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displacement result in direct transient analysis

akash0990

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

<?xml version="1.0" encoding="UTF-8"?>

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Altair Forum User

Hi Akash,

Thats because you requested to output till 1 sec in TSTEP card.

Edit the card and try again.

akash0990

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.

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

Akash,

OK, I will check and update to you soon.

akash0990

Thanks prakash

Altair Forum User

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 /emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20'>

akash0990

Hi,

from help file - w3 is the frequency of interest. but how can we calculate this value?

is it the 1st natural frequency?

Altair Forum User

Akash,

Run a normal mode analysis to calculate the natural frequency.

akash0990

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

Altair Forum User

Hi Akash,

Can you please share the modified file with W3?

akash0990

hi,

PFA

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Altair Forum User

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.

akash0990

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.

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

hi akash,

Can you share the modified file of modal transient analysis.

akash0990

hi,

pfa

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Altair Forum User

@akash0990

Isn't it should be reverse?

Altair Forum User

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.

Altair Forum User

How to use damping in explicit analysis ?

Altair Forum User

Hi @ank

Can you post more about your analysis and your end objective so that we can help you with necessary information?

Karthic_22061

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?