## General Mechanical

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#### Verification of viscous damping through inverse FRF

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• HA
Subscriber

I run a simulation in which there is 2 spring (Combin14) with viscous damping. Then, i got velocity FRF. To verify the viscous damping value that i define, i took the inverse of velocity FRF. I expected that real part of inverse FRF equals to damping value that i define.But results are not the same.

How ansys explictly calculate the imaginary part of FRF function? The explanations in ANSYS Help are not enough.

• Ashish Kumar
Forum Moderator

Hi,

What you noted can be true. Please refer to the following note from Ansys help:

Note:  The FRAC calculator feature is removed from the current release. The methodology adopted for FRAC calculation in the previous release has some drawbacks as per research literature. Work on implementing improved methodologies is in progress and this feature will be available again in a future release.

2.5. FRF Calculator (ansys.com)

Regards,

Ashish Khemka

• HA
Subscriber

• Ashish Kumar
Forum Moderator

Hi,

You can compute the damping ratio from the frequency response function (FRF) using the amplification factor (Q), which is a measure of how undamped a system is. When analyzing a frequency response plot with multiple peaks at natural frequencies, you can determine the damping ratio at each peak:

1. Extract the natural frequency (f_n) where the curve has the maximum amplitude (A_max).
2. Compute the value of A_max divided by the square root of 2.
3. At this amplitude value, you will find two intersections with the curve, and extract the respective frequencies (f_1 and f_2).
4. Compute the amplification factor (Q) using the equation: Q = (f_n) / (f_2-f_1).
5. Calculate the damping ratio using the formula: damping ratio = 1 / (2*Q).

Are you using this approach?

Regards,

Ashish Khemka