PSD Plot Data Interpolation

[mark.solway2NMMC]

I'm trying to run a Random Vibration analysis. The PSD input is plotted on a log scale with 4 break points, I've input the table and enabled log X and log Y data interpolation. However when I check the PSD (acceleration) results at the node where the excitation occurs, the interpolation is incorrect, the slope on the plot is curved. See attached images.

I could fix this by interpolating lots of points manually but I would expect the tool to do this. Am I doing something wrong or is this a known issue?

[John_Holtz]

Hi @mark.solway2NMMC 

Thanks for the information. Can you confirm that the result that you show in the results plot is for "Start Set > PSD 1" through "End Set > PSD n". Correct?

Next, try creating the results plot using Log X but not Log Y. (That is, create a semi-log plot. Log of X but linear on Y) Does that graph look like the input? In my test, it looks like the "log-log" input is being interpreted as "log-linear".

[John_Holtz]

Hi,

I found the problem. The Nastran file is not written correctly. The two "LOG" texts are in the wrong column becaues the TABRND1 command is using 2 columns for each entry (the wide field format, as indicated by the * on the command name) instead of 1 column for each entry.

You can edit the TABRND1 line in the Nastran File tab and move the "LOG" texts to the proper columns.

Correction needed for PSD input that does not use Linear interpolation.

[mark.solway2NMMC]

Yes, if I amend the TABRND1 command in the Nastran file as you have detailed the data interpolation works as expected.

If I change anything in the 'dynamics setup' window I have to go back and correct the table as it gets reset, but at least I can make it work with some manual intervention. Will this get put right with the next release? I'm currently running Version 15.0.0.187

Many thanks for your help.

Mark