Mechanical APDL Structural Analysis Guide

• If you use the QR damped mode-extraction method, you must specify any damping (ALPHAD,BETAD,
  MP,ALPD,MP,BETD, or element damping including gyroscopic) that you want to include during the
  modal analysis. During the harmonic analysis, you can define additional damping, including a constant
  modal damping ratio (DMPRAT), constant structural material damping coefficients (DMPSTR,
  MP,DMPR), or the modal damping ratio as a function of mode (MDAMP). For more details about
  damping definition, see Damping (p. 2).


• If you need to apply harmonically varying element loads (pressures, temperatures, accelerations, etc.),
  specify them in the modal analysis. ANSYS ignores the loads for the modal solution, but calculates a
  load vector and writes it to the mode shape file (Jobname.MODE) and also writes the element load
  information to the Jobname.MLV file. You can generat e multiple load vectors (see the MODCONT
  command in Generating and Using Multiple Loads in Mode-Superposition Analyses (p. 68)). The load
  vectors created can then be scaled and used in the harmonic solution.


• To include the contribution of higher frequency modes in the subsequent harmonic analysis, you can
  calculate the residual vectors in the modal analysis (see the RESVEC command in Using the Residual-
  Vector Method to Improve Accuracy (p. 66)).


• If the harmonic excitation is coming from the support, you can generat e pseudo-static modes needed
  in the subsequent harmonic analysis (see the MODCONT command in Enforced Motion Method for
  Mode-Superposition Transient and Harmonic Analyses (p. 70)).


• You should expand the modes and calculate the element results to save computation time in the
  subsequent expansion of the harmonic results (MXPAND,ALL,,,YES,,YES). Do not use this option if
  you are applying thermal loads, or if you desire to postprocess energies. Additionally, ensure that
  you output element nodal loads (OUTRES,NLOAD,ALL) if you want to postprocess total reaction
  forces.


• Do not change the model data (for example, nodal rotations) between the modal and harmonic
  analyses.


• You can select the modes of interest for the expansion. Because the expansion is performed on the
  selected modes only, you will save computational time in the subsequent mode superposition har-
  monic analysis. See Using Mode Selection (p. 71) for information about this procedure.

4.8.2. Obtain the Mode-Superposition Harmonic Solution

In this step, the program uses mode shapes extracted by the modal solution to calculate the harmonic
response. The following requirements apply:

• The mode shape file (Jobname.MODE) must be available.


• If the unsymmetric eigensolver is used, the left mode shape file (Jobname.LMODE) must also be
  available (ModType = BOTH on the MODOPT command). Unlike the Jobname.MODE file which
  contains participation factors, load vectors, and other information, the Jobname.LMODE file only
  contains the left eigensolutions.


• The full file (Jobname.FULL) must be available if linear acceleration (ACEL) is present in the mode
  superposition analysis.


• The database must contain the same model from which the modal solution was obtained.

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Harmonic Analysis