Dear @Yingxin.Lv,
The structural damping matrix of the tower as used by ElastoDyn is explained in the forum topic I linked to above: Natural frequency and damping ratio calculation. That is, if K_ij is the tower elastic stiffness matrix, the tower structural damping matrix C_ij is:
C_ij = K_ij * beta_j
where
beta_j = zeta_j / ( pi * f_j )
with
f_j = 1 / ( 2 * pi ) * SQRT( K_jj / M_jj )
Carrying out the math to eliminate beta_j and f_j separately for the tower-fore aft (TFA) and tower side-side (TSS) components:
CTFA_ij = KTFA_ij * 0.01 * TwrFADmp(j) * 2 * SQRT( MTFA_jj / KTFA_jj )
CTSS_ij = KTSS_ij * 0.01 * TwrSSDmp(j) * 2 * SQRT( MTSS_jj / KTSS_jj )
So, your math, is close, but not exact. Just a few clarifying points:
- The tower in ElastoDyn has 4 DOFs, not 20, the 1st and 2nd TFA and 1st and 2nd TSS modes.
- If the tower elastic stiffness matrices have off-diagonal terms (nonzero terms for i /= j), so, will the tower structural damping matrices, as implied by the math above.
- I’m not sure I understand your point about axial load.
For the blades, the process is similar, but the effect of structural pretwist must be accounted for, which coupled in-plane and output-of-plane bending, as discussed in other forum topics.
Best regards,