Learning Forum

[ga83sas]

Hello everyone,

I would like to run a Full Transient simulation with a constant damping ratio.

In the analysis settings, only Rayleigh damping is offered in terms of [Betad] and [Alphad] commands, which are computed from two (main) frequencies of the system. This damping declaration is not useful for my study case, as it originates underdamped and overdamped regions.

Structural damping [DMPSTR] (or [DMPS] for multiple materials), as well as the Damping coefficient ratio [DMPR], can also be considered by the solver by specifying the [DMPRFreq] command. However, once again these are depending on the dominant frequency of the system.

Could anyone make the implementation of this constant damping ratio clear to me? I think I am missing something here, I would expect this declaration to be straightforward, such as the case of the Transient Analysis with Modal Superposition, where a constant damping coefficient [DMPRAT] can be chosen directly in the damping controls.

Thank you in advance. I would appreciate any input regarding this matter.
Best,

[sk_cheah]

I'm curious, how do you write an equation for damping ratio for something other than single degree of freedom (i.e. with mass and stiffness matrix)?

Kind regards,
Jason

[April Wang]

 Hi ga83sas,

 

There two source of damping for Full harmonic and transient analysis, Rayleigh damping (alpha and beta) and Structural damping (g). Below is a simplifed equation for damping matrix in full analysis.

There's no constant damping ratio in above equation.

Since R18.0, Constant damping ratio (DMPRAT) can only be used for Mode-superposition harmonic/transient analysis. There's no such definition of constant damping ratio in Full analysis. 

For Full harmonic/transient analysis, one should use DMPSTR and DMPS, which basically defines g. 

 

Regards,

April