Following may help:

1. Consider a linear system say  e= L*didt +R * i

2. let e= vm*sin(wt)  and  z= sqrt((wL)^2 +R^2)    thz = atan (wL/R)

3. The response i(t)= K*e^-L/R *t   + vm/z *sin(wt -thz) that is natural + steady state response

4. When excited by e we must wait for transient to die down to access the steady state ie at least 5 or 6 times L/R time constant

5 Note if we take time wt0 = thz we obtain ss right away.

6. Now if we take FT of 3 clearly it will not be equal FS (ie steady state ) except in case  5 holds .

For the  case given in question  we may have a more complex circuit of parallel and series configuration of spring mass system (linear assumed), still we have natural and steady state  response as above. Note however we can obtain the linear parameters L and R by an impulse response to a delta function or maybe a step function ,leading to system identification . We can then use this to obtain steady state harmonic response.

This might help explain your result.

Good Luck

Cheers

Thank you all for the answers.

Q Narasim ramesh :  I am not sure I completely followed your answer. Could you please explain it more? Or some reference material woudl help..

@ Ibrahim                 : I made sure I have the same constant damping ratio in both the analyses. I am more interested in knowing if the approach is correct?

@ Hatem Hadi         :  As mentioned in the question, I did the FT of the results of the transient analysis. I am not sure why I have to do FFT of the 2 load cases as you suggested (1st way). It would be great if you could please elaborate?

Regards,

Shree

Hello Shreeraman,

I am facing a problem similar to yours. My effort is to compare the results from my pre-stressed harmonic analysis of a cable vibration with the results obtained from nonlinear transient analysis.

My load in the transient case is a sinusoidal force applied at the same frequency as the harmonic analysis case. Theory states that if we wait for the transient simulation to reach SS, it should match the prestressed harmonic response results.

I have turned on large deflection in transient, and I am aware that I am attempting to compare a linear sim with a nonlinear sim. However, if the forced perturbation wrt equilibrium is small, the results should be comparable. I am nevertheless not getting a good match in ANSYS.

Note that I am applying forcing on a cable already sagging (large deformation) under self weight. In the harmonic case, ANSYS imports the stresses and conducts a pre-stressed analysis using the updated stiffness (hence linear). In the transient case, ANSYS captures the entire nonlinear results including the sag.

You said your doubt got solved. If so, would you please explain how it got solved and help provide me some insight into my problem.

Thank you,

Roshan

Hi Roshan,

If I understood it correctly,

I don't know if it helps, but my thoughts are

• 'However, if the forced perturbation wrt equilibrium is small, the results should be comparable.' - This is what I would expect as well (at least theoretically).

-- Check, double check and make sure damping is defined coherently in both the analyses.

-- Make sure your analysis is converged to the correct (steady state) solution. Cannot stress this enough.

• 'Note that I am applying forcing on a cable already sagging (large deformation) under self weight' - This I don't get. Now you say, you have large deformations and then you are bound to get different results.

• 'My effort is to compare the results from my pre-stressed harmonic analysis of a cable vibration with the results obtained from nonlinear transient analysis.' - I don't get this as well. For the sake of comparison, either both of them have to be pre-stressed or both should not. Corrections are welcome.

• I would suggest to verify the procedure with a simple linear model and get further with pre-stressed once that is done.

Just a tip. It is nicer to post a new question (as opposed to answering) to get better ideas from others as well

Hi Shreeraman Swamynathan

Did you finally solve this issue? I have the same issue as I cannot match the natural frequencies of the same model with transient frequencies? would you please advice if you could solve this?

Ali Ghiasi , Are you doing a FT of the transient results to compare with harmonic or what difference do you see? Could you be more specific in explaining what you are trying to achieve and also maybe post a new question. That way you can expect more answers.

Shreeraman Swamynathan

Hi Sheeraman,

My issue is firstly I do the modal analysis of a simple 2D beam in ANSYS. The modal analysis gives me some natural frequencies say 6# between 0 and 50HZ. Then, I do the transient analysis using a load at a node and that gives me the time domain data. I then do the signal processing using fft to obtain the frequency-domain data. I also calculate the SVDs. The problem is only 4# of the natural frequencies are exactly the same as modal and always fft gives me some other peaks which are different from modal analysis output. Theoretically, these should be exactly the same peaks, however I have tested several complex and simple models in ANSYS and the same issue exist. I change transient time steps and loadings, still the same issue exist in all models. Do you know what the problem is here? why we cannot match 100% the data from modal with the transient analysis?

Thanks,

Ali