Typical harmonic system. Fo and Ω are known. uo and Φ are unknown (a). Transient and
steady-state dynamic response of a structural system (b).Harmonic analysis is a linear analysis. Some nonlinearities, such as plasticity, are ignored, even if they
are defined. You can, however, have unsymmetric system matrices such as those encountered in a fluid-
structure interaction (FSI) problem. For more information, see the Acoustic Analysis Guide and the Coupled-
Field Analysis Guide.
Harmonic analysis can also be performed on a prestressed structure, such as a violin string (assuming
the harmonic stresses are much smaller than the pretension stress). See Prestressed Full Harmonic
Analysis (p. 104) and Linear Perturbation Analysis for more information on prestressed harmonic analyses.
4.2. Commands Used in a Harmonic Analysis
You use the same set of commands to build a model and perform a harmonic analysis that you use to
do any other type of finite element analysis. Likewise, you choose similar options from the graphical
user interface (GUI) to build and solve models no matter what type of analysis you are doing.
Sample Harmonic Analysis (GUI Method) (p. 90) and Example Harmonic Analysis (Command or Batch
Method) (p. 96) show a sample harmonic analysis done via the GUI and via commands, respectively.
For detailed, alphabetized descriptions of the ANSYS commands, see the Command Reference.
4.3. Two Solution Methods
Two harmonic analysis methods are available:full and mode superposition.(A third, relatively expensive
method is to do a transient dynamic analysis with the harmonic loads specified as time-history loading
functions; see Transient Dynamic Analysis (p. 107) for details.) The ANSYS Professional program allows
only the mode-superposition method. Before we study the details of how to implement each of these
methods, let's explore the advantages and disadvantages of each method.
4.3.1. The Full Method
The full method is the easiest available method. It uses the full system matrices to calculate the harmonic
response (no matrix reduction). The matrices may be symmetric or unsymmetric. The advantages of the
full method are:
- It is easy to use, because you don't have to worry about choosing mode shapes.
- It uses full matrices, so no mass matrix approximation is involved.
- It allows unsymmetric matrices, which are typical of such applications as acoustics and bearing
problems. - It calculates all displacements and stresses in a single pass.
- It accepts all types of loads: nodal forces, imposed (nonzero) displacements, and element loads
(pressures and temperatures). - It allows effective use of solid-model loads.
A disadvantage is that this method usually is more expensive than the other method when you use the
sparse solver. However, when you use the JCG solver or the ICCG solver, the full method can be very
efficient in some 3-D cases where the model is bulky and well-conditioned.
Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationHarmonic Analysis