Learning Forum

[Tianyuan Wang]

Dear all:

I would like build a coaxial double buoy in AQWA workbench, but I am a little confused about the lid method in AQWA.

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Q1:

In workbench, does the structural surface of the floating body on the SWL need to be preserved? I am not sure if the structural surfaces on the SWL are involved in the calculations.

This essentially relates to the principle of the internal lid method. If I turn on the internal lid method, does AQWA generate additional lid elements that are duplicated with the structural surface meshes on the SWL.

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Q2:

For a torus with a moon pool, assume that the structural surface on the SWL is deleted in the DM, does the internal lid method just generate the internal lid of the toroidal surface, or generate the lid including the toroidal surface and moon pool?

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Q3:

As for a coaxial double buoy, how could I generate a external lid of the narrow gap between inner and outer bodies? 

(1) Which type is right?
A. a lid attached to inner body

B. a lid attached to outer body

C. a separate lid

(2) Is there a clearance required between the lid and the other body?

(3) I found AQWA USER MANUAL highlights that lid normal should be pointing up (away from the fluid). Do all the lids have to meet this requirement? And this operation needs to be done in DM?

Looking forwards to hearing from you

Best regards,

Tianyuan😀

 

[Mike Pettit]

Hello Tianyuan,

To answer your questions:

1. Generally, it does not hurt to include the structural surface on the SWL.

If you are only running a Hydrodynamic Diffraction calculation, it does not make any difference whether the structural surface on SWL is included or not - Aqwa will consider it as non-diffracting, and ignore it for the radiation-diffraction calculation. The same can be said for any other parts of a vessel above the mean waterline. However, if you are also going to run a Hydrodynamic Response stability analysis or time domain analysis (without slow drift), then you should include the structural surface. For these analysis types, Aqwa will calculate the hydrostatic pressure over the instantaneous wetted surface, so it is more accurate to include all of the structural surfaces in this case.

The existence (or not) of the structural surface on the SWL does not affect the internal lid. Once Aqwa decides that the structural surface is non-diffracting, it does not matter if the internal lid elements are overlapping.

2. The automatic internal lid method will create lid elements inside the toroidal surface only. The moon pool will not be covered.

3. For the external lid surface, either A or B is right. You should have a small clearance between the structural surfaces and the external lid surface - any clearance is fine, as long as they are not touching (i.e. not sharing nodes in the mesh). The Aqwa Workbench editor will automatically correct the lid normal if necessary, so you don't need to worry about it. (Generally, you need to use a Surface Flip operation in DM if you do need to change the surface normals for any other reason).

I hope this helps!

Mike

 

[Tianyuan Wang]

 

Dear Mike

Thanks for your reply.

For the outer buoy, specifing the structural surface as the internal lid is the same as generate internal lid, am I right?

If a internal lid is created on the moon pool surface for the outer buoy, will this have an effect on the results?

Best regards,

Tianyuan😊

 

[Mike Pettit]

Hi Tianyuan,

You can specify the structural surface on the SWL as an internal lid, but you should modify the geometry so that there is a gap between the lid surface and the other structural surfaces (similar to the external lid).

This is not exactly equivalent to using the 'Generate Internal Lid' option - the latter roughly fits square elements into the cut waterplane area inside the buoy, while the former will use the elements created by the Mesh. Here is an example of automatic (left) vs manually-defined (right) internal lids (visualised using AqwaGS):

You should not use an internal lid over the moon pool, as this will overdamp the water surface - it will remove all vertical wave motion from inside the buoy, which is not realistic. If you need to, you should rather create an external lid, which will allow you to tune the damping frequency (via the Gap value) and amount (Damping Factor value).

Cheers, Mike

 

[Tianyuan Wang]

 

 

Dear Mike:

I recently conduct the simulation in AQWA-Workbench as you suggested. The external lid established in DM has a inner radius of 6.5 and a outer radius of 7.25. The actual gap between inner and outer bodies is 6.25-7.5, thus the Gap for External Lid is set to 1.25 m

The added mass and radiation damping coefficients of single platform and multi-body cases are shown below, with different Lid Dmaping Factors of 0.5 and 1 of external lid. The hydrodynamic coefficients are in the main diagonal (i,i), i = 1:24.

I’m wondering if the subsidiary objects affect the hydrodynamic coefficient of the main body (platform)? From the above figures, the subsidiary objects indeed change the hydrodynamic coefficients of the platform. Is it related to the reflected wave?

In addtion, It is difficult to determind the Lid Dmaping Factor. The Lid Dmaping Factor of 0.5 even induces the negative added mass cofficients in the surge and sway directions, while the Lid Dmaping Factor of 1 does not seem to be desirable either! 

Do you have any insights into this issue?

Looking forwards to hearing from you

Best regards,

Tianyuan

 

 

[Mike Pettit]

Hello,

The lid damping factor is an empirical value - generally we recommend that you adjust the damping factor so that the simulation results match the experimental data. That being said, damping factors of 0.5 and 1.0 are quite large - we would usually expect values in the range [0.01 - 0.1]. It may be useful to run the hydrodynamic analysis with the External Lid surfaces suppressed, to gain a better understanding of their effect on the solution.

The Hydrodynamic Diffraction Structure Selection allows you to collect structures into one or more Interacting Structure Groups. If all of the structures are grouped together (this is the default behaviour), then the radiation and diffraction effects of the subsidiaries will affect the hydrodynamic properties of the main platform, as you observe.

I hope this helps.

Cheers, Mike

[Tianyuan Wang]

Dear Mike

I understood the AQWA workflow. Thank you very much for your guidance.

Best regards,

Tianyuan