Learning Forum

[Sathya Shanka Vasuki]

Hi,

I have some update with this simulation. I ran both hydrodynamic diffraction (frequency domain) and hydrodynamic response (Time domain) analysis.

Note: The reference article used in this thread has also made use of Ansys AQWA and has provided the frequency dependent RAOs as a result of both frequency domain and time domain analysis and I am trying to validate the same (doi: https://doi.org/10.1016/j.oceaneng.2022.112754). 

Hydrodynamic Diffraction (Frequency domain) analysis:

I did the following in the diffraction analysis.

 

• I removed the connection stiffness matrix from the HD simulation
• I added an additional damping matrix in the surge and the pitch DoF (as shown in the image below) as the paper used the PQ method to calculate the viscous damping matrix which is not considered in AQWA.

 

The result of the Diffraction analysis is as follows which now seems to be matching decently well:

Hydrodynamic Response (Time domain) analysis:

I used the following input parameters to evaluate the time response of this simulation. I ran 5 time domain simulations with regular, 2nd order stokes equation with different wave frequencies and amplitudes as follows:

Freq (rad/sec)	Amplitude (m)
0.33	1.12
0.415	1.47
0.522	0.98
0.628	0.84
0.785	0.84

As a result, I extracted the RAOs from the time domain analysis as follows:

 

• I first plotted results by going through :– solution –> insert result –> structure motions –> strucutre position –> RAO-based response
• Then Calculated the RAO as the ratio of pitch amplitude from AQWA to the wave amplitude as shown in the image below

 

I did the same for all the frequencies for pitch, heave and surge and I compared the results with the RAOs I obtained using the diffraction analysis. From the plots below the variation of surge is significant compared to heave and pitch.

Based on these results I have some questions which needs some clarification:

1.  When the results from the diffraction analysis is matching the reference why is the results from the hydrodynamic response analysis not matching?
2. Why I am getting huge differences in the surge motions in the HR analysis? 
3. What am I missing in the hydrodynamic response analysis?
4. Also in the hydrodynamic response analysis, the RAO based response is in [deg] although its RAO, should it be [deg/m]?
5. What do the different responses mean – RAO-Based Response, Low-Frequency Based Response, Wave Frequency Based Response, Actual Response?
6. Also, in the end how do I know if my simulation is correct? because the RAOs match in the HD analysis but they dont in HR analysis, So I dont know what is wrong.

I kindly please ask you to answer all of the above questions because I have spent a lot of time trying to solve this problem and not able to validate it. Please help me out with this.

Thanks