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akash0990

displacement result in direct transient analysis

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Hi,

 

i was trying to run direct transient analysis for a simple model. while checking the displacement, i got a continuous displacement as shown in fig. as you can see here, the displacement keeps on increasing . but after 0.8 sec, the load in the table is zero. i have also attached .fem file for your reference.

 

Regards

Akash

 

trans.fem

displacement vs time.PNG

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Hi,

Thanks for the reply.

I changed the tstep time to 10 sec and ran the analysis. and got following results. 

my query is why there is no damping in displacement even if the load is zero.

 

 

Capture.PNG

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Hi Akash,

 

You cannot use structural damping as it is for transient analysis. This has to be converted to viscous damping.

 

Use PARAM>>W3 to convert structural damping to viscous. 

 

Please refer to OptiStruct help on how calculate W3 :) 

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hi,

 

i have already run that. and i got the 1st mode at 21.7 hz. then i converted it into radians per seconds and applied the same value at PARAM, W3 along with PARAM, G. But still, it is not damping. so, i would like to know the procedure what i followed is correct or not.

 

regards

akash 

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Hi Akash,

 

Generally, in transient analysis the proper selection of the damping value becomes less important for very short duration events, such as crash impulse or a shock blast. The specification of the damping value becomes more important for long duration loadings (such as earthquakes) and is critical for loadings (such as sine dwells) that continually add energy into the system.
 

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Hi,

 

thanks for the reply.

 

if you see my model, i have not applied any load after 0.8 sec. still there is an increase in displacement. so if you take the final displacement 4.27mm , which is very high as compared to the load applied. that means, the displacement is neither steady nor damped and the result is completely wrong.

 

again if you run the same model with modal transient analysis, damping is working. so my query is, how can i apply proper damping in direct transient anlysis.  for your information, i have attached the result from modal transient analysis.

modal_trans.PNG

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Hi Akash,

 

Modal transient results are absoulute and using ENFMOTN REL we convert to real. This is same with direct transient and the reuslts you see in transient are absolute. 

 

You can remove ENFMOTN in modal transient and check for yourself and you will see the graphs match. 

 

Also after looking at your modal transient model, the G value in TABDMP should be half of PARAM G and on top of that use KDAMP -1.

 

ENFMOTN works only with modal transient and not with direct transient analysis.

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Hi @Prakash Pagadala ,

 

The results from Direct transient are absolute without using ENFMOTN>>REL and also ENFMOTN  REL does not work with Direct transient analysis.

 

If so, is there any way for direct transient to give the real result?

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