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April 8, 2024, 14:57 
Big difference in Potential flow vs CFD

#1 
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maximiliano
Join Date: Jun 2020
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Hello everyone,
I'm currently conducting ship simulations using CFD, specifically Star CCM+, following initial calculations using potential flow analysis software. In the potential flow analysis, the total resistance is calculated to be 560 kN at a speed of 13.5 knots. However, in the CFD simulations with a half hull model, I'm observing nearly double the resistance, approximately 2,000 kN. Typically, I achieve reliable results using the Hull Performance plugin, and this case isn't particularly complex. I intended to share the geometry file in .dbs format for reference, but it exceeds 1 MB. Does anyone have any insights on how to address this issue? 

April 8, 2024, 21:03 

#2  
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Peresvet
Join Date: Mar 2021
Location: USSR
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Quote:
I solve problems using an overset grid, which means I observe real movement in a virtual pool. When setting the correct surface roughness below the waterline, the difference in speed with a real ship is no more than 0.5 km/h at a speed of 20 km/h. When setting the required temperature, the difference in speed is no more than 0.2 km/h. You can imagine how accurately the towing resistance of a vessel is calculated!!! I'm working on version 17. I build automatic grids with correction. Only with an overset grid, an accurate center of mass and moment of inertia, can you get an accurate result. If it doesn't work, look for trouble in the mesh or in the physics of the model. I abandoned “dead” calculations from version 10. 

April 8, 2024, 22:35 

#3 
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maximiliano
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Sorry but I don't understand how this is related to my problem.


April 9, 2024, 00:00 

#4  
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Peresvet
Join Date: Mar 2021
Location: USSR
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Quote:
Send an archive of your problem, cleared of the solution. Let's see. What version are you working on? 

April 9, 2024, 02:08 

#5 
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I believe the "Hull Performance plugin" implies DFBI + overset, so the same approach that Peresvet seems to be suggesting.
The first thing I'd check is how the symmetry is handled in the force report. It's pretty easy to make a factor of two error by double accounting for the symmetry etc. But it can be a milion other things of course. BTW you seem convinced that the potential solver result is more reliable. Have you tried comparing both solvers to a measured case? 

April 9, 2024, 02:34 

#6  
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Peresvet
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Quote:
I prefer geometry in .step or .iges. Constructing geometry is not interesting. The structure itself is not interesting and will not provide an answer. 

April 11, 2024, 09:45 

#7 
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maximiliano
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I conducted some simulations in the past comparing Star and Potential Flow software (Maxsurf in this case, MS). For this hull in MS, I obtained a resistance of 500kN, while in Star, the mean resistance values were around 10321033kN. This value was obtained for half hull (where the wetted area is half of the total area), and the simulations converged on these values, albeit with residual discrepancies as usual.
However, a practical comparison can be made by considering the power required for a given speed and comparing it with similar ships. In this context, if the power requirement aligns better with the resistance value from Maxsurf, it suggests that Maxsurf provides a more realistic value. I used version 15.02.009R8 of Star CCM+ for this analysis. I'll share the geometry file via wetransfer because the files are much heavier than 193kb. https://we.tl/tykYSK0sJz8 

April 13, 2024, 23:32 

#8 
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Peresvet
Join Date: Mar 2021
Location: USSR
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Доброго дня.
Ваш проект я получил слишком поздно. Вы долго думали. Сейчас супер ПК занят расчётом, а на ноутбуке я не стану время терять. Готовлюсь в отпуск  путешествие до октября. Осенью сделаю расчёт сопротивления и выложу ссылку здесь на видео анимацию. Даже если этого уже не будет надо. Извините, но шевелиться надо по жизни резво. Last edited by Peresvet; April 30, 2024 at 18:21. 

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cfd, potentialflow, ship, starcmm 
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