Slicing a Solid with a Contoured Surface

[shm21284]

I am attempting to run a CFD analysis on a car. I have been able to convert the 3ds Max file into a *.dxf, which imports to NX3 as a faceted body. I then "sewed" the multiple faces together, extracted geometry, and got a surface/sheet, which imports to Fluent.

The car's body is essentially one sheet, but it is approximated by flat surfaces instead of smooth curves, as it was built with polygons.

What I am trying to do is build a very large block of air (10x the characteristic length of the vehicle) around the vehicle, and have the sheet that represents the car's body slice the block of air, so a mesh and boundary conditions can be applied to it.

I can not split the block of air, it gives me the error "non manifold solid." I can not trim the block of air. I can not thicken the sheet, it tells me to lower the thickness. I can not use the sheet to solid assistant, as it tells me to use a smaller tolerance in relation to the thickness, but i went down several orders and this didn't do anything.

A picture can be uploaded later.

 

[ewh]

Have you run "Analysis -> Examine Geometry" on it? This may tell you where your model is having problems.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[shm21284]

Self intersecting face detected.
How am I supposed to find it with 16000 faces in the body???

 

[hudson888]

After you run examine geometry then tick the "Highlight Results" box next to self intersecting and it will give you some idea. There may be more than one location. The thing to do in cases like this one is to exclude affected areas from the sew and sneak up on adding portions until you find the culprit and can apply some kind of remedy. Often with imported data it is to do with how meeting edges are trimmed to one another.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

 

[shm21284]

I did a test on a layer that had passed the all the areas of the geometry analysis, and I am still unable to have the sheet/surface split the block.

 

[hudson888]

What error do you get?

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

 

[shm21284]

With the failure to thicken the sheet, I get the general error: Unable to apply thicken.

I did a test case, one with a sinusoidal sweep that resulted in a 3-dimensional curve, and the other with a bunch of triangles (much like my geometry of the car) that were turned into sheets, sewed, and then attempted to thicken. The one like my car i'm trying to model, with all the triangles, will NOT thicken, and the complex 3-D curve will.

 

[shm21284]

http://files.engineering.com/getfil...nusoidal_Shape_from_a_Thickened_Sheet_(1).jpg

 

[hudson888]

It is probably the case that the faceted type of model when thickening is attempted totally consumes one or more of the faces and can't construct an offset vertex as a result. Such things frequently fail in that kind of manner.

Now as I said if you can find the area/s that cause problems sometimes it may be as simple and effective as removing an offending sheet from the sew in order to get the thicken to work. Depending on your specific needs you may be happy enough with that or easily able to fix the problem.

In examine geometry analysis alone is not sufficient to meet your needs then by all means trimming the body with planes in half and half again until you have isolated the offending area sometimes helps to focus in on small problems with large models.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

 

[shm21284]

Update:

The thicken surface feature will work if i apply edge blends to each edge in the faceted body, creating a "smooth flow" from one face to the next.

I think what is happening is it attempts to thicken one face and it collides with the next face over, and creates discontinuities in the geometry. The edge blend worked for my test model, although I had to do most of the blending manually and separately. This will take a long time to do on a model that has 16,000 faces.

Once a solid is created, i can then subtract the geometry from the fluid block and apply my mesh!

http://files.engineering.com/getfil...-9757-b56330f229d6&file=Test_Faceted_Body.jpg

http://files.engineering.com/getfil...-9bcd-4ccf-8934-e3550a240924&file=240sx_1.jpg

 

[cowski]

Was the surface actually modeled as polygons or was that the result of the .dxf export?

I have tried to export a few parts out of NX with the .dxf option, but the result was unusable. If the surface was modeled with smooth curves, perhaps a different export option would take care of the problem. The first choice would be parasolid (.x_t) if it is available, otherwise STEP (.stp or .step) or IGES (.igs or .iges).

 

[hudson888]

Cowski,

Conversely in the same vein I have seen surfaces from other systems come in to NX in such a fashion though I have never seen NX create such a thing because of a translation. The only cases where I found NX making the equivalent of faceted surfaces was a degree 1 cloud of points surface with multiple patches.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

 

[shm21284]

Cowski, the car was built for a video games using polygons. The only method I have found to get it into NX was through the DXF option.

 

[hudson888]

Well that explains the crap geometry then. Try for a start doing one half of the vehicle at a time. The other half is (probably?) going to be symmetrical.


Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum