Learning Forum

[Kursat Guner]

Hello all,
I have some problems with AQWA.

I have modelled a cylinder, cube and spherical object. The sphere and cube work fine but there are some problems with the cylinder. I am using 2023 R2.

On time domain analysis, the cylinder's critical damping is around %0.07. I perform free decay tests. If I test the cylinder, it oscillates forever. If I perform with the cube and sphere, it is fine. I am able to validate the sphere's results with an experiment, that fits.

What is the problem with the cylinder, what should I check?

[Yu Bi]

Hi, you can have a check of the stiffness of the cylinder in the oscillating direction to see if it is too small. Besides, if the gravity center is too high to make the hydrostatic righting moment negative, the cylinder also cannot return to its original position. You can add in additional damping manually to cover the viscous effect if you think the current radiation damping is not enough. Hope these give you some hits to fix the problem.

[Kursat Guner]

Thank you for your reply.

I see but problem is I should not try to fix the problem. Is there a general problem about it? Because I am not able to add additional damping for all type of structures I will try. Some experiments lack of informations to fill all datas... 

[Yu Bi]

Hi, Aqwa uses potential theory and calculates the first-order radiation damping automatically. For other types of damping, Aqwa offers non-linear roll damping and frequency-independent additional damping, which require the user to define the damping coefficient. Those damping coefficients can be obtained from model tests or CFD analyses. If you don't have any data to indicate how much the damping should be and the damping effect is also obvious and should not be neglected, then Aqwa might not be the right tool for you. Instead, you can use Aqwa Fluid to do CFD analysis, however, it will be much more time-consuming.