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*MODAL DAMPING

Keyword type: step

This card is used within a step in which the *MODAL DYNAMIC or *STEADY STATE DYNAMICS procedure has been selected. There are two optional, mutually exclusive parameters: RAYLEIGH and MODAL=DIRECT (default).

If MODAL=DIRECT is selected the user can specify the viscous damping factor $ \zeta$ for each mode separately. This is the default. Direct damping is not allowed in combination with nonzero single point constraints.

If RAYLEIGH is selected Rayleigh damping is applied in a global way, i.e. the damping matrix $ \left [ C \right ]$ is taken to be a linear combination of the stiffness matrix $ \left [ K \right ]$ and the mass matrix $ \left [ M \right ]$:

$\displaystyle \left [ C \right ] = \alpha \left [ M \right ] + \beta \left [ K \right ].$ (209)

The coefficients apply to all modes. The corresponding viscous damping factor $ \zeta_j$ for mode j amounts to:

$\displaystyle \zeta_j=\frac{\alpha}{2 \omega_j}+ \frac{\beta \omega_j}{2}.$ (210)

Consequently, $ \alpha$ damps the low frequencies, $ \beta$ damps the high frequencies.

The *MODAL DAMPING keyword can be used in any step to redefine damping values defined in a previous step.


First line:

Second line if MODAL=DIRECT is selected (or, since this is default, if no additional parameter is entered):

Repeat this line if needed.

Second line if RAYLEIGH is selected:

Example:

*MODAL DAMPING,RAYLEIGH
,,0.,2.e-4

indicates that the damping matrix is obtained by multiplying the stiffness matrix with $ 2 \cdot 10^{-4}$


Example files: beamdy3, beamdy4, beamdy5, beamdy6.


next up previous contents
Next: *MODAL DYNAMIC Up: Input deck format Previous: *MATERIAL   Contents
guido dhondt 2012-10-06