Differentiating output request between inertance, mobility and receptance

Altair Forum User

Hello,

 

I am using Optistruct to numerically calculate some FRFs. I have test data to validate my numerical results. Measured test data is an FRF data in the form of Inertance (acceleration/force). I see that there is a big gap in frequency components' amplitudes between the measured data and numerically calculated data, though calculated and measured natural frequencies align very well.

 

I have followed the modal frequency response tutorial and nowhere in the tutorial is it shown that we're specifically selecting the type of FRF we're wanting (e.g. inertance or mobility or receptance). GLOBAL_OUTPUT_REQUEST is used with DISPLACEMENT enabled and that makes me think that, what I am calculating in Optistruct is Receptance and not Inertance. I tried to request ACCELERATION instead of DISPLACEMENT, but ACCELERATION doesn't let me select the output node set which I used for DISPLACEMENT.

 

How can I make sure in Optistruct that the numerical data I'm getting is inertance and not mobility (velocity/force) or receptance (displacement/force)?

 

Best regards

Altair Forum User

Altair Forum User said:

but ACCELERATION doesn't let me select the output node set which I used for DISPLACEMENT.

HI,

 

DId you provide the specific node in the SID for acceleration?

 

Could you check if you can plot using HyperGraph?

 

You can output HGFREQ which writes a session file with plots that can be viewed in HyperGraph.

 

 

Altair Forum User

Hello Prakash,

 

Thank you for your response. I have a node set for response calculation and I am able to select this node set for the SID entry in DISPLACEMENT. However, I am unable to select this same node set for the SID entry in ACCELERATION. When I enable ACCELERATION and click on Option -> SID, the window that pops up showing available node sets for selection is empty. So it doesn't let me select that node set for ACCELERATION.

 

I'm outputting using HGFREQ and I am able to solve the model and plot the results in HyperGraph if I use DISPLACEMENT. However, since it doesn't let me select the same response node set in ACCELERATION, I'm unable to run an analysis using ACCELERATION. Thank you.

 

Update: I have just noticed that I can't select any node sets in DISPLACEMENT too. This is happening in version 2017.2. Yesterday I was able to select node sets in version 2017. I will check back with my 2017 build as well as on different machines with 2017.2 build and update my answer.

Altair Forum User

Hi,

 

I am using 2017.2 and I am able to select Node set for SID both displacement and acceleration. 

 

Could you please clear the setting files and check if that helps?

 

Altair Forum User

Hello Prakash,

 

I have tried as you said, the problem still exists. On the machine with 2017 build, there are no problems, everything works both with DISPLACEMENT and ACCELERATION. However in 2017.2, I am unable to select response node sets for DISPLACEMENT and ACCELERATION entries. I have tried two machines with 2017.2 build installed, the problem was seen on both machines. Thank you.

Altair Forum User

  HI,

 

Is it specific to your model?

 

Can you check with another model on the same machine with the same version?