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Old   September 3, 2012, 00:52
Default Problem with ship motion analysis?
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shameem
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Hi
I am simulating ship motions (3DOF, but concentrating on Roll studies). I am getting comparable results around the natural frequency of the ship model (compared with experiment) but after this the roll response is not matching with the experiment. The CFD results are decreasing with only a very small decrement after the natural frequency (let us say if the natural period is 1.6 sec, in 1.8 sec also am getting almost same result. But this was not the case with experiment) I tried with different models still the same problem persists. Can anyone suggest what may be the problem?
Thanks and regards!
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Old   September 3, 2012, 04:36
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Maybe you should describe your problem more such as what have you done in your simulation (correct boundary condition, model, etc), and is that your own experiment or some published result?
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Old   September 3, 2012, 06:19
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Velocity inlet on all 5 sides and pressure outlet at end. Now I am trying with our own experiment. I tried with 3 models (experiment+CFD) and also i tried with one published paper. In all cases I experienced the same problem.
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Old   September 3, 2012, 06:25
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And also I am using DFBI morphing.
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Old   September 6, 2012, 15:54
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There's much more information needed. What about domain sizes, time step size, mesh quality, does the sink match experimental data (might be slightly different due to meshing etc.), did you apply the correct center of gravity, are the time steps fully converged, how many 6DOF iterations do you allow...
I think it's best to post some pictures of your domain, the mesh and some plots like residuals, 6DOF orientation and / or translation over iterations (not over time steps!) etc. That will help us to help you!
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Old   September 7, 2012, 01:47
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Abdul, thanks for the respond.
Domain: size calculated according to Wave length. My model natural period is around 1.6 sec (Wave length = 4m)
and I made the domain with respect the wave length 4.5m, i.e., corresponding to 1.7 sec and made it constant for all the cases till 1.8 sec wave period.
Domain size 2L(9m upstream) and 3L(13.5m downstream) respectively for beam sea condition.
Mesh: Water surface has been created with 20 cells on wave height, 24 cells on wave length (w.r.t 4.5 m WL). Kindly find the attached image.
Sink matches the experimental data. But there is a little difference after the natural period as I said earlier.
I have given the exact CG which we calculated using experiment. I tried with changing the VCG in an another model to know its response.
Time step = 0.01
In Stopping criteria, Inner iterations given was only 5
In 6DOF solver the maximum number of iterations was default value, i.e., 10
The plot I am having is with time step which is attached. This is for near natural freq.
Again thanks and regards
Attached Images
File Type: jpg naturalfreq.jpg (42.7 KB, 54 views)
File Type: jpg Residuals.jpg (55.5 KB, 53 views)
File Type: jpg Mesh Scene.jpg (79.8 KB, 69 views)
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Old   November 20, 2013, 07:55
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Dear Shameen,

I am dealing with a similar kind of problem. DFBI of a brage. Is it possible to share your model? I just want to see the settings. I tried to run a 2-d problem of a cylinder in waves and seems to be working fine. when I do a 3-D of the barge it is behaving very erratically specially the roll. After a few seconds the body capsizes and simulation blows up.

Is it possible for you to share your model please?
My email is arun7328@gmail.com

Thanks
Arun
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Old   November 25, 2013, 16:33
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Have you ever checked your wave time history? I mean maybe you should first check whether the input wave is stable since I am currently doing some thing on numerical wave tank, the simulated wave is far from satisfied.....
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