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Water on top of Rigid Body during Decay Test

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Old   June 9, 2018, 04:06
Default Water on top of Rigid Body during Decay Test
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Hello everybody,

I have some struggle with my CFD-Decay test. Like you can see in the pictures, there is more and more water on top of my rigid body (it's a box 60x30x15 cm). The natural period of the body is 0.9 s and in still water the freeboard is 5 cm. Only heave motion (translatoric y axis) is activated. There is no overtopping, because I just lifted the body 1 cm over its still water level.

The pictures ar here: https://imgur.com/a/cg0eKJQ

Uploading as attachment didn't work, because I have no permission?!

Has anybody an idea how the water is getting on top of the RB?

Thanks in advance,

mavo
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Old   June 9, 2018, 06:51
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Glenn Horrocks
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* Mesh too coarse
* Wrong advection scheme on volume fraction
* Convergence not tight enough
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Old   June 12, 2018, 05:18
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Hello Glenn,

thanks for the reply. The Box is meshed with an Inflation-Layer with a thickness of first layer =0.4 mm and 25 layers in total. The boundary walls of the box are meshed with 4 mm and the maximum cell size is 2cm

The advection scheme is high resolution and the time step size is 0.01s. Convergence is reached after ~3 iterations (max coeff. loops = 15). So, in my opinion, the mesh should be fine enough, as well as the time step size. But I'm not an expert

I still have absolut no idea how the water is getting on top. I#M glad for any help, ideas or suggestions
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Old   June 12, 2018, 18:52
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Unless you have done a mesh sensitivity study and shown your mesh is adequate you should assume it is not adequate. Just because the mesh looks sort-of fine is not acceptable.

You should also check your convergence tolerance. Try a tighter convergence tolerance and see if it helps.

The reason for the water on the top will be one or more of the reasons I stated before. So do checks on those parameters and work it out. And do not assume that because your settings look sort-of OK that is OK. Until you do a proper check using a sensitivity study you are just guessing.
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Old   June 13, 2018, 04:16
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Ok, thanks for you help, I'll check that.
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