Learning Forum

[aavash paudel]

Hello all, I wanted to find damping ratio from my experimental data so that i could use it in the simulation. I used half bandwidth method to find the Zeta and then used raleign daping to find mass and stiffness coefficient. However i have two peaks and none of the method above is yielding correct result where both the peaks are on par with the experimental data.

I would like to know how to calculate damping from FRF so that we can get the exact result on the Ansys for both the peaks with less or equal to 10% error. I have a thesis due this semester so any help would be highly appreciated

[Erik Kostson]

 

 

Hi

 

One way to measure damping could be the 3dB method – there are some suggestions on this method if you search on the internet, e.g.,:

 

https://www.google.com/search?q=find+damping+ratio+from+test+data&rlz=1C1GCEA_enDE1032DE1032&oq=find+damping+ratio+from+test+data&aqs=chrome..69i57j33i160.9959j0j15&sourceid=chrome&ie=UTF-8

 

To learn how to run a harmonic response analysis in WB/Mechanical please see here:

https://courses.ansys.com/index.php/courses/harmonic-response-analysis-in-ansys-mechanical/

https://courses.ansys.com/index.php/courses/damping-effects/

 

All the best

Erik

 

 

[Bill Bulat]

Further to what Erik said, it may be helpful to review this Help section:Please note the tables in this section. There are different ways to define damping depending on whether your harmonic response analysis is FULL or MSUP (mode superposition). When you say calculated results are not "on par" with measured test data, do you mean both the natural frequencies and the response amplitudes differ, or just one or the other? If the natural frequencies are not in agreement, I would guess that in the model, some aspect of the way the structure is constrained is inconsistent with the way it is physically constrained in test. If just the response amplitudes differ, I might start by defining only Rayleigh stiffness (beta) damping using the equation below to determine beta from the damping ratio (don't define mass (alpha) damping):It occurs to me you may want to define material dependent damping rather than global damping (the latter applies to the entire structure rather than just parts of it). Another possibility is that the damping exists in propeller shaft bearings and/or packing glands or similar structures used to prevent water intrusion. Maybe a joint with damping may be defined to support the shaft in your model. It may also help to consider damping due to fluid present at the propeller surfaces (I confess I don't have an intuitive feel for how significant this affect might be).