viscous damping in NE/Nastran? 1

[cannett]

Hi,

I'm doing a ball drop simulation in NE/Nastran and am confused with how to handle the damping. Is the parameter 2*C/Co when defining the material the viscous damping coefficient? If it is, the tutorial supplied with the software says to enter the frequency of the first mode for the w4 parameter. This will cause viscous damping at that frequency right? Don't you really want damping at all frequencies or whatever frequency the object vibrates at after being hit by the ball?

Also, after reading some of the threads here I realize it is quite hard to obtain a damping coefficient. If I were to use structural damping how do you come up with the damping value if I don't have a physical model?

I know that you guys have a lot of knowledge and any help with these issues would be greatly greatly appreciated! Or if you could point me where to find this info that would be great!

Thanks

Chad

 

[pjhype]

Chad,

In linear finite element analyses, there are three types of damping: (1) structural damping, which creates damping forces that are constant w.r.t. frequency, (2) viscous damping, which creates damping forces that increase linearly w.r.t. frequency, and (3) modal damping, which "artificially" applies damping to specific modes (used in modal dynamic analysis solutions). There are different situations where you would use the different types of damping, and no one approach is better than the other. If you are entering a structural damping coefficient on your material card, the damping will be independent of frequency. It seems like you are attempting mix the structural damping with modal damping. If you are using a direct transient solution method, I would recommend that you implement structural damping.

pj

 

[brad]

pj--clarification--don't you instead mean "structural damping, which creates forces that are INDEPENDENT OF frequency"?

Based on your other comments (and my understanding of structural damping--"param,g" or GE" ), I think you may have meant this rather than "constant w.r.t frequency"? Or am I confused?

Best regards,
Brad

 

[pjhype]

Brad,

I don't understand your distinction between "independent of frequency" and "constant w.r.t. frequency". Perhaps you could clarify.

The damping force due to structural damping is equal to:

i*G*k*u

where i=sqrt(-1)
G=structural damping coefficient
k=stiffness
u=displacement

Structural damping is therefore not a function of the forcing frequency.

On the other hand, the damping force due to viscous damping is equal to:

b*v = i*b*w*u

where i=sqrt(-1)
b=viscous damping coefficient
v=velocity
w=omega (radian frequency)
u=displacement

Therefore, viscous damping will increase linearly w.r.t. forcing frequency.

pj

 

[cannett]

thanks pj for your help!

If I was to go ahead and use the structural damping does anyone know where to find these values! They are extremely hard to find. Also, Brad you were mentioning the structural damping parameters param, G and GE. Are these the same?
If you enter a structural damping parameter I believe you have to enter a frequency for W3, or W4...why would this be? Isn't structural damping just = i*G*k*u as PJ mentioned.

Any insight would be great!

I love this site. Thanks again for the help!