7. Modal AnalysisModal analysis is used to determine a
structures vibration characteristics natural frequencies and mode
shapes.It is the most fundamental of all dynamic analysis types and
is generally the starting point for other, more detailed dynamic
analyses.In this chapter, we will describe how to do a basic modal
analysis via the following topics:A. ProcedureB. WorkshopFor
details, refer to your Structural Analysis Guide or Dynamics
Training Manual.
Modal AnalysisA. ProcedureThe steps to do a modal analysis are
as follows:PreprocessingGeometryMeshingSolutionAnalysis Type and
OptionsLoadingSolvePostprocessingReview results
Modal Analysis...ProcedureGeometry & MeshingSame
considerations as a static analysis:Include as many details as
necessary to sufficiently represent model geometry.A fine mesh will
be needed to resolve complex mode shapes.Both Youngs modulus and
density are required.Specify mass density instead of weight density
when using British units.weight density (lb/in3) g (in/sec2) = mass
density (lb-sec2/in4)For example, density of steel = 0.283/386 =
7.3E-04 lb-sec2/in4Linear elements and material properties only.
Nonlinearities are ignored.
Modal Analysis...ProcedureAnalysis TypeThe analysis type is
modal.Solution > -Analysis Type- New AnalysisOr issue
ANTYPE,MODAL
Modal Analysis...ProcedureAnalysis OptionsMode extraction
optionsMODOPT command or Solution > Analysis Options...Method:
Block Lanczos is generally recommended.Number of modes: Specify the
number of modes you want to extract.Mode expansion optionsMXPAND
command or Solution > Analysis Options...Expanding modes allows
you to view them in the postprocessor.Number of modes is usually
the same as the number extracted.Prestress effectsPSTRES command or
Solution > Analysis Options...calculate mode shapes of a
prestressed structure, such as a spinning turbine blade.Default
settings are usually sufficient
Modal Analysis...ProcedureLoading & SolveThe only loads
valid in a modal analysis are displacement constraints:Solution
> -Loads- Apply > Displacement >Or use the D family of
commands: D, DK, DL, DA, etc.If you dont specify constraints, ANSYS
will calculate rigid body modes (zero frequency).After all
constraints have been specified, obtain the solution:First save the
database (SAVE command or Toolbar > SAVE_DB).Then issue SOLVE
(or Solution > -Solve- Current LS).
Modal Analysis...ProcedureReview ResultsPOST1 (the general
postprocessor) is used to review modal analysis results.Typically,
the first step is to list the natural frequencies:General Postproc
> Results SummaryOr issue SET,LISTNote that each mode is stored
in a separate substep.
Modal Analysis...ProcedureTo plot a mode shape:First read
results for that substepGeneral Postproc > Results Summary...Or
use the SET commandThen plot the deformed shapeGeneral Postproc
> Plot Results > Deformed ShapeOr use the PLDISP command
Modal Analysis...ProcedureYou can also animate mode
shapes:Utility Menu > PlotCtrls > Animate > Mode ShapeOr
use the ANMODE commandAnother option is to plot contours stresses
and strains. Note, however, that displacement and stress values are
simply relative numbers and have no real meaning.
Modal AnalysisB. WorkshopThis workshop consists of the following
problem:W6. U-BracketPlease refer to your Workshop Supplement for
instructions.
