frequency

[ibry]

Hello everyone,

I want to use rayleigh damping, but I don't know the value of frequency (omega) i and j that should be known to calculate a0 and a1 in the formula C=a0*M+a1*K.
Could I obtain the frequency result by adding the command in the input file?
Thank you.

[Prof. R.L. Taylor]

The specification for a0 and a1 can be input as solution commands in two forms

1.  RAYLeigh,, a0, a1  where you compute the values

2.  RAYLeigh FREQuency zeta omega1 omega2  where omega1 to omega2 is the range you want to approximate and zeta the damping ratio.

a1 = 2*zeta/omega1 + omega2

a0 = a1*omega1*omega2

[ibry]

Thank you for your reply, Professor.
If I don't know the values of both omega1 and omega2, how could I calculate a0 and a1 respectively?

[Prof. R.L. Taylor]

Why not compute the normal modes for the problem?  That should give you a good estimate of the frequencies involved in the solution.  Read some in a book to see about uses of Rayleigh damping (e.g., Chopra: Dynamics of Structures)