nastran manual


Dynamics

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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.

Modes Analysis

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.

        

Remarks

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.

Dynamic Design

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.