Meshing error visibility

DaveStuppleDaveStupple
Just an observation that might be useful. The three yellow images show a small part of a much larger mesh. The element highlighted in blue is one of two tet4 that I placed in error with their shared edge laying across the diagonal of the hex8 face beneath. With all linear elements, the error does not show up even with 'Open cracks' activated. It was only when I converted the whole mesh to quadratic elements with mid-edge nodes that I spotted the error. With 'Show element surfaces' switched off, the misplaced elements show up in bold as isolated model edges (centre of image) - this is very useful, and I don't think I would have spotted the error without it. I am sure that 'Open cracks' would reveal this error with quadratic elements, but I would need pretty forensic use of the cutting plane and hide features to expose it. Lesson learned.

Comments

  • VictorVictor
    That's a good point. Neither open cracks or model edges is really bulletproof for this kind of subtle interior problem. I'd like to add a more powerful tool in future.

    Another way you might catch it is with:

    1) Select faces mode
    2) Show element surfaces off
    3) Edit -> Select all
    4) Move the cutting plane slider to open up a hole to look inside and you should see extra selected faces, possibly from behind.
  • DaveStuppleDaveStupple
    That's useful, as (I assume) it doesn't rely on quadratic elements. I'll try that.
  • DaveStuppleDaveStupple
    This works with both linear and quadratic models. The images are from the linear model. From above the problem faces are highlighted blue and easy to spot. From below (i.e. behind the faces), the blue highlight is not visible but the faces are dark against the lighter background. For this reason it is important to choose your viewing angle so that faces seen from behind do not have other dark faces as backdrop, as the error faces are very difficult to see. Adjusting angle at high zoom is tricky, could there be a fine adjust option, or scale the sensitivity with zoom? But the really helpful thing about this inspection trick is that the error can be seen from a distance through the mesh, and precision with the cutting plane is not critical. Great tip.
  • VMHVMH
    edited August 2016
    Is this mesh generated by Netgen in Mecway or imported? I haven't encountered this before.
  • SergioSergio
    Me neither. I have found sometimes in tetra meshes done in another programs that Mecway shows some bad elements were the other program show no errors. Guess that this problem is more frequent with second order elements.

    Would be nice a feature to make some checks and stats in the mesh, maybe with the option to create groups of bad elements to clearly show it and isolate.

    Regards
  • DaveStuppleDaveStupple
    VMH and Sergio,

    No, no, no. This mesh is generated manually by a fallible man at a desk in the south of England, who has imperfect concentration and is prone to making mistakes. I laid an edge across a face (several instances) in a linear model, and then converted to quadratic. Once converted, the errors showed up as model edges. Victor's trick is better as it shows them up in both the linear and quadratic models.

    Well done VMH and Sergio for the many responses you both put onto this forum, it is appreciated.
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