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Hello everybody,

I've been having some trouble trying to simulate the flow through a nozzle. The nozzle is located inside a box, which outlet is defined on the upper side of the box. At the nozzle's outlet I've made some mesh refinement: That's what it looks like a cone (see pictures).

Regardless of the time step, alpha is after 2-3 iteration smaller than 0 and bigger than 1. I guess, that's why the simulation at some point diverges. I've tried to change the fvscheme, fvsolution and boundary condition but nothing seems to help. The output from checkMesh was ok.

These are one of the first iterations and the one when everything starts to go really wrong:

Courant Number mean: 0.001011663 max: 33.73018
Interface Courant Number mean: 3.233757e-06 max: 1.547393
Time = 0.0006

PIMPLE: iteration 1
smoothSolver:  Solving for alpha.water, Initial residual = 0.002851213, Final residual = 5.817432e-09, No Iterations 3
Phase-1 volume fraction = 4.606276e-06  Min(alpha.water) = -8.110569e-37  Max(alpha.water) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 4.606276e-06  Min(alpha.water) = -5.658737e-29  Max(alpha.water) = 1
smoothSolver:  Solving for Ux, Initial residual = 0.2224137, Final residual = 5.36794e-08, No Iterations 15
smoothSolver:  Solving for Uy, Initial residual = 0.05766709, Final residual = 8.711863e-08, No Iterations 14
smoothSolver:  Solving for Uz, Initial residual = 0.005021781, Final residual = 6.344698e-08, No Iterations 11
GAMG:  Solving for p_rgh, Initial residual = 0.0002026823, Final residual = 1.482498e-06, No Iterations 65
GAMG:  Solving for p_rgh, Initial residual = 8.477441e-06, Final residual = 9.10022e-08, No Iterations 3
time step continuity errors : sum local = 3.456393e-08, global = 1.273059e-08, cumulative = 1.984811e-08
GAMG:  Solving for p_rgh, Initial residual = 1.145649e-05, Final residual = 9.459702e-08, No Iterations 68
GAMG:  Solving for p_rgh, Initial residual = 3.933028e-06, Final residual = 5.324668e-08, No Iterations 2
time step continuity errors : sum local = 1.621167e-08, global = 2.118172e-09, cumulative = 2.196628e-08
GAMG:  Solving for p_rgh, Initial residual = 3.024072e-06, Final residual = 4.312977e-08, No Iterations 66
GAMG:  Solving for p_rgh, Initial residual = 9.835354e-07, Final residual = 4.512249e-08, No Iterations 1
time step continuity errors : sum local = 1.245904e-08, global = 1.925232e-09, cumulative = 2.389151e-08
smoothSolver:  Solving for epsilon, Initial residual = 0.03281331, Final residual = 4.027295e-08, No Iterations 11
bounding epsilon, min: -11.98792 max: 64604.16 average: 3.231076
smoothSolver:  Solving for k, Initial residual = 0.1275502, Final residual = 8.044356e-08, No Iterations 12
ExecutionTime = 106.59 s  ClockTime = 110 s


Courant Number mean: 0.01542766 max: 4305.785
Interface Courant Number mean: 0.001508509 max: 4305.785
Time = 0.2244

PIMPLE: iteration 1
smoothSolver:  Solving for alpha.water, Initial residual = 0.0003461995, Final residual = 9.638341e-09, No Iterations 67
Phase-1 volume fraction = 0.002594258  Min(alpha.water) = -9.943067e-05  Max(alpha.water) = 1.000747
MULES: Correcting alpha.water
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 0.002594258  Min(alpha.water) = -14.12184  Max(alpha.water) = 14.91034
smoothSolver:  Solving for Ux, Initial residual = 0.28712, Final residual = 6.795894e-08, No Iterations 22
smoothSolver:  Solving for Uy, Initial residual = 0.3240182, Final residual = 7.216942e-08, No Iterations 21
smoothSolver:  Solving for Uz, Initial residual = 0.05525987, Final residual = 5.901969e-08, No Iterations 20
GAMG:  Solving for p_rgh, Initial residual = 6.635285e-05, Final residual = 5.24964e-07, No Iterations 257
GAMG:  Solving for p_rgh, Initial residual = 6.930181e-08, Final residual = 6.930181e-08, No Iterations 0
time step continuity errors : sum local = 0.002720822, global = -2.326503e-06, cumulative = -4.235363e-06
GAMG:  Solving for p_rgh, Initial residual = 1.175393e-07, Final residual = 2.416806e-08, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 3.561633e-08, Final residual = 3.561633e-08, No Iterations 0
time step continuity errors : sum local = 0.001397755, global = -1.039455e-07, cumulative = -4.339308e-06
GAMG:  Solving for p_rgh, Initial residual = 6.183298e-08, Final residual = 6.183298e-08, No Iterations 0
GAMG:  Solving for p_rgh, Initial residual = 6.183298e-08, Final residual = 6.183298e-08, No Iterations 0
time step continuity errors : sum local = 0.002426621, global = 9.706378e-08, cumulative = -4.242245e-06
smoothSolver:  Solving for epsilon, Initial residual = 0.03335731, Final residual = 5.528391e-08, No Iterations 10
bounding epsilon, min: -3.069506e+08 max: 2.650014e+14 average: 1.599839e+09
smoothSolver:  Solving for k, Initial residual = 0.1363255, Final residual = 8.208013e-08, No Iterations 12
bounding k, min: -20.41971 max: 2.017263e+08 average: 4944.138
ExecutionTime = 18425.42 s  ClockTime = 18737 s

I've also attached here the case files and the complete log file.

On the pictures you can also see an example for alpha at 0.1 ms, where the negative values and the ones greater than 1 are shown.

Am I doing something wrong here?
I'm grateful for any help!

box_nozzle.png

01ms_alpha.jpg

01ms_alpha_smaller_bigger_than.jpg

NOZZLE_CASE.zip

mesh_nozzle.jpg