Deviation of Mode shape frequency values between UG NX Nastran and Ideas NX

[ninjaz]

Why there is a deviation of results even if I solve a same FE model in NX Nastran and Ideas Model solution solver.
What has to be done to match the results. Are there different types of algorithms used for each solver.

In UG NX , the first mode is about 190.5 HZ. The same FE model is exported into Ideas and there , the first mode is 183 Hz.

What is the exact reason for this deviation.

 

[JBlack68]

check param k6rot & coupmass to see if there are the "same"

 

[BlasMolero]

Hello!,
This is reasonable, I understanbd one run goes with NX NASTRAN (SOL103) and the other with Model Solution solver. Both solvers are good FEA solvers, then for ccomparisson make sure you are using exactly the same eigenvalue extraction algoritm, in NX NASTRAN the default is Lanczos. Seven methods of real eigenvalue extraction are provided in NX Nastran. These methods are numerical approaches to solving for natural frequencies and modes shapes. The reason for seven different numerical techniques is because no one method is the best for all problems. While most of the methods can be applied to all problems, the choice is often based on the efficiency of the solution process. The recommended real eigenvalue extraction method in NX Nastran is the Lanczos method. The Lanczos method combines the best characteristics of both the tracking and transformation methods. The Lanczos method is the preferred method for most medium- to large-sized problems, since it has a performance advantage over other methods.

Also check also how you manage the mass, in NX NASTRAN the default is lumped, you can choose between lumped or coupled.Lumped mass matrices contain uncoupled, translational components of mass. Coupled mass matrices contain translational components of mass with coupling between the components. Coupled mass can be more accurate than lumped mass. However, lumped mass is more efficient and is preferred for its computational speed in dynamic analysis.

Also check the NX NASTRAN Bulk Data Options, important parameters here are the K6ROT and WTMASS, as well as RESVEC to account for residual vectors. Well, you see many factors can influence when determining the natural frequencies and mode shapes of the structure with damping neglected.

Best regards,
Blas.



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Blas Molero Hidalgo
Ingeniero Industrial
Director

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