Determining Hz from a modal anaylsis

[JOAKES24]

Hello there.

First post, just new to the forum. Was hoping someone could help out.

I have ran a modal analysis on a plane model in FEMAP, and also carried out experimental work to determine the mode shapes. I then ran the analysis in Abaqus 6.7-1 as I prefer it, and I know how to have more control over meshing. The problem I get is, the mode shapes correspond with FEMAP and my experimental work.

However the frequency is giving in cycles/time. I know cycles/second = Hz. What correlation does the cycles/time give? As the values output do not match either my FEMAP or experimental expectations. I have looked in the INP file but I cannot find any time to calculate it from.

Hoping someone can shed some light on how to convert from this cycles/time into Hz.

Thanks
Jonathan

 

[corus]

The units of the frequency will depend on the units you've used in the model. If you've used SI units of kg, m, seconds, Newtons then the frequency will be in cycles per second.

corus

 

[JOAKES24]

That was my initial thought. I have used SI units, and therefore it should have been cycles/second?

Thanks for your feedback

 

[corus]

A common mistake is to use mm instead of m when using SI units. If so your answers will be a factor of 30 out, roughly.

corus

 

[JOAKES24]

Thanks Corus, I thought that was the problem when you mentioned it. However I went into Abaqus, checked the units and the model was indeed set up for m, no mm. Stumped as to why this is not working out. I am sure its got to be a time problem, but I cannot see where the time is to be setup/.

thanks again

 

[abaqusn00b]

I'm having the same problem. I'm just learning ABAQUS. I've tried a couple of static problems and matched with hand calculations.
I'm just moving towards dynamic response and I've been stuck in modal analysis for some time now. The modal frequencies given by ABAQUS and that obtained from hand calculations does not match.
I use Step>Linear Perturbation>Frequency and use either Lanczos or sub-space solver.
I stick with the SI units and am very meticulous about it.

Please help, anyone?

 

[abaqusn00b]

I figured it out
There was a factor of about 6.28 by which the results from hand-calculations was greater than the result from ABAQUS. Turns out, ABAQUS gives the results in Hz and hand-calculations is in rad/sec.