# Strange Results on Ship Hull Resistance CFD simulation

Hello everybody!

I’ve been trying to run a multiphase CFD simulation for a few days to calculate the resistance (later the coefficient of resistance) of a ship. The project can be viewed at: https://www.simscale.com/projects/leonardolombar/grupo_5_-_cfd_de_resistencia_ao_avan-o_de_navio

Honestly, I have little experience with CFD and therefore to diagnose what might be wrong with my project. But clearly there are errors since in “Run 2” of the “Simulations Runs” it is verified that:

1. In the “Force Plot”, the pressure force at x is of the order of 14000 kilo newtons. The total resistance force (pressure + viscous) expected for the ship studied is around 2000 kilo newtons. (on the other hand, the viscous force of the simulation is of the order of 800x2 = 1600 kilo newtons, which is quite reasonable)

2. There appears to be no water movement, as the Phase Fraction remains constant in the domain when viewing the “Solution Field”.

I tried to copy the project setup of https://www.simscale.com/projects/dheiny/ship_hull/ - domain of similar size, similar initial and boundary conditions.

I do not know what to do. Can anybody help me?
Thank you very much!

Hi Leonardo, and thanks for using the forum!

First of all: sorry for the late reply. Somehow your post slipped through the cracks of notifications.

Did you already check this guide to correctly postprocess results of multiphase simulations?

Please check the guide and in the meantime I will check your simulation setup and see if I find some suggestions.

Hi! Thank you very much for answering!

No problem! I’m finding SimScale amazing, thanks for the support.
I just really really want to get nice results from my simulation.

Somehow the suggested post slipped through me . I thought that by assigning the ship surface on “Results Control”, I would be able to analyze the forces inside Simscale… I’ll try the procedure recommended by the article, thanks!

Seizing the moment: actually what I want to extract from the simulation is the drag coefficient. I calmly read the article at How To Analyze the Pitch, Lift, and Drag Coefficients | SimScale, but it was not clear if I could get reliable results considering the reference length and area of a ship.

For the case of resistance to a ship, we have:

C_T = {R_T \over \frac{1}{2} ρS V^2}

Where C_T is the drag coefficient; R_T is the resistance force; ρ is the fluid density; S is the wetted area of the ship; and V is the flow velocity of fluid/ship.

In that case, should I use S as the reference area and L, the ship’s length, as the reference length?

Hello @leonardolombar , reference area and lengths are only used to non-dimensionalize resultant force and moments, that should be totally up to you or reference documentations.

By the description you provided though, the answer is yes. That’s also how the non-dimensional coefficients are formulated within the SimScale platform. You can use your S value as the reference area.

Reference length is used only to obtain moment coefficients. As far as I know, the length or width dimensions can be used depending on which moment you’re interested in. Perhaps it’s best to benefit from a reference article if there is any that you’re following.

Hope these are helpful. Could you also please share your final results once you achieve successful computations. I had previously tested a few similar cases by using the multiphase solver, sadly without any success. I’d appreciate the help. Thanks in advance!

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