lsdyna, R13.x, structural-mechanics
March 17, 2023 at 8:59 amFAQParticipant
Eigenvalue analysis is performed by selecting IMFLAG=1 on *CONTROL_IMPLICIT_GENERAL and specifying a nonzero value for NEIG on *CONTROL_IMPLICIT_EIGENVALUE. If NEIG>0, the lowest NEIG eigenvalues will be computed at time 0 and LS-DYNA will terminate. If NEIG<0, intermittent eigenvalue analyses will be performed to compute the modelâ€™s eigenvalues at specified times during the analysis. A detailed description of NEIG<0 can be found in the Manual Vol I. When an eigenvalue analysis is performed the eigenvectors are written by default to the binary database d3eigv. This can be viewed in LS-PrePost in the same way as the d3plot database. The database d3eigv contains NEIG states with each state corresponding to the computed eigenvector (mode shape) at an eigenfrequency. The mode shape displacement at a node can be accessed through history plot. The horizontal axis of the plot corresponds to the eigenfrequency which is given in units of cycles per unit time. Tables with the eigenvalues, modal participation factors, modal effective mass are written in the ascii file eigout by default. Eigenvalues and eigenvectors can be also written to the file Eigen_Vectors by using a nonzero value for EVDUMP on *CONTROL_IMPLICIT_EIGENVALUE. This feature is available only in SMP. If EVDUMP>0, an ascii file is used, while if EVDUMP>0, a simple binary format is used. When the ascii file is used, the file is organized as follows. The first line shows the number of eigenvalues extracted (NEIG). The second line shows the number of the independent degrees of freedom. The first NEIG entries starting from line 3 show the computed eigenvalues (omega^2). The remaining lines show the nodal displacements and rotations of each eigenvector starting from the one with the lowest eigenvalue. For each eigenvector the degrees of freedom are read row-wise starting from degree of freedom 1. The eigenvectors are orthonormalized to the mass matrix. This is different than the scaling used in LS-PrePost. The mass matrix is written in an ascii file when MTXDMP>0 in *CONTROL_IMPLICIT_SOLVER. The numbering of the degrees of freedom can be found in the ascii file Node_Data_xxxx_yyy when MTXDUMP>0. The file lists the independent degrees of freedom at each node with the zero entries corresponding to the constrained degrees of freedom. If the model has rigid bodies, their IDâ€™s and corresponding degrees of freedom are listed after those of the final node. The degrees of freedom assigned to the rigid bodies correspond to the center of mass. After the list of the rigid bodies, the file shows the coordinates of each node, and then the coordinates of the center of mass of each rigid body.
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