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Modal Analysis

Module 7. Modal Analysis. 7. Modal Analysis. Modal analysis is used to determine a structure’s vibration characteristics — natural frequencies and mode shapes.

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Modal Analysis

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  1. Module 7 Modal Analysis

  2. 7. Modal Analysis • Modal analysis is used to determine a structure’s 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. Procedure B. Workshop For details, refer to your Structural Analysis Guide or Dynamics Training Manual. October 30, 2001 Inventory #001571 7-2

  3. Modal AnalysisA. Procedure • The steps to do a modal analysis are as follows: • Preprocessing • Geometry • Meshing • Solution • Analysis Type and Options • Loading • Solve • Postprocessing • Review results October 30, 2001 Inventory #001571 7-3

  4. Modal Analysis...Procedure Geometry & Meshing • Same 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 Young’s 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/in4 • Linear elements and material properties only. Nonlinearities are ignored. October 30, 2001 Inventory #001571 7-4

  5. Modal Analysis...Procedure Analysis Type • The analysis type is modal. • Solution > -Analysis Type- New Analysis • Or issue ANTYPE,MODAL October 30, 2001 Inventory #001571 7-5

  6. Modal Analysis...Procedure Analysis Options • Mode extraction options • MODOPT 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 options • MXPAND 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 effects • PSTRES command or Solution > Analysis Options... • calculate mode shapes of a prestressed structure, such as a spinning turbine blade. Default settings are usually sufficient October 30, 2001 Inventory #001571 7-6

  7. Loading & Solve The 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 don’t 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...Procedure October 30, 2001 Inventory #001571 7-7

  8. Modal Analysis...Procedure Review Results • POST1 (the general postprocessor) is used to review modal analysis results. • Typically, the first step is to list the natural frequencies: • General Postproc > Results Summary • Or issue SET,LIST • Note that each mode is stored in a separate substep. October 30, 2001 Inventory #001571 7-8

  9. Modal Analysis...Procedure • To plot a mode shape: • First read results for that substep • General Postproc > Results Summary... • Or use the SET command • Then plot the deformed shape • General Postproc > Plot Results > Deformed Shape • Or use the PLDISP command October 30, 2001 Inventory #001571 7-9

  10. Modal Analysis...Procedure • You can also animate mode shapes: • Utility Menu > PlotCtrls > Animate > Mode Shape • Or use the ANMODE command • Another option is to plot contours stresses and strains. Note, however, that displacement and stress values are simply relative numbers and have no real meaning. October 30, 2001 Inventory #001571 7-10

  11. Modal AnalysisB. Workshop • This workshop consists of the following problem: W6. U-Bracket Please refer to your Workshop Supplement for instructions. October 30, 2001 Inventory #001571 7-11

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