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Modal analysis

I am performing a modal analysis of a tube with the two ends completely fixed, the results showed me that the first mode of vibration has a frequency of more than 130 Hz, and a maximum deformation of more than 90 meters, what does not make sense since the tube has a little bit more than 150 millimeters of length.


Anyone could help me on this?


Re: Modal analysis

Dear Halipio,

Displacements results of a normal modes/eigenvalue analysis (SOL103) are useless, not more meaning that to show the deformed shape of the mode shape at a specific natural frequency. The displacement results are normalized results based in unit modal mass.


In normal modes analysis using NX NASTRAN (SOL103) we solve for the eigenvalues and eigenvectors of the model. For each eigenvalue, which is proportional to a natural frequency, there is a corresponding eigenvector, or mode shape.


Each mode shape is similar to a static displaced shape in that there are displacements and rotations for each grid point. However, there is one important difference between the mode shape and the static displacements: the scaling. In static analysis the displacements are the true physical displacements due to the applied loads. However, because there is no applied load in normal mode analysis, the mode shape components can all be scaled by an arbitrary factor for each mode. In NX Nastran this scaling can be done so that each mode has a unit modal mass, so that the maximum displacement in any mode is 1.0, or that any user specified degree of free­dom has a modal displacement of 1.0. The first option, unit modal mass, is generally preferred, though the scaling of a maximum displacement to 1.0 is useful for comparison to modal test data.



Element forces and stresses and reaction forces are computed in the same manner as for static analysis, with each mode shape treated the same as a set of static displacements. Due to the scaling of each mode, the resulting modal forces and stresses are on a per mode basis and can­not necessarily be compared from one mode to another.


Best regards,


Blas Molero Hidalgo, Ingeniero Industrial, Director
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