Mechanical APDL Structural Analysis Guide

lysis problems, the Mechanical APDL Verification Manual does not present them as step-by-step examples
with lengthy data input instructions and printouts. However, most ANSYS users who have at least limited
finite element experience should be able to fill in the missing details by reviewing each test case's finite
element model and input data with accompanying comments.

The Mechanical APDL Verification Manual includes a variety of harmonic analysis test cases:

VM76 - Harmonic Response of a Guitar String

VM86 - Harmonic Response of a Dynamic System

VM87 - Equivalent Structural Damping

VM88 - Response of an Eccentric Weight Exciter

VM90 - Harmonic Response of a Two-Mass-Spring System

VM176 - Frequency Response of Electrical Input Admittance for a Piezoelectric Transducer

VM177 - Natural Frequency of a Submerged Ring

VM183 - Harmonic Response of a Spring-Mass System

VM203 - Dynamic Load Effect on Simply-Supported Thick Square Plate

4.8. Mode-Superposition Harmonic Analysis

The mode-superposition method sums factored mode shapes (obtained from a modal analysis) to cal-
culate the harmonic response. It is the only method allowed in the ANSYS Professional program. The
procedure to use the method consists of five main steps:

1. Build the model.


2. Obtain the modal solution.


3. Obtain the mode-superposition harmonic solution.


4. Expand the mode-superposition solution.


5. Review the results.

Of these, the first step is the same as described for the full method. The remaining steps are described
below.

4.8.1. Obtain the Modal Solution

Modal Analysis (p. 39) describes how to obtain a modal solution. Following are some additional hints:

• The mode-extraction method should be Block Lanczos, PCG Lanczos, Supernode, Subspace, QR
  damped, or unsymmetric. (Other methods do not apply to mode superposition.)

If your model has damping and/or an unsymmetric stiffness or damping matrix, use the QR Damp

mode-extraction method (MODOPT,QRDAMP) and do not request the calculation of the complex

eigenmodes (Cpxmod = OFF on MODOPT).

If your model is undamped with an unsymmetric stiffness and/or mass matrix, use the unsym-

metric mode-extraction method (MODOPT,UNSYM) and request both left and right modes

(ModType = BOTH on MODOPT).

• Be sure to extract all modes that may contribute to the harmonic response.

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Mode-Superposition Harmonic Analysis