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Nirav Modi

Random Response Analysis

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Hello sir,

I am currently using MSC Nastran, Patran for random response anlysis because it is quite easy. I want to do that in hypermesh/optistruct. but the tutoril does not match with my requirement.

1. I have input in terms of acceleration as base excitation.

2. I have Frequency Input 20 to 2000 Hz with 100Hz increment.

3. I have a critical damping condition of 0.3.

4. I have PSD Input as follow

    20 Hz- 0.056

    100Hz - 0.28

     700Hz- 0.28

     2000 Hz- 0.0343


5. I want peak g rms and g rms at few nodes. I also want Von-Mises stress.


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Will you Please elaborate about cards like PARAM,G? and

How to specify the value of that?

can you please make an FEM file which is consistent with my inputs? So, It would be very helpful and I can co-relate easily. 


-Thank you

-Nirav Modi

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Hi Nirav Modi,


G is the Structural damping coefficient. Here G= 2* C/C0 (2 times the critical damping ratio)

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Could you please ask for the below output request



Acceleration (psdf) = all



This should write out both PSD and RMS responses to .h3d file which can import to HG to plot the curves.


In option set it to SID for Selected node set.See attached screenshot.Refer user guide for acceleration output.




Rahul R

G r.m.s



Nirav Modi likes this

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I want to do for Acceleration(PSDF)=All .


but I also want PSDF-acceleration  response curve for the base node and some selcted nodes. in a single graph for comparison .

In my friend's pc, using the same version of hypermesh, it doesn't show the option of Random(1) in acceleration and displacement ( in Global_Output_request ). It only shows for stress only. I also tried to change solver template but it was of no use. Please help me with that ! 


and your attachment Gr.m.s is not properly attached. I am unable to open it. 

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