[piotr.mecht]

I've been learning OF for a while and I'm still struggling with meshing cad imported geometries. The snappyHexMesh fits my shape well, but somehow produces meshes, that are unusable (?) in transient simulations.

I'm trying to simulate transient hot water tank discharge in order to estimate cold inlet jet range in different dimensional configurations (buoyantPimpleFoam to account buoyancy forces with >20°C temperature differences).
The geometry seems to be quite simple, but I find it very hard to mesh it properly [Picture02]. I tried blockMesh at first [Picture03], but a huge aspect ratio between connectors and mantle diameters makes the number of blocks to increase dramatically. Requirement of consistent number of cells between blocks is also a strong limitation, so I had to use sHM.

My sHM output always finishes without any errors at faces, so it meets my meshQualityDict requirements, but checkMesh always proves it wrong, showing that there are concave cells, underdetermined cells, warped faces and so on [Picture04]. Sometimes some of these cannot be avoided, but if it happens on the surface at snapping stage it's nearly impossible to add layers without errors like negative cell volumes or warped shapes. But it happens! All the time! Especially between refinement levels, and my refinement regions just have to cut the surface.

I had this idea that it is enough to have no skewed cells and no cells with negative volume, but my simulations quickly went unstable (omega values shoot into infinity after second or third iteration, exactly on the defective cells). Reducing time step only increased Courant number, so I assumed it is definetly related to mesh quality. I ran the same simulation with different axisymmetric shape, but with the same settings and BCs to make sure my setup works okay and it did [Picture 06].

Can I leave any of these mesh checks failed? How "perfect" does my mesh need to be in order to perform such a simulation? I can't prepare a single working solution, so how i'm supposed to check my grid convergence?

I think I slightly understand what these check errors mean (as they easily can be displayed in Paraview), but i have no clue how to avoid them. Is there any explaination or general tips for this?
It doesn't seem to be that hard, but it is really ashaming to say for how long I'm trying to solve this problem :< Where am I failing at?