CAD preparation struggle

Hello everyone, this is my first post here. Glad to meet you all.

I’m Jaime, I’m 23 years old. I’m a mechanical engineer.

3 months ago I discovered this fantastic software, and as I am a motorsport aerodynamics fan I started my journey to learn how to use a CFD software properly.
I have created a rough design of a CAR with CATIA v5 to try to simulate the effects of MRF zones (wheels) and the porous media by my own, with no step by step videos (I’ve watched the FSAE tutorials and videos to get on with the software).

Now, without wasting more your time, let me explain my problem:

My problem is regarding the geometry preparation.

I use CATIA, and this is how my Product looks like (Product=Assembly). Each Part is a solid e.g the body, wheels…

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What I do is I generate a 1 single solid from all the objects that I want them to behave as a future face in Simscale navigator geometry tree.

In CATIA, I use the boolean operation of Join, then I save this all join, e.g. body with rear wing, diffuser… as STL. (Having 1 solid containing the body, rear wing, sidepod. Then another 1 body for each wheel…)

The workflow idea is: When I have all the bodies joined as a single 1 in CATIA, I save each general body joined in CATIA as STL files.

Therefore, my last step should be to merge all the STL as 1 STL body and below having the surface (being each surface a body from CATIA, right?). For this what I use is the Merge PowerShell script used in FSAE tutorials.
From my understanding, the body and the wheels now should look as faces inside the body merged by the script once I upload it to Simscale, like for example Yosuke’s project.

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But instead of that, what I got is this:

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I’m very frustrated because I don’t know what to do to solve it because with this CAD representation is really difficult to generate a proper mesh and evermore run a simulation.
All the bodies are assembled together and saved as STL as I mentioned, and joined correctly…

I beg for your help…

Thanks a lot, we keep in touch,


Hi Jaime,

I downloaded your Model.stl file and checked it with a text editor.
Your Model.stl file includes only two solids, “Body” and “Wheel”.

Please check your wheel model STL files and the names of the wheel solids each other and then merge them again.


Hi Yosuke,

I’ve reuploaded the geometry to try if I had fixed the problems but… no way.
I’ve checked what you write but I do not understand it at all.

Here there are the STL files used, wheel, body and merged.

Thanks for your time and help!


Hi Jaime,

Is your problem that your “Body” and “Wheel” are not nested in “solid_0” ?
It seems that it depends on when a model was uploaded recently or in the past, don’t it, SimScale team?
These days there is no solid_0, isn’t it? @jousefm

Or your problem is that your model does not includes 3 wheels other than your front left wheel,
please check your solid models of your front right, rear right and left wheels.
Your front left wheel STL looks fine.


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Yes, my problem is that the bodies I create an STL files are not seen as surfaces nested “solid_0”. I was wondering if this is because I’m doing the CAD preparation wrong.

For this model I only wanted to try using 1 wheel and the body. I just wanted to check if the “solid_0” was created and therefore nested the “solid_0” the surfaces (1 surface being the full joined body and other surface being the full joined Wheel).



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Hi Jaime ,

That sounds fine. Maybe you can create a simulation mesh with your model.