Difference between Transient direct and Transient modal

Altair Forum User

Hi all,

could someone explain me the difference between a Transient direct and a Transient modal analysis? Which may I use to analyze the vibrational behavior of a system?

 

Thanks

Pranav Hari

Hi @Ferrari248F1AQ

 

You can try getting an idea about Transient direct and a Transient modal analysis from Optistruct Help tutorials.

 

Refer 

OS-T: 1310 Direct Transient Dynamic Analysis of a Bracket

OS-T: 1315 Modal Transient Dynamic Analysis of a Bracket

 

 

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Rahul Rajan_21763

You can refer attached pdf from OptiStruct user guide.

Use below link to download free eBook on Dynamic analysis using OptiStruct.

https://altairuniversity.com/free-ebook-learn-dynamic-analysis-with-altair-optistruct/

 

Unable to find an attachment - read this blog

Altair Forum User

Thanks

Altair Forum User

I am getting a huge difference in result for the same problem solved in modal transient and direct transient respectively. Can some explain how much difference we can expect and what are factor affecting the difference? 

FaroukM

Altair Forum User said:

I am getting a huge difference in result for the same problem solved in modal transient and direct transient respectively. Can some explain how much difference we can expect and what are factor affecting the difference? 

 

It's normal to have a difference between direct transient and modal transient ... Just make sure that your synthesis modal basis is large enough to ensure to convergence and normally you will approach the results of the direct transient /emoticons/default_wink.png' srcset='/emoticons/wink@2x.png 2x' title=';)' width='20' />

Adriano A. Koga

Like mentioned by Farouk, in order for your modal transient to represent well your problem, you need to take care with the timestep used and also the frequency range used in your modal basis.

As a rule of thumb, if your frequency of interest in the transient analysis is fn, them your modal analysis should cover at least 1.5*fn.

 

Modal frf/transient needs enough modes to represent the structure behavior as a combination of the normal modes.

Also make sure RESVEC is on. (Residual vectors increase the accuracy of the modal transient)

FaroukM

Well done.

 

Altair Forum User said:

least 1.5*fn

I just want to add some details: when you have a dumped system, we recommand  to increase the multiplying coefficient. In my last modal simulation with a dumped system, the convergence is reached with \alpha = 5 ( 5*fn) !