nastran manual
MENU SEQUENCE:
The Dynamics section is used to set different parameters of dynamic analysis problems and for eigenvalues extraction (modes analysis).
This window consists of two parts: Modes Analysis and Dynamic Design.
Defines data needed to perform real eigenvalue analysis.
Figure 22 – Modes Analysis Section |
-Method of eigenvalue extraction: Sets which method will be used to find eigenvalues. See the following table:
INV | Inverse power method, symmetric matrix operations. |
DET | Determinant method, symmetric matrix operations. |
GIV | Givens method of tridiagonalization. |
MGIV | Modified Givens method (see Remark 2). |
FEER | Tridiagonal reduction method, symmetric matrix operations. |
FEER-Q | See Remark 1. |
FEER-X | See Remark 1. |
UINV | Inverse power method, unsymmetric matrix operations. |
UDET | Determinant method, unsymmetric matrix operations. |
1. The rigid body frequencies are set to zero, unless FEER-X is requested. If FEER-Q is requested, certain key areas in FEER computations are done in quad precision (Real*16) for 32-bit word machines and in double precision for 60- and 64-bit word machines. The FEER-Q request would yield much better rigid body eigenvalues, but it may take two to three times longer to compute than FEER or FEER-X.
2. Givens method requires the mass matrix not to be singular. The MGIV method allows a singular mass matrix. The bigger dynamic matrix will require more CPU time and more memory.
-Frequency
range of interest:
(Required for METHOD = DET, INV, UDET, or UINV) (Real 0.0; F1
F2). If METHOD = GIV, frequency range acts over desired
eigenvectors. The frequency range is ignored if ND > 0. In this case the ND
number of first eigenvectors associated to positive roots is found. (Real, F1
F2).
If METHOD =
FEER, F1 is the center of range of interest (Default is F1 = 0.0) (Real 0.0), and F2 is an
acceptable relative error of tolerance expressed as percentage on
frequency-squared (Default percentage is 0.1/n, where n is the order of the
stiffness matrix) (Real > 0.0).
-Estimate number of roots in range (Required for METHOD = DET, INV, UDET, or UINV. Ignored for METHOD = FEER) (Integer > 0).
-Desired number
The value of this field always has to be a positive integer.
This field has the following functions:
- For methods DET, INV, UDET, or UNIV: represents desired number of roots. If left blank, the default of '3 * Estimated number' is taken.
- For method GIV: represents desired number of eigenvectors.
- For method FEER: represents desired number of roots and desired number of eigenvectors. If left blank, an automatic numbers are calculated to extract at least one accurate mode.
-Mass orthogonality test: Method for normalizing eigenvectors.
The following are the available methods:
MASS | Normalizes eigenvectors to unit value of the generalized mass. |
MAX | Normalizes eigenvectors to unit value of the largest component in the analysis set. |
POINT | Normalizes eigenvectors to unit value of the component defined in fields NodeID and DOF. If the defined component is set to zero, the method is changed to MAX by default. |
-NodeID: Node or scalar point identification number (Required if and only if Mass orthogonality test is set to POINT). The value of this field always has to be a positive integer.
Provides the solution type and control information for Dynamic Analyses. Each dynamic load used in a Dynamic Analysis must have the appropriate Solution Method activated.
Figure 23 – Dynamic Section |
- Solution Method and Domain of Solution: chooses the type of Dynamics Solution to be performed. The four following options are available: (1) Direct Transient, (2) Modal Transient, (3) Direct Frequency, and (4) Modal Frequency. The inappropriate boxes for each Solution Method will be inactivated automatically.
-Equivalent Viscous Damping: These statements provide damping information of a structure. The Overall Structural Damping Coefficient is an input for all four-solution methods.
-Equivalent Viscous Damping Conversion: Information for both System Damping and Element Damping is provided in this box. These values are only inputs for Direct and Modal Transient Analysis.
-Transient Time Step Interval: For transient analyses, these options control the number of steps, steps sizes, and the output interval.
-Frequency Step: The Solutions Frequencies are defined in this section. These statements define analyzed frequencies for both Direct and Modal Frequency Analysis.
-Response Based on Modes: For modal solution methods, these options allow choosing the number and/or range of modes to include in the frequency response or transient formulation.
-Select Range of Frequencies for Output: Allows to set which frequencies will be printed in the output file. In some cases it is interesting to run analysis for more frequencies that can be studied later.