Aqwa quadratic damping

[jmpape]

Hi everyone,

I use Ansys Aqwa to perform an hydrodynamic&stability analysis of a floating structure.

I need to input a quadratic damping matrix but i don't find where. I tried with 'additional damping matrix' but this matrix is frequency undependant, a quadratic damping matrix is.

Maybe someone could help me, i cant go further in my study because the RAO i will calculate would be wrong without that matrix !

Merci pour vos aides,

Jean-Maxime

[CWTeebs]

Are you talking about a non-linear drag coefficient? Or are you talking about quadratic transfer functions? Are you analyzing this in the frequency or time domain?

- CWTeebs

[jmpape]

I analyze in the frequency domain. The quadratic damping is : B(w), with w=2*pi*f (f=freq).

The quadratic damping terms are non linear. Linear damping terms are defined by B=0,5*ro*Cd*S*V².

I don't know if it deals with drag coefficient, it may be, what have you in mind ?

[CWTeebs]

Hi jmpape,

Thanks for clarifying what your problem is. You cannot directly specify a non-linear drag coefficient in the frequency domain. So, your options are to linearize your drag term using the "additional damping matrix" functionality in Workbench, but as you correctly stated, this matrix is frequency independent.

You can linearize the drag term such that the work done by the linear damping coefficient is the same work that would be done by the non-linear drag term, integrated over a complete cycle. I've linearized the drag term by calculating it at the RMS peak frequency of the structure.

Alternatively, you can simulate your structure in the time domain and supply the program the Morison drag matrix. Search for MDIN and MDSV cards in "AQWA Reference.chm"

EDIT: I don't feel comfortable with the careless mixing and matching of terms. Re-reading this thread, I realized one could deduce that the non-linear drag term you're talking about, and the damping term in the frequency domain, are somehow the same thing. There is actually a significant difference between the two, and I feel I should make that difference clear, in case others that are new to the subject chance upon this thread.

Frequency domain potential flow stipulates that the part of the solution that is out of phase with the solution (e.g. is imaginary) is the damping term. The part that is in phase, in front of the acceleration term, is added mass. Potential flow is inviscid and irrotational. The non-linear drag term is *not* from inviscid, irrotational flow, rather, higher order drag emerges from things like eddy shedding and viscosity.

It's important to understand the difference, and to know that there *is* a difference!

- CWTeebs

[jmpape]

Hello CW teebs,

Thank you for responding me. I understand the fact of using MDIN/MDSV card but unfortunaltly i must use Ansys AQWA for the analysis and not AQWA GS alone.

I choosed to solve the study with current drag coefficients in order to replace the quadratic damping coefficient. This is the matrix i use :

X Y Z RX RY RZ
X 0 0 0 0 0 0
Y 0 0 0 0 0 0
Z 0 0 627900kg/m 0 0
RX 0 0 0 5088000000 kg.m²/rad
RY 0 0 0 0 4897000000 kg.m²/rad
RZ 0 0 0 0 0 0

I will get the result in few hours, i keep in touch with you if its relevant or not.

[jmpape]

I finally used the additional damping frequency independant matrix to input quadratic damping. Someone from Ansys told me ' the whole of AQWA in to Workbench is not complete yet, and we are concentrating on those features that are most commonly used.'
it seems that quadratic damping is not commonly used...!

[khabiran]

Hi Jmpape
your privet message is banned. so open it or be in contact with khabiran@gmail.com