Module 7 Modal Analysis 7 Modal Analysis Training

Module 7 Modal Analysis

7. Modal Analysis Training Manual • 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. INTRODUCTION TO ANSYS 6. 0 - Part 2 • Modal analysis is used to determine a structure’s vibration characteristics — natural frequencies and mode shapes. October 30, 2001 Inventory #001571 7 -2

Modal Analysis A. Procedure – Preprocessing • Geometry • Meshing – Solution • Analysis Type and Options • Loading • Solve – Postprocessing • Review results INTRODUCTION TO ANSYS 6. 0 - Part 2 • The steps to do a modal analysis are as follows: Training Manual October 30, 2001 Inventory #001571 7 -3

Modal Analysis . . . Procedure Training Manual • 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/in 3) g (in/sec 2) = mass density (lb-sec 2/in 4) • For example, density of steel = 0. 283/386 = 7. 3 E-04 lb-sec 2/in 4 • Linear elements and material properties only. Nonlinearities are ignored. INTRODUCTION TO ANSYS 6. 0 - Part 2 Geometry & Meshing October 30, 2001 Inventory #001571 7 -4

Modal Analysis . . . Procedure • The analysis type is modal. – Solution > -Analysis Type- New Analysis – Or issue ANTYPE, MODAL INTRODUCTION TO ANSYS 6. 0 - Part 2 Analysis Type Training Manual October 30, 2001 Inventory #001571 7 -5

Modal Analysis . . . Procedure Training Manual • 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 INTRODUCTION TO ANSYS 6. 0 - Part 2 Analysis Options October 30, 2001 Inventory #001571 7 -6

Modal Analysis . . . Procedure Training Manual • 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). INTRODUCTION TO ANSYS 6. 0 - Part 2 Loading & Solve October 30, 2001 Inventory #001571 7 -7

Modal Analysis . . . Procedure Training Manual • POST 1 (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. INTRODUCTION TO ANSYS 6. 0 - Part 2 Review Results October 30, 2001 Inventory #001571 7 -8

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 INTRODUCTION TO ANSYS 6. 0 - Part 2 • Training Manual October 30, 2001 Inventory #001571 7 -9

Modal Analysis . . . Procedure You can also animate mode shapes: – Utility Menu > Plot. Ctrls > 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. INTRODUCTION TO ANSYS 6. 0 - Part 2 • Training Manual October 30, 2001 Inventory #001571 7 -10

Modal Analysis B. Workshop Training Manual W 6. U-Bracket Please refer to your Workshop Supplement for instructions. INTRODUCTION TO ANSYS 6. 0 - Part 2 • This workshop consists of the following problem: October 30, 2001 Inventory #001571 7 -11

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