Warning: Back CFACE search did NOT Find Intersection

[phart1]

I am getting this warning that is related to GEOMs. I have searched the user manual, technical manual and the FDS github and could not find anything.

What is its meaning and is there a resolution?

[marcosvanella]

Hi Paul, this warning means that for a boundary face of your geometry, an intersection with a triangle in the back (looking in the normal into the solid direction) was not found. This calculation is done to find a back temperature when doing 1D heat transfer with exposed Backing. The backing boundary condition is set to the default in these cases (VOID).
Can you share your input file? Also, I have not forgotten about you intersection issue. It is on my list.

[phart1]

No worries. The FDS and geom files are attached.

There are two "bulbs". One causes the error (Bulb7) but the other does not (Bulb8). They should be the same as I created one and copied it.

Description of how I created the bulb in Blender in case it is relevant.
https://www.youtube.com/watch?v=ZTPUgFuoYm4

1. Extrude and scale circle based on a bulb reference.
2. Select the top edges (top opening into bulb) go to Faces and select Grid Fill (repeat for bottom)
3. Select the bulb and modify it using Modifier, Solidify
4. Select the bulb and modify it using Modifier, Generate, Subdivision Surface.

GB2.fds.txt

Circle.013.bingeom.txt
Circle.014.bingeom.txt

[phart1]

I did a bit more investigating. This may be due to a malformed geometry. In Blender I used the X-Ray feature and found some indication of defects.

I recreated the bulb using a coarser mesh and check it with X-Ray feature. All good. So far no errors. So I would not spend anytime on investigating this at this point.

Is there a limit to the number of vertices or faces an individual GEOM can have? I did find a vertices limit for Geometries Extruded From Simple Planar Polygons (User Guide, p58). Does this apply to GEOMs?

[marcosvanella]

Hi Paul, there is no limit on the number of faces/vertices but ideally you want their size of the order of the cell size. There will be no accuracy gain in having sub-cell features and it will make the calculation more expensive. I tracked the warning to a boundary face stemming from a blocked cell. Turns out that you have a THICKNESS defined in the surf ID and the code tries to find the back surface of the bulb to apply the boundary condition. For this it uses ray tracing and the geometry triangles which are not in the line of sight of this blocked cell boundary face (CFACE). Now, you probably want to set the BACKING to "INSULATED" for your 2 mm thick SURFs, as using the surface temperature on the other side of the bulb for heat transfer in those 2 mm is not realistic, and the default "VOID" will consider ambient temperature in the back.
I noted a lot of blocked cells in the original geometries (see below). These are blocked for example when the cut-cell boundary polyhedrons cannot be defined by the algorithm, among other issues (intersections, etc,).
Can you post the new bulb geometry you developed? Thanks

[phart1]

Changing backing to "INSULATED" appears to solve the problem with the bulbs. However I also encountered the warning with three GEOMs. It may have something to do with the intersections between them. If I run the case with only one of these GEOMs I do not get the error. But once I include the slanted wall and one of the side walls I get the warning.

Is there a way to identify these blocked cell boundary faces so they can be corrected? Or is there guidance on how to avoid the problem.

Associated FDS files attached.

Thanks,
Paul

Cface.fds.txt
Circle.014.bingeom.txt
Cube.003.bingeom.txt
Cube.009.bingeom.txt
New OBST.007.bingeom.txt

[marcosvanella]

Hi Paul, we need to keep track of CFACEs stemming from blocked cells when searching for back faces. This way we can address the case where no back side of geometry is found. I'll look into this.
Try merging in Blender the slanted wall and sides into a single geometry. This way FDS will not have to compute a linearized intersection for these.

[phart1]

Regarding the Bulb - In Blender, I removed the Solidify Modifier and no longer received the warning. Solidify thickens the object to a desired thickness. I tried adjusting the Solidify Offset and thickness but still got the warning. But less instances as I reduced the thickness. I am guess the process results in blocked cells. I presume if it is not thickened it would be treated as a thin obstruction as described in User Guide, 7.2.2.