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FRA analysis

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Guest

hello again,

we made a FRA for a system (attached picture) with enforced displacement of 1mm. The first plot shows the displacement of the mass without damping. The results are the same as we expected. The second plot shows the results for the same system with damping. we did not expect these results and can not understand them. we expected the maximum displacement at the eigenfrequency. is the second plot right and how can we understand it?

Thanks

We uploaded our model to your dropbox (LTC_FRA)
kv817-wegerr_ged.jpg
o7948-FRA_undamped.jpg
7n8gj-FRA_damped.jpg

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Guest

Hi,


with no damping the maximum displacement is expected at or near the natural frequency of the component, this is what the first graph says.


but this is the not case in when you have damping.


Please run the same analysis with Structural damping instead of viscous damping and you will notice, the peaks reduces as the damping increases.


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Guest

we changed the viscous damping to a structural damping with a value of 0.1768, and we got this plot. We got maximum displacement (5.7478mm) at the eigenfrequency 1.125 s^(-1). We expected the maximum at a lower frequency, because of damping. Our calculated displacement is only 3.037mm.

Why are there these differences?

Thanks
4f614-FRA_damped_structural.jpg

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Guest

Hi,

what is the 0.1768 value mean? is it the damping ratio (C/Co)?
what are the equations used to calculate?

if you are using global structural damping parameter, then you should multiply the damping ratio with 2 and apply the same value in Control Cards-->PARAM-->G.

z9ozz-Capture.JPG

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Guest

Hi,

the value of 0.1768 is the damping ratio (C/Co). We multiplied it with 2, changed it in HyperWorks and got the same plot as you. Do you know why we have to multiply with 2? You can see our calculation in the picture. For the frequence 1.125 s^(-1) we calculate the same value as Radioss.

Our problem is, that if you calculate a maximum of V, it has to be at a frequence of 1.093 s^(-1). You can see the calculation in the second picture. Are our thoughts correct?

Thanks a lot

2g87d-20140423_092338.jpg
hc18g-20140423_100935.jpg

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Guest

 think you are calculating the Transmissiblity, instead of amplitude.


can you please check? there is some reading available online for forced vibration with damping.


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Guest

yes, but we have an enforced displacement of 1mm. So the displacement and the transmissibility have the same value. Is it right?


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Guest

Hi,


your formula is for an excitation by a force. We excite the system by an enforced displacement of 1mm. So we cant use your formula.


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Guest

Hi,


Damping will affect the amplitude but not the frequency. you can with your model with different damping values. you will notice that the amplitude value changes but no frequency change. Frequency shift can be possible during any preload or dynamic+static superimpose.


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As far as i know there is no direct way to output transmissibility from OptiStruct/HyperGraph. We have shared some scripts id from scripts exchange for similar request in other post.I will check internally & let you know if we have any alternate solution to output same.

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Guest

Hi Ajay,

The h3d should normally run in HyperView. Which HyperWorks version are you using?.

Else you can try running the model file shared above.

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Dear Mr. Rahul

I am able to get FRF plot but for force transmissibility I apply force via DAREA but I do not know how to get output force

(I usd GPFORCE but I got nothing)

can you help me?

 

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We don't have a direct output for TL. But, certainly for radiated power (ERP).Since TL is a logarithmic measure of Incident power to the radiated power, you can measure/output the radiated power from OptiStruct and then manually calculate the TL.I have attached a sample tutorial which walks you through the steps to output ERP using OptiStruct.

For ERP refer online help for detailed information.

 

 

OS-T 1380 Computation of Equivalent Radiated Power.pdf

cover.fem

durukanbdilek likes this

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thank you Mr. RAHUL solve my problem. and then I obtained acc(out)/acc(in) transmissibility curve depend on drive frequency

but now, my problem is optimize this curve

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