Learning Forum

[r.abbasi]

Hi all,

I was comparing RAO and evaluate that in time domain with frequency domain. I confront with this question that, why when we consider a specific freq and in that freq consider a regular wave with T(regular)=Tp(irregular),and H(regular)=Hs(irregular), but the RAO will be different. maybe to understand my question is better to see attached picture:

[Shuangxing Du]

Not sure what H(regular)=Hs(irregular) means.

In your time domain simulation in regular wave, did you use the Airy wave or the second order Stokes wave theory and is there any other nonlinear damping applied? 

Is the structure as a free floating structure (without any constraint) in the time domain analysis? If yes, the coupling effects of other DOFs' large motions may affect the heave motion. 

From the curve of the heave RAO vs frequency, the interested frequency point  (1.2Hz) is close to the heave natural frequency. Near the natural frequency, the amplitudes of RAOs are sensitive to the damping applied. 

[r.abbasi]

Thank you for your response.

maybe I didn't ask my question clearly.

we have RAO curve in specific irregular wave with Hs and Tp, which calculated by FD of AQWA.
Now, I want to validate

this RAO curve with my experiments.

so,I have to do my tests in different wave frequencies with regular wave with amplitude=Hs/2 and T=Tp? 
if yes,so why the results will not match!

thank you

[Shuangxing Du]

No sure what are the definitions of Hs in regular wave and irregular waves in you cases. You also did not mention the wave direction. I have given several possibilities on why the results could be different in the previous reply.

 

[r.abbasi]

Thank you for your response. and other question is that why when I get RAO from Hydrodynamic Diffraction analysis , the RAO starts from 1 at freq=0 (which is correct), but when I get RAOs in Hydrodynamic Response in irregular wave, RAO at freq=0 is not 1!!! and also, the Natural frequency( I mean the peak of curve) are not align?

in HD part, what is the specification of wave?

[Shuangxing Du]

In Hydrodynamic Response in irregular wave, the motion RAOs are calculated from the lower frequency to the upper frequency of the wave spectrum. Outside this frequency range, the RAOs are set to be zero, as it is assumed that there is no the first order wave exciting force at the wave frequency outside this frequency range of the wave spectrum.  The lower and upper wave frequencies are listed in (your HR in irregular wave project)\FREQUENCY.LIS, in the format of

Please check in Hydrodynamic Response in irregular wave if there is any nonlinear drag being defined, which will be included in Hydrodynamic Response analysis if the linearized damping option is switched on. 

In Hydrodynamic Response in irregular wave, it is automatically set 0.5% of critical damping if the total damping is less than it, which may also make slight difference of the peak of curve near the resonant frequency.