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[ICEM] Nuclear fuel assembly, Mesh Check: "missing internal faces, uncovered faces"

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Old   June 23, 2017, 01:52
Default Nuclear fuel assembly, Mesh Check: "missing internal faces, uncovered faces"
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parth swaroop
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Dear All,

I want to mesh a nuclear fuel channel assembly. It consists of two outer concentric tubes, within which, moderator is flowing through. Inside this tube, there are 28 fuel pins arranged in fashion as shown the figure1 (They are fixed together with the help of an endplate of either side, which is not yet considered). Coolant flows through these sub channels. Inside this fuel, there is an heating element which produces constant power in order to simulate heat production from fission events. I have tried two meshing approach, both of which yield different set of problem.

1) Since the fuel pins are placed eccentric to outer tube, the geometry is symmetric about y plane. Hence I first create a 2D blocking in one half of the inlet plane as shown in figure 2. I convert the pre-mesh into mesh. Mirror this mesh (figure3) and extrude the mesh along z direction to complete the mesh in 3D domain. I run check mesh twice, one prior to mirror and one after the extrude ( all parts inherited and extruded along the curve). For check mesh prior to mirror, there are no issues expect for single edges on the periphery of the domain. However, after extrusion, I getting errors : " missing internal faces" and " uncovered faces" as shown in the figure. Another thing that I noticed was my mesh quality went down from 0.6 to 0.2 after extruding.

For this, initially I pushed the fix button ( which would create line elements at those surfaces). Now, I import this mesh into CFX it generates an error message : " hexahedral element 77352 will degenerate to an invalid element ". I assumed that this would be due to " missing internal faces" and " uncovered faces" error or either due to mesh quality. So, I tried 1) mesh smoothing to improve the minimum quality of mesh from 0.2 to 0.4 which still would get the same error message. Hence for these elements i tried the associate mesh with geometry by selecting these particular elements, but then nothing happened (These elements in the subset are just shown for visualisation?). I realised that I was mirroring and extruding only shell elements and I could fix " missing internal faces" and " uncovered faces" by choosing "line" elements as well. But this now generates " multiple edges" during check mesh which only a "possible error" (figure4) but I still get the same error mesh from cfx : " hexahedral element 136197 will degenerate to an invalid element ".

Now, I do not know how to fix this error, in the current approach. I guess, longer approach would be to mirror and extrude blocking ( as well my curves and points). I would like to know if there is any other fix, before trying this tedious approach? ( I have tried this as well, and will post the message a bit later)

P.S I am new to ICEM, and this my understanding from the past 2 months. Hence, is there something very basic that I am missing?

Link to my model :

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File Type: jpg multipleedges-fig4.jpg (79.8 KB, 29 views)
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File Type: pdf geometry-fig1.pdf (93.0 KB, 10 views)
File Type: pdf blocks-fig2.pdf (43.3 KB, 9 views)
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Last edited by swparth; June 23, 2017 at 02:13. Reason: Link to model added
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Old   June 23, 2017, 04:32
Default Attempt 2
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parth swaroop
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2) So as I said, in my second variation, I also tried mirroring the blocking and then extruding it. I only associate all the edges to curve and vertices to points in the "inlet" and "outlet " domain but, along the z-direction, I do not associate the geometry (I think they are just too many edges too do one-one association with the curves) Is there a simple trick to do this?

Unfortunately, also partly expected, I do not get the desired pre- mesh. I have attached the figure. Just associating the edges and point on the 2D plane does not guarantee the extrusion to be along the curve since the edges could get attached to other curves and surfaces. I would like to know what are the possible alternatives here?

Link to this model:
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File Type: jpg 3Dpremesh-fig2.jpg (188.1 KB, 16 views)
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Old   June 27, 2017, 07:12
Default Volume Mesh
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parth swaroop
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I think I figured out why mesh is getting distorted in my second approach i.e when I mirror and extrude my blocks.

It is due to the fact that between two surface elements there is a volume element (or fluid domain) and there is no corresponding surface to it. As a simple example, in my model, I checked this for moderator flow through the outer cylinder ( CT) and inner cylinder (PT).

Initially I associated all the edges along the z direction for these three parts ( CT, PT and moderator region) but the distortion still remain. Hence, I deleted all the face association for these three parts. To my surprise, no mesh was generated ( since i thought there is still edge to curve association, it would generate pre- mesh). So now i associate all faces of CT and PT to the corresponding surfaces of part ( i.e inlet , outlet and along z) and I got the mesh that I expected in these two region. Unfortunately, for the moderator there is no corresponding surface in the inlet and outlet region. Hence, by default it associates these faces to the nearest possible surface. Further, this overwrites my edge association.

Does this mean that face\edge to surface supersedes edge to curve association ?

Further, more complicated would be : how will I associate the faces of the subchannels? i.e fluid path inbetween fuel pin.

Please help.
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missing internal faces, multiple edges, uncovered faces, volume meshing

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