Chapter 3: Modal Analysis
Use modal analysis to determine the vibration characteristics (natural frequencies and mode shapes)
of a structure or a machine component while it is being designed. It can also serve as a starting point
for another, more detailed, dynamic analysis, such as a transient dynamic analysis, a harmonic analysis,
or a spectrum analysis.
The following modal analysis topics are available:
3.1. Uses for Modal Analysis
3.2. Understanding the Modal Analysis Process
3.3. Building the Model for a Modal Analysis
3.4. Applying Loads and Obtaining the Solution
3.5. Reviewing the Results
3.6. Applying Prestress Effects in a Modal Analysis
3.7. Modal Analysis Examples
3.8. Comparing Mode-Extraction Methods
3.9. Modal Analysis Tools for Subsequent Mode Superposition Analysis3.1. Uses for Modal Analysis
You use modal analysis to determine the natural frequencies and mode shapes of a structure. The nat-
ural frequencies and mode shapes are important parameters in the design of a structure for dynamic
loading conditions. They are also required if you want to perform a spectrum analysis or a mode-super-
position harmonic or transient analysis.
You can perform a modal analysis on a prestressed structure, such as a spinning turbine blade. Another
useful feature is modal cyclic symmetry, which allows you to review the mode shapes of a cyclically
symmetric structure by modeling just a sector of it.
Modal analysis in the ANSYS, Inc. family of products is a linear analysis. Any nonlinearities, such as
plasticity and contact (gap) elements, are ignored even if they are defined. You can select from among
several mode-extraction methods: Block Lanczos, Supernode, Subspace, PCG Lanczos, unsymmetric,
damped, and QR damped. The damped and QR damped methods allow you to include damping in the
structure. The QR damped method also allows for unsymmetrical damping and stiffness matrices. Details
about mode-extraction methods are covered later in this section.
3.2. Understanding the Modal Analysis Process
The general process for a modal analysis consists of these primary operations:
- Build the model.
- Apply loads and obtain the solution.
- Review the results.
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