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[balaji.bp]

Hi all,

I am performing a MSUP Random Vibration (RV) analysis and before solving for the random response, I want to reduce the number of modes used in the analysis. For this, I used the MXPAND and MODSELOPTION commands during the modal analysis, which works well for reducing the modes in the modal analysis itself. However, when I proceed to the RV analysis, an error occurs stating: 'The mode coefficient values are not on the .mode file. The mode selection cannot be performed.'

the command snippet which i am using for this is given below,

! Obtain the Modal Solution
/solu
antype, MODAL
modopt,LANB,20,,,,,,,,,,! Block Lanczos
solve
finish
! Perform the modal selection and expansion
/solu
antype, MODAL, RESTART
mxpand,,,, yes,,, MODM ! Mode selection is based on the modal effective mass, and element
! results are requested
modseloption, .90, no,no, no,no,no ! Only direction X is selected, a minimum of 90% of the total
! mass is requested in this direction
solve
finish
! Post-process the modal results or perform a mode superposition analysis

 

Using this code if i am solving Harmonic it working fine but MSUP RV its not working, is there any extra step i need to add in this code?

please help

 

[Stella Peloni]

Hello,

Please refer to the Documentation for MXPAND command and check the syntax of this command. I believe the problem stems from the fact that the method for mode selections is not properly written. 

Kind Regards,

Stella

[harshvardhan.negi]

Hi,
You may want to look into how you have setup the MXPAND and MODSELOPTION commands.
Refer to the following links:
MXPAND
MODSELOPTION

I hope this helps.

Regards,
Harshvardhan
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[balaji.bp]

 

Hi all,

i did some modification as per the syntax, i run the simulation again but still same error,

the code which have used is given below 

antype,modal
modopt,lanb,20
mxpand,none
SOLVE
FINISH
!
/SOLU
antype,modal,restart
mxpand,,,,YES,,YES,EFFM,,
modseloption,0.9,no,no,no,no,no,
SOLVE

following is the error

for above code i have referred this discussion content

https://innovationspace.ansys.com/knowledge/forums/topic/how-can-we-consider-only-specific-modes-in-the-harmonic-analysis-based-on-the-modal-participation-factor-in-modal-analysis/

[harshvardhan.negi]

Hi,
Please check the MODSELOPTION command also.

You need to define the limit in the MXPAND command itself. Ansys Help page says:
"If dir1 = YES, then any mode in this direction is expanded if its modal effective mass divided by the total mass (modal effective mass ratio) is greater than SIGNIF on the MXPAND command."

Regards,
Harshvardhan
Ansys Help
Ansys Learning Forum (Rules & Guidelines)

[balaji.bp]

thanks for your reply,

as per your suggestion i made the changes in the code as given below,

antype,modal
modopt,lanb,20
SOLVE
FINISH
!
/SOLU
antype,modal,restart
mxpand,,,,YES,0.1,YES,EFFM,,
modseloption,yes,yes,yes,no,no,no,
SOLVE

as i told previously also that this code is working for modal analysis that means it expanding only those modes which are above significance threshold given in the code. But when i am running a Random vibration analysis its throwing same error as mentioned in last replies. I feel there some extra step i am missing which is required for RV analysis.

[harshvardhan.negi]

Hi,
Generally, we use the *GET command to select the modes for further analyses. However, I can see that here you are using MODSELOPTION to select the modes. Maybe you can share the code for the spectrum analysis you are solving.

From the error screenshot you have shared, I can see that solver has started the spectrum analysis. So, the issue might be with commands preceeding the ANTYPE, SPECTR command.

Regards,
Harshvardhan
Ansys Help
Ansys Learning Forum (Rules & Guidelines)

[balaji.bp]

 

HI,

i am running this command in mechanical and doing MSUP random vibration analysis. Could you please tell me what exactly i will have to share? means from mechanical database which file i have to share so that you will get an idea about code.

 

[harshvardhan.negi]

Hi,
Since you have shared APDL codes for Modal analysis, I thought you are using Ansys APDL to solve this problem and not Mechanical.

Are you using a stand-alone Random Vibration analysis, using APDL code to run the Modal analysis?

Regards,
Harshvardhan
Ansys Help
Ansys Learning Forum (Rules & Guidelines)

[balaji.bp]

HI, 

i am using Ansys mechanical MSUP Random Vibration system, not stand alone.