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[snappyHexMesh] [Problem] Addlayer worsen Mesh quality

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Old   October 25, 2017, 05:16
Unhappy [Problem] Addlayer worsen Mesh quality
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Edoardo
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Hi guys.
I'm encountering problems with snappyHexMesh. I'm trying to generate the mesh of a centripetal turbine (scroll and impeller).
The snap phase seems ok, the edge are well defined and nothing goes wrong.
But when I try to add the boundary layer cells, the edges of the impeller became really bad.
I've already tried different edge/surface/region refinements as well as different shrinking parameters.
The problem is that the solutor (rhoSimpleFoam) will not do more than 5 iters (I attached the log file).
I'm also sure that the problem are these boundary layer cells in the impeller, since if I do not add here the layers the solutor won't have any issues.
I attach the images of the problem as well as the dictionaries.
I really need help since this is the basis of my master thesis project and it seems I can't find the solution

blockMesh.jpg

impelleraddLayer.jpg

impellersnapped.jpg

snappyHexMeshDict.txt

logrhoSimpleFoam.txt
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Old   October 26, 2017, 10:27
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Pascal Balz
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HI edoardo,

regarding your solver crash: this could either be due to bad boundary conditions or a really bad mesh/wrong fvSchemes. Could you please post the output for
Code:
checkMesh -constant -allGeometry
If the solver converges without boundary layers an error in the definition of boundary conditions is more unlikely.

Layer generation with snappyHexMesh is really a pain in the ass for propeller geometries. The edges will most times look like yours and layers will likely collapse at the trailing edge. Thats due to the algorithm and I don't know a solution to that.
What you should do:
- post the log file of snappyHexMesh so that we can see whats going on there
- some changes in snappyHexMeshDict:
Code:
maxLocalCells: highly increase this value as it will likely stop any refinement steps and layer additions.
maxGlobalCells: increase this one too.
nCellsBetweenLevels: 3 or more.
nLayerIter: increase this value up to 200, start with 50

Most likely your mesh quality settings are to strict for generating the layers.
Try setting the following values:

maxNonOrtho 75;
maxInternalSkewness 10;
maxConcave 180;
minVol 1e-20;
minTetQuality -1e30;
minTwist -1;
This should generate at least a few layers.
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Pascal
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