[KAYANO]

Hello everyone! An error occurs while solving the problem.

Task in the movement of air in the sinuses.

Time step 5e-7, Courant number <1.

The solution is stable until 0.025 seconds, after which the Courant number decreases and the solution falls apart.

Flow velocity 1 m/s, supersonic areas appear at the exit (which is very strange).

Mesh quality log here:

Create time

Create mesh for time = 0

Time = 0

Mesh stats 
    points:           5401673
    faces:            14022342
    internal faces:   12965953
    cells:            4389800
    faces per cell:   6.147955
    boundary patches: 3
    point zones:      1
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     3710992
    prisms:        120467
    wedges:        0
    pyramids:      0
    tet wedges:    381
    tetrahedra:    1
    polyhedra:     557959
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            4   141179
            5   106526
            6   88992
            7   13
            8   14
            9   110298
           10   2
           11   4
           12   61431
           14   5
           15   44639
           17   2
           18   4854

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   PointsSurface topology
                   inlet     7393     7893  ok (non-closed singly connected)
                  outlet     5251     6139  ok (non-closed singly connected)
                    nose  1043745  1205189  ok (non-closed singly connected)

Checking faceZone topology for multiply connected surfaces...
    No faceZones found.

Checking basic cellZone addressing...
    No cellZones found.

Checking basic pointZone addressing...
               PointZone  PointsBoundingBox
            frozenPoints       0(1e+150 1e+150 1e+150) (-1e+150 -1e+150 -1e+150)

Checking geometry...
    Overall domain bounding box (-0.06548677 -0.06728039 -0.02183478) (0.03593177 0.03602325 0.02079412)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (3.222643e-16 1.024171e-15 -4.319472e-17) OK.
    Max cell openness = 4.320483e-16 OK.
    Max aspect ratio = 3.730799 OK.
    Minimum face area = 2.814233e-09. Maximum face area = 4.129001e-06.  Face area magnitudes OK.
    Min volume = 4.700838e-13. Max volume = 8.307965e-09.  Total volume = 3.295746e-05.  Cell volumes OK.
    Mesh non-orthogonality Max: 63.17777 average: 12.60157
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 2.252324 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End

The task itself is available at the link