有限元模态分析ANSYS

1、modal analysismodule 2dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-2module 2modal analysisa. define modal analysis and its purpose.b. discuss associated concepts, terminology, and mode extraction methods.c. learn how to do a m

2、odal analysis in ansys.d. work on one or two modal analysis exercises.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-3modal analysisa. definition & purposewhat is modal analysis?a technique used to determine a structures vibrati

3、on characteristics:natural frequenciesmode shapesmode participation factors (how much a given mode participates in a given direction)most fundamental of all the dynamic analysis types.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #00211

4、02-4modal analysis definition & purposebenefits of modal analysisallows the design to avoid resonant vibrations or to vibrate at a specified frequency (speakers, for example).gives engineers an idea of how the design will respond to different types of dynamic loads.helps in calculating solution cont

5、rols (time steps, etc.) for other dynamic analyses.recommendation: because a structures vibration characteristics determine how it responds to any type of dynamic load, always perform a modal analysis first before trying any other dynamic analysis.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dyna

6、mics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-5general equation of motion:assume free vibrations and ignore damping:assume harmonic motion ( i.e. )the roots of this equation are i2, the eigenvalues, where i ranges from 1 to number of dof. corresponding vectors are ui, the eigenv

7、ectors. 0umk2 0ukum tfukucum modal analysisb. terminology & concepts) tsin(uudynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-6modal analysis terminology & conceptsthe square roots of the eigenvalues are i , the structures natural

8、 circular frequencies (radians/sec). natural frequencies fi are then calculated as fi = i /2p p cycles/sec . it is the natural frequencies fi that are input by the user and output by ansys.the eigenvectors ui represent the mode shapes - the shape assumed by the structure when vibrating at frequency

9、fi.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-7modal analysis terminology & concepts (cont.)mode extraction is the term used to describe the calculation of eigenvalues and eigenvectors.mode expansion has a dual meaning. for

10、the reduced method, mode expansion means calculating the full mode shapes from the reduced mode shapes. for all other methods, mode expansion simply means writing mode shapes to the results file.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inven

11、tory #0021102-8modal analysis - terminology & conceptsmode extraction methodsseveral mode extraction methods are available in ansys:block lanczos (default)subspacepowerdynamicsreducedunsymmetricdamped (full)qr dampedwhich method you choose depends primarily on the model size (relative to your comput

12、er resources) and the particular application.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-9modal analysis - terminology & concepts mode extraction methods - block lanczosthe block lanczos method is recommended for most applica

13、tions.efficient extraction of large number of modes (40+) in most modelstypically used in complex models with mixture of solids/shells/beams etc.efficient extraction of modes in a frequency range handles rigid-body modes welldynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1tra

14、ining manualjuly 22, 2004inventory #0021102-10modal analysis - terminology & concepts mode extraction methods - subspacewhen extracting a small number of modes (40) in similar size models, the subspace method can be more suitable.requires relatively less memory but large diskspacemay have convergenc

15、e problems when rigid body modes are present.not recommended when constraint equations are present.generally superseded by block lanczosdynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-11modal analysis - terminology & concepts mod

16、e extraction methods - powerdynamicsfor large (100k+ dof) models and a small number of modes ( by pick” in the general postproc menu.notice that each mode is stored in a separate substep.dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #00

17、21102-34modal analysis procedure review resultsviewing mode shapes:first read in results for the desired mode using first set, next set, or by load step.then plot the deformed shape: general postproc plot results deformed shapenotice that the graphics legend shows mode number (sub = ) and the freque

18、ncy (freq = ).dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-35modal analysis procedure review resultsviewing mode shapes (continued):you can also animate the mode shape: utility menu plotctrls animate mode shape.dynamics 8.1dyn

19、amics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-36modal analysis procedure review resultsparticipation factors:calculated for each mode in global translation and rotation directionshigh value in a direction indicates that the mode will be excit

20、ed by forces in that directionvalues are relative based on a unit displacement spectrumthe final participation factor value (rotz) can be retrieved into a parameter using *get command. a spectrum analysis with a specified direction (sed,0,1,0) could be used to obtain other valuesalso printed out (to

21、 the output file) is the effective mass. ideally the sum of the effective masses in each direction should equal total mass of structureeffective mass = (participation factor)2dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-37moda

22、l analysis procedure review resultsmodal stresses:available if element stress calculation is activated when choosing analysis options.stress values have no real meaning, however these can be used to highlight hot spotsif mode shapes are normalized to unity, you can compare stresses at different poin

23、ts for a given mode shapedynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-38modal analysis procedure review resultsmode shapes normalized to unitydynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1dynamics 8.1training manualjuly 22, 2004inventory #0021102-39modal analysisprocedure build the mode