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multiphaseEulerFoam: unexplained oscillation on phase interface

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Old   August 3, 2021, 07:21
Question multiphaseEulerFoam: unexplained oscillation on phase interface
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Thomas
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Dear Foamers,

I am attempting to build a model for a axi-symmetric problem involving two phases (liquid and gas) in OF8 with the multiphaseEulerFoam solver. The liquid rests at the bottom of a cylinder and contains two species. One of these species can cross the phase boundary via mass transfer, then react with the species in the gas. A gas jet located centrally and being directed downward sweeps away the reaction products and out via an outlet at the top. Image of the mesh & geometry: https://www.dropbox.com/s/ytpcaq37srgxt0q/mesh.png?dl=0 (the phase interface is where the mesh becomes much finer.)

checkMesh results:

Code:
Mesh stats
    points:           33525
    internal points:  0
    faces:            116212
    internal faces:   49500
    cells:            33188
    faces per cell:   4.993130047
    boundary patches: 5
    point zones:      0
    face zones:       1
    cell zones:       1

Overall number of cells of each type:
    hexahedra:     0
    prisms:        32732
    wedges:        0
    pyramids:      228
    tet wedges:    0
    tetrahedra:    228
    polyhedra:     0

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
    Patch               Faces    Points   Surface topology                  
    front               33188    16877    ok (non-closed singly connected)  
    walls               312      627      ok (non-closed singly connected)  
    outlet              15       32       ok (non-closed singly connected)  
    inlet               9        19       ok (non-closed singly connected)  
    back                33188    16877    ok (non-closed singly connected)  

Checking geometry...
    Overall domain bounding box (-3.447584875e-18 0 -0.002626563908) (0.1504757455 0.959235076 0.002626563908)
    Mesh has 2 geometric (non-empty/wedge) directions (1 1 0)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Wedge front with angle 1 degrees
    Wedge back with angle 1 degrees
 ***Number of edges not aligned with or perpendicular to non-empty directions: 443
  <<Writing 886 points on non-aligned edges to set nonAlignedEdges
    Boundary openness (2.593575995e-17 -5.370361972e-19 3.107126343e-15) OK.
    Max cell openness = 2.808108818e-16 OK.
    Max aspect ratio = 2.515983483 OK.
    Minimum face area = 1.299236627e-08. Maximum face area = 6.104149172e-05.  Face area magnitudes OK.
    Min volume = 4.022322116e-12. Max volume = 2.342403203e-07.  Total volume = 0.0003758036516.  Cell volumes OK.
    Mesh non-orthogonality Max: 28.5678042 average: 4.291466937
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 0.3998834454 OK.
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.

End
The fact that the non-perpendicularity check fails is related to the fact that the geometry is a wedge and should be ignored (correct me if I'm wrong!).

After setting up the problem, all seems to work fine (as in, the results make sense from a physics standpoint) - mass transfer occurs, chemical reactions take place, flow is reasonable. However, the phase interface exhibits small, unexplained and jagged waves. The problem begins where the interface touches the axis and the outer wall boundary - unexplained oscillation begins there, which can propagate to the middle as normal waves would. These unwanted oscillations destabilize the solution, causing scalars to diverge near them (i.e. temperature). The surface looks like this after a while: https://www.dropbox.com/s/zpcxie0gcbdjqaw/surf.png?dl=0 Ultimately, this is what crashes the solution after an unspecified time (can be very early on in the simulation or much later depending on the particular case).

Theses oscillations manifest even if:
- the mesh is made finer multiple times
- the case is run in a plane geometry + the axis is made to be a wall
- chemistry is turned off
- mass transfer across the boundary is turned off
- impinging gas flow is turned off
- temperature is set much lower (operating temperature is 1500 K, checked at 400 K; the liquid phase is molten silicon in this case)

Another clue is that the simulation produces any results only if the gas phase flow is modeled as laminar. If RAS k-epsilon is enabled, the solution immediately diverges as soon as k or epsilon is being calculated in the first step. I reckon this occurs at those odd oscillation points near the boundaries and the phase interface.

The case has been checked under identical circumstances and BCs with the interFoam solver and no such oscillation at the surface can be seen. This leads me to think that the boundary conditions are also not the problem.

I am currently out of ideas what could be causing this and feeling somewhat desperate. Perhaps someone has an idea where the problem could be coming from and could help me out here.

Here are links to fvSolution and fvSchemes if those can help.

If anyone deems it necessary, I will upload any of the case files as well.
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divergence, multiphase boundary, openfoam 8

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