GUI Path Command
Option
Main Menu> Solution> Analysis Type> New Analysis ANTYPE
New Analysis
Main Menu> Solution> Analysis Type> New Analysis>
Modal ANTYPE
Analysis Type: Modal (see Note below)
Main Menu> Solution> Analysis Type> Analysis Options MODOPT
mode-extraction Method
Main Menu> Solution> Analysis Type> Analysis Options MODOPT
Number of Modes to Extract
Main Menu> Solution> Analysis Type> Analysis Options MXPAND
No. of Modes to Expand (see Note below)
Main Menu> Solution> Analysis Type> Analysis Options LUMPM
Mass Matrix Formulation
Main Menu> Solution> Analysis Type> Analysis Options PSTRES
Prestress Effects Calculation
Note — When you specify a modal analysis, a Solution menu that is appropriate for modal analyses ap-pears. The Solution menu will be either “abridged” or “unabridged,” depending on the actions you took prior to this step in your ANSYS session. The abridged menu contains only those solution options that are valid and/or recommended for modal analyses. If you are on the abridged Solution menu and you want to access other solution options (that is, solution options that are valid for you to use, but their use may not be encouraged for this type of analysis), select the Unabridged Menu option. For details, see Section 3.11.1: Using Abridged Solution Menus in the ANSYS Basic Analysis Guide.
Note — In the single point response spectrum (SPOPT,SPRS) and Dynamic Design analysis method (SP-OPT,DDAM), the modal expansion can be performed after the spectrum analysis, based on the significance factor SIGNIF on the MXPAND command. If you want to perform modal expansion after the spectrum analysis, choose NO for mode expansion (MXPAND) on the dialog box for the modal analysis options (MODOPT).
3.5.2.1. Option: New Analysis [ANTYPE]
Choose New Analysis.
Note — Restarts are not valid in a modal analysis. If you need to apply different sets of boundary conditions, do a new analysis each time (or use the "partial solution" procedure described in Chapter 3, “Solution”
in the ANSYS Basic Analysis Guide).
3.5.2.2. Option: Analysis Type: Modal [ANTYPE]
Use this option to specify a modal analysis.
3.5.2.3. Option: Mode-Extraction Method [MODOPT]
Choose one of the extraction methods listed below. (For more detailed information, see Section 3.13: Comparing Mode-Extraction Methods.)
• Block Lanczos method (default)
The Block Lanczos method is used for large symmetric eigenvalue problems. You can use this method for the same types of problems for which you use the subspace method, but you achieve a faster convergence rate. The Block Lanczos method uses the sparse matrix solver, overriding any solver specified via the EQSLV command.
• Subspace method
The subspace method is used for large symmetric eigenvalue problems. Several solution controls are available to control the subspace iteration process.
When doing a modal analysis with a large number of constraint equations, use the subspace method with the frontal solver instead of the JCG solver, or use the Block Lanczos mode-extraction method.
• PowerDynamics method
The PowerDynamics method is used for very large models (100,000+ DOFs), and is especially useful to obtain a solution for the first several modes to learn how the model will behave. You can then choose the most appropriate extraction method (subspace or Block Lanczos) for running the final solution. This method automatically uses the lumped mass approximation (LUMPM,ON)
To use the PowerDynamics method when running in batch or command mode, you first issue MOD-OPT,SUBSPACE, followed by EQSLV,PCG. (The PCGOUT solver can also be used, but it is very slow.)
• Reduced (Householder) method
The reduced method is faster than the subspace method because it uses reduced (condensed) system matrices to calculate the solution. However, it is less accurate because the reduced mass matrix is approx-imate. (See Section 3.13: Comparing Mode-Extraction Methods.)
• Unsymmetric method
The unsymmetric method is used for problems with unsymmetric matrices, such as fluid-structure inter-action problems.
• Damped method
The damped method is used for problems where damping cannot be ignored, such as bearing problems.
• QR Damped method
The QR damped method is faster and achieves better calculation efficiency than the damped method. It uses the reduced modal damped matrix to calculate complex damped frequencies in modal coordinates.
For most applications, you will use the Block Lanczos, subspace, reduced, or PowerDynamics method. The un-symmetric, damped, and QR damped methods are meant for special applications.
When you specify a mode-extraction method, ANSYS automatically chooses the appropriate equation solver.
Section 3.5: Apply Loads and Obtain the Solution
Note — The damped, unsymmetric, and QR damped methods are not available in the ANSYS Professional program.
3.5.2.4. Option: Number of Modes to Extract [MODOPT]
This option is required for all mode-extraction methods except the reduced method.
For the unsymmetric and damped methods, requesting a larger number of modes than necessary reduces the possibility of missed modes, but results in more solution time.
3.5.2.5. Option: Number of Modes to Expand [MXPAND]
This option is required for the reduced, unsymmetric, and damped methods only. However, if you want element results, you need to turn on the "Calculate elem results" option, regardless of the mode-extraction method. In the single point response spectrum (SPOPT,SPRS) and Dynamic Design analysis method (SPOPT,DDAM), the modal expansion can be performed after the spectrum analysis, based on the significance factor SIGNIF on the MXPAND command. If you want to perform modal expansion after the spectrum analysis, choose NO for mode expansion (MXPAND) on the dialog box for the modal analysis options (MODOPT). If you want the mode shapes normalized to unity for the Block Lanczos or subspace methods, you will need to expand the modes as well.
3.5.2.6. Option: Mass Matrix Formulation [LUMPM]
Use this option to specify the default formulation (which is element-dependent) or lumped mass approximation.
We recommend the default formulation for most applications. However, for some problems involving "skinny"
structures such as slender beams or very thin shells, the lumped mass approximation often yields better results.
Also, the lumped mass approximation can result in a shorter run time and lower memory requirements.
3.5.2.7. Option: Prestress Effects Calculation [PSTRES]
Use this option to calculate the modes of a prestressed structure. By default, no prestress effects are included;
that is, the structure is assumed to be stress-free. To include prestress effects, element files from a previous static (or transient) analysis must be available; see Section 3.11: Prestressed Modal Analysis. If prestress effects are turned on, the lumped mass setting [LUMPM] in this and subsequent solutions must be the same as it was in the prestress static analysis.
Note — You can use only axisymmetric loads for prestressing harmonic elements such as PLANE25 and SHELL61.
3.5.2.8. Additional Modal Analysis Options
After you complete the fields on the Modal Analysis Options dialog box, click OK. A dialog box specific to the selected extraction method appears. You see some combination of the following fields: FREQB, FREQE, PRMODE, Nrmkey, RIGID, SUBOPT. Refer to the MODOPT and RIGID command descriptions for the meaning of these fields.
3.5.3. Define Master Degrees of Freedom
In a modal analysis, you also need to define master degrees of freedom. These are required only for the reduced mode-extraction method.
Master degrees of freedom (MDOF) are significant degrees of freedom that characterize the dynamic behavior of the structure. You should choose at least twice as many MDOF as the number of modes of interest. We recom-mend that you define as many MDOF as you can based on your knowledge of the dynamic characteristics of the
structure [M,MGEN], and also let the program choose a few additional masters based on stiffness-to-mass ratios [TOTAL]. You can list the defined MDOF [MLIST], and delete extraneous MDOF [MDELE]. For more details about master degrees of freedom, see Section 3.14: Matrix Reduction.
Command(s): M
GUI: Main Menu> Solution> Master DOFs> User Selected> Define
3.5.4. Apply Loads
After defining master degrees of freedom, apply loads on the model. The only "loads" valid in a typical modal analysis are zero-value displacement constraints. (If you input a nonzero displacement constraint, the program assigns a zero-value constraint to that DOF instead.) Other loads can be specified, but are ignored (see Note below). For directions in which no constraints are specified, the program calculates rigid-body (zero-frequency) as well as higher (nonzero frequency) free body modes. Table 3.2: “Loads Applicable in a Modal Analysis” shows the commands to apply displacement constraints. Notice that you can apply them either on the solid model (keypoints, lines, and areas) or on the finite element model (nodes and elements). For a general discussion of solid-model loads versus finite-element loads, see Chapter 2, “Loading” in the ANSYS Basic Analysis Guide.
Note — Other loads - forces, pressures, temperatures, accelerations, and so on - can be specified in a modal analysis, but they are ignored for the mode-extraction. However, the program will calculate a load vector and write it to the mode shape file (Jobname.MODE) so that it can be used in a subsequent mode-superposition harmonic or transient analysis.