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Guidelines for damping schedule of flexbody?

The default Adams CRATIO for a flexbody is the following:

1% damping for all modes with frequency lower than 100.
10% damping for modes with frequency in the 100 to 1000 range.
100% critical damping for modes with frequency above 1000.

Lets say I run a flexbody with a 200Hz cutoff frequency in the EIGRL card. What would be the best way to adjust the default damping schedule to reflect my flexbody? Running with a flat X% damping percentage doesn't seem to run well as the residual vectors add a lot of high frequency noise to the sim.

0 Jesper Slattengren over 2 years ago

Turn off default damping ratio and specify your own function as a function of FXFREQ. For example:

STEP(FXFREQ, 50, 0.05, 200, 1)
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0 michael.w.clarke1 over 2 years ago

@Jesper Slattengren Is there any recommended criteria on selecting the frequency to begin to apply critical damping etc? Would love guide that maps the Nastran inputs for EIGRL cutoff etc and relates them back to what you should specify in the Adams CRATIO.
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0 Jesper Slattengren over 2 years ago

Not to my knowledge. Trial and error. But this is not my specialty either so someone else might chip in.
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0 Jan Sotnik over 2 years ago

Hello Michael,

say, for your NVH study you are interested in frequencies up to 6 kHz then apply modal damping of 0.02 (2% of critical damping) up to that frequency and then step up to 1.0 of modal damping (critical damping) over some frequency range. My tutor used to say, have at least 6 modes over that range if possible, to let solver have a smooth transition from low damping to critical damping.

STEP(FXFREQ, 6.0E3, 0.02, 1.0E3, 1.0)
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0 michael.w.clarke1 over 2 years ago

Thanks appreciate it!
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