Learning Forum

[r.abbasi]

I am analyzing a cylinder in waves with AQWA and have 2 questions:

In the time domain, when I want to obtain the response of the buoy in heave (using a joint that only allows it one degree of freedom), I simply deactivate the other degrees of freedom. Is it accurate to say that the buoy is constrained to only heave in reality?

However, in the frequency domain, when calculating the Response Amplitude Operator (RAO), there is no deactivation option. Does this mean that our buoy has degrees of freedom in different directions?

What should I do to introduce a joint that allows our buoy to only have motion in the Z direction? Additionally, what types of joints should I use to incorporate a damper in the system, as shown in the attached photo

[Shuangxing Du]

As shown below, please use 8 fenders to restrict the motions except heave in the Hydrodynamic Response analysis. The fenders' stiffness should be large enough, for example, about 100 * omega(heave)**2 * m, where omega(heave) is the natural frequency (in rad/s) of heave, m is the mass of the structure. The frequency independent damping for heave can also be defined to model the damping of the damper.  These fenders will have no effect on the RAOs obtained by the Hydrodynamic Diffraction analysis.  But you can run the Hydrodynamic Response -> Freuqncy Statistic analysis to have the RAOs including these fenders's effect.

[r.abbasi]

Many thanks for your response.

in this method,how can I find the radiation damping just in heave?when in HD consider all degree of freedom and no damper!are those Radiation dampings correct?

[Shuangxing Du]

The radiation damping coefficients are determined by the radiation waves due to the unit structure's motions. In the Hydrodynamic Diffraction analysis, the hydrodynamic damping is of 6x6 matrix form.  In your model, there is only heave motion in the time domain analysis, the radiation damping force will be contributed by the heave motion only, all the other damping coefficient elements, except heave-heave term, will have no effect as the motions in other DOFs are zero (or nearly zero due to fenders).

[r.abbasi]

Thanks for your explanation. I did but I have some problem:

there should be 8 point near the structure and 8 point on that,and connectivity and setting like bellow?

also I cand see stiffness for that

[Shuangxing Du]

Coefficient A is the stiffness of the fender as you can set the fenders' stiffness is linear.

[r.abbasi]

Thank you.by this wY I can just deactive the motion in one direction of heave!

but what about a appropriate PTO damping which we need in our system?

the damping for heave should be (N/m/s).how can I model the damping in heave direction?

[Shuangxing Du]

The value of the appropriate PTO damping depends on your system. Normally it could be estimated by the output power, power=damping*velocity^2.

To define the frequency independent damping of heave-heave in Aqwa-Workbench Editor, see below

[r.abbasi]

Thanks for your explanation.

But it would be like an additional damping which comes from a water,but we need a PTO damping like a previous picture, outside of the system.

And the second question is that, how does AQWA calculate radiation damping?because radiation damping is not in the presence of incident waves, so what is the encounter frequency in Radiation damping graph?

[Shuangxing Du]

The frequency independent damping could come from water or any other sources, such as PTO.

Please read the Aqwa Theory manual, 4.1.1. General Formula in Zero Forward Speed Case, for the hydrodynamic damping calculation. If the structure in travelling with a constant speed, the encounter frequency, which is the function of the incident wave frequency, incident wave direction and the forward speed, will be calculated (see Aqwa theory manual, 4.9. Radiation Wave Properties at Negative Encounter Frequency). If the forward speed is zero, the encounter frequency will be the same as the incident wave frequency.  In the time domain analysis, the hydrodynamic damping is used in the radiation force calculation (see 13.1. Radiation Force by Convolution Integration).