[kandelabr]

Hello,

I've been struggling to make my pump case run for well over six months now and now I decided to split my problem(s) into smaller ones, starting with meshing.

I made the simplest case I could think of: a sudden expansion joint, see attached pictures. To eliminate errors on the inlet edge I declared a 'slip' BC on a short section after inlet patch. I used k-omega SST turbulence model and wall functions. Boundary conditions are 1m/s inlet velocity and 0 outlet pressure, noSlip walls (except the mentioned patch).

I tried with a few different meshes:
- snappyHexMesh with 1, 0.5, 0.25 and 0.125mm 'smallest' cell without boundary layers
- cfMesh with 1x 1mm, 3x 0.5mm and 5x 0.3mm boundary cells
- cfMesh with 10x 8um boundary cells without wall functions

I also tried the cfMesh cells without turbulence modelling. All except one laminar case converged. Here are a few numbers I think are important:

mesh                min size    N*y+    Umax    pmin
snappy coarse       1mm                 1.074   -0.217
snappy medium       0.5mm               1.073   -0.213
snappy fine         0.25mm              1.083   -0.221
snappy extrafine    0.125mm             1.088   -0.225

coarse layers       1x 1mm      110     1.243   -0.237
medium layers       3x 0.5mm    54      1.1     -0.247
fine layers         5x 0.3mm    33      1.103   -0.241

no wall functions   10x 9um     1       1.087   -0.301

y+ = 9um

You can check all other results in attached PDF.
I now have no idea what the results mean. Are the results reasonable and which ones should be trusted most?

Thanks!