Subract fails on region border
 

Hello,
At first I describe the scenario.
I have a huge block as Windtunnel. Because I want to split the regions I created a block inside the region. On one side the blocks are intersecting.
e.g.:
Block one:
-5,0,0
10,5,5

Block two:
0,0,1
2,1,2

So the blocks are intersecting at y=0, between and .

The subract would be block one - block two.
At the border (the described intersection) the subract fails and block one have a weird, random looking pattern.

I tried to work with it and made the an interface there in the regions. But the volume mesh fails telling me there is an self intersecting surface in block one.

I tried some thing, but nothing worked.

Any suggestions?

-FFD

Hi FFD,

so you want to model a block inside a windtunnel, right?

Then first create the windtunnel-Block.
Then make a sketch with the other rectangle. Then rightklick the sketch and select extrude-cut.

Hello,
thank you for the suggestion.
I explain you my problem a bit more in detail:
In my windtunnel I have an object. Block two is to create a region around a part of the object.

So I create the block. Subtract the object and the block from the windtunnel, subtract the object from block two and create an interface between these subtracts.

The object is symmetrical, so I just simulate the half object. The object is cut by the windtunnel. And there I do have the described problem.

I never worked with the 3D tool in StarCCM+ and I have some problems on the first try.
I cannot create a sketch plane which is huge enaugh. The sketch plane is like 1m^2 big.
And I cannot create the rectangle , because I cannot change the size of it.

I dont really understand what you want to do ?! ^^

can you upload a Screenshot or something like that

ps:
Maybe you create your bodies, select as extrusion mode: Body interaction: None. Then you get 2 bodies. Multiselect them, rightklick -> boolean -> substract

It sounds like you are using Geometry Parts with Parts Operations to subtract. Try imprinting the two blocks before doing the subtract. Since CCM+ stores triangles with double precision, it's possible that two surfaces that line up in precise engineering units have some strange behavior when converted to doubles (https://en.wikipedia.org/wiki/Double...g-point_format for more info). In that case, imprinting the surfaces / blocks first will ensure that the faces which are "on top" of each other are treated as one representation.

Then, after the imprint you can perform your subtract operation as needed.

As an alternative (and learning exercise), duplicate the two blocks, then right click > combine, and do the work in Surface Repair (by selecting Merge Imprint tool and deleting the appropriate faces). Hopefully this helps you understand better what CCM+ is doing "behind the scenes".

Deranda's suggestions are also great for working in 3D CAD - that is where I prefer to create these simple shapes as it's much easier to visualize and make parametric. As for the sketch plane, it will resize once you start drawing on it and you'll get a grid 2-3x bigger than your largest dimension.

Thank you.
The idea with imprinting was really helpfull.
The CAD Method would be good too i think, but I never worked with the StarCCM CAD tool and had a few problems.

If you are using a relatively recent version, you do not need to use 3D-CAD to do CAD booleans. In the subtract operation there should be an option to 'perform CAD boolean'.