Rayleigh damping coefficients

[vibract]

Dear members,
In our present task, we have to evaluate Rayleigh damping coefficients.
--> Could anyone suggest how they can be evaluated?
--> Do Rayleigh damping co-efficients vary with modes?
Suggestions are most welcome.
Regards

 

[GregLocock]

1) by curve fitting to the real data

2)No, they are a bodge.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

First obvious hit from google:

http://www.algor.com/news_pub/tech_white_papers/dynamic_analysis/default.asp

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[vibract]

Hello Greg locock,
Thanks for the reply. I couldn't understand the answer for my 2nd query.
As per the calculations which I peformed, I am with varying coefficients (varying with the modes). Which means,
mode1 has one set of coefficients and likewise for other modes.
Regards

 

[GregLocock]

Well then, you aren't using rayleigh coefficients, which by definition are 'constants' that are adjusted to give a good fit to observed data.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[spongebob007]

Rayleigh Damping:

D=1/2*(alpha*omega+beta/omega)

alpha and beta are the stifness and mass proportional damping constants. In order to determine these constants you need to solve two simultaneous equations. From experimental data you should have the damping ratio for two frequencies. The right way to do it is to use the lowest natural frequency and the highest frequency of interest and the correspinding damping ratios.

Then solve the following:

alpha=2*(D2*omega2-D1*omega1)/(omega2^2-omega1^2)
beta=2*omega1*omega2*(D1*omega2-D2*omega1)/(omega2^2-omega1^2)

 

[vibract]

Hello Spongebob,
I thank you once again for your response. In my task, I have to evaluate damping co-efficients. For this purpose, I did an experiment with the aim of extracting first 6modes and resp modal parameters.
All this is done to evaluate damping coefficients.
So, now I am with 6 equations, 2 constant unknowns ( alpha and beta). These 2 constants have to satisfy all the 6 equations. But in my case, I am with different values of alpha and beta. Can anyone suggest??

 

[spongebob007]

In evaluating the constants, you only use two modes: the lowest and the highest. Rayleigh damping is still an approximation.