Scaling issue

[Luke Cleary]

Hello, 

I am using the dagmc toolchain to run neutronics on a cad model in openmc. When I import my model into cubit as a .step file, it seems that dimension information is lost.  My cad model workspace is in dimensions of centimetres. As an example, a part from my model measures ~ 100 centimetres across, however once it gets into openmc, it's brought down to ~ 1 cm. 

A couple of questions:

- How exactly does dagmc interpret dimensions from a .step file (if at all)?

- Is there a way to uniformly scale a volume to correct this issue? Or better yet, to import it with dimensions preserved?

Let me know if I can clarify anything. 

Thanks, 

Luke 

[tim....@wisc.edu]

Hi Luke,

Cubit does not "use" units.  Thus you may have to scale the model when you import it.  You can use the cubit command scale to perform this.  For example, to scale all volumes by 100, use:

cubit> vol all scale 100.

[Luke Cleary]

Hi Tim, 

Thanks, that scale command will be helpful. Do you mind expanding on that (Cubit does not "use" units)? How then does it handle dimensions of volumes relative to each other when reading from the cad file? Does it just use arbitrary numbers? I'm wondering, if the software isn't using units, how I'll be able to know how much to scale it by. 

Thanks, 

Luke 

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[Paul Wilson]

Hello Luke

Perhaps it is better to say that Cubit doesn’t imply or infer any units.  It is up to the user to imply units.  If your simulation software assumes units of cm (as most do), then you should build your model so that the dimensions are interpreted in cm.

If you want a cube that is 10 cm on a side, then you want to make sure that Cubit indicates a side length of 10.  Many CAD software packages use mm as a default, so that same cube will be exported with a side length of 100. If you import it into Cubit, you’ll still see a side length of 100, but since you know that you want implied units of cm, you will need to scale by 0.1.

I hope that’s a little more clear.

Paul

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On Aug 19, 2021, at 02:07, Luke Cleary <luke....@copenhagenatomics.com> wrote:



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[Andrew Davis]

Hi

Not meaning to polute this thread, but are you for instance worried about the fact that STEP files can have components listed each one in different units?

Thanks

Andy

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