FEMAP Modal Analysis and Mesh size

[gianmarco]

Hi, I have a problem. When I did a modal analysis on a plate (780x400 and t=0,002), I got 305 Modes in a range 0-5000 Hz. I created a mesh size 0,01. After I did the same analysis but with a new mesh size of 0,005 and I got 294 Modes. I thinked to find the same number of modes, or some more for a lower mesh size. Can you explain me why this appened?

 

[BlasMolero]

Hello!,
If you refine your mesh then the NX NASTRAN (SOL103) solver will extract more local modes, but the main modes (those with energy) will basically remains the same. I always plot the modal mass participation factor vs. mode number, this way you will see the amount of mass captured, the minimum for any dynamic response analysis is to achive the 85% of the masss of the direction of the excitation.

For dynamic analysis the mesh must be fine enough to accurately represent the deformed shape of the modes of interest. If many modes are to be considered in the analysis, then the model must be fine enough to accurately represent the mode shapes of the highest modes of interest.

A general rule is to use at least five to ten grid points per half-cycle of response amplitude.

Another way to verify the accuracy of the mesh density is to apply static loads that give a deformed shape the same as the mode of interest and perform stress discontinuity calculations. This process can be laborious and is not recommended as a general checkout procedure.

Best regards,
Blas.

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Blas Molero Hidalgo
Ingeniero Industrial
Director

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