[Backstein]

Hello together!
This is my first question here on the board and I hope somebody can give me a hint on this one:
I am simulating a vertical axis water turbine with a sliding mesh using pimpleDyMFoam.
My simulation is working well in serial but in parallel turns out to become instable with the current fvSchemes setting (using Gauss linearUpwind for div). Changing to a more robust first order Upwind it also seems to run in parallel so far. (as omega seems to be the "responsable for the floating point exception i changed only the scheme for omega) . Changing it back at any time, the simulation breaks off almost immediately with floating point exception.

My question is how can it be explained that the same setting becomes more instable in parallel than in serial?
And second where could be the "weak spot" that causes this instability? I admit that the Mesh is not the best one (as we want to do automatic mesh generation for several geometries), but a quite similar mesh worked fine so far for smaller dimensioned cases. How could I maybe improve stability apart from relaxationFactors? Increasing the nCorrectors for PISO maybe? Or a gradient limiter?
I am using k-w-SST, having an unstructured 2D mesh with simple decomposition (had similar problems using Scotch or Metis, so stayed with Simple). TImeStep is variable, using smaller Courant does not seem to improve stability (did one quick test). As it can be seen in the last time step log the omega seems to be the one making problems:

Time = 0.00014

volume continuity errors : volume = 1.060668678, max error = 5.100053917e-07, sum local = 4.872359146e-16, global = 6.628585127e-18
time step continuity errors : sum local = 2.961667034e-12, global = -9.488543584e-14, cumulative = -4.645558234e-08
DICCG:  Solving for pcorr, Initial residual = 1, Final residual = 0.0009904684412, No Iterations 207
DICCG:  Solving for pcorr, Initial residual = 0.02454862805, Final residual = 0.000995627169, No Iterations 14
time step continuity errors : sum local = 6.982447501e-11, global = -2.43461651e-13, cumulative = -4.64558258e-08
Courant Number mean: 0.004522383496 max: 3.997331289 velocity magnitude: 18.83099822
BiCGStab:  Solving for Ux, Initial residual = 3.370098715e-05, Final residual = 2.747417836e-10, No Iterations 1
BiCGStab:  Solving for Uy, Initial residual = 0.0003091516009, Final residual = 3.314199687e-09, No Iterations 1
DICPCG:  Solving for p, Initial residual = 0.1095220538, Final residual = 0.001057952323, No Iterations 97
DICPCG:  Solving for p, Initial residual = 0.02453989547, Final residual = 0.0002384790543, No Iterations 113
time step continuity errors : sum local = 1.939135728e-11, global = -4.548477981e-13, cumulative = -4.645628065e-08
Moving mesh time step continuity errors : sum local = 1.939135728e-11, global = -4.548477981e-13, cumulative = -4.64567355e-08
DILUPBiCG:  Solving for omega, Initial residual = 0.5384700888, Final residual = 1.305115403e-17, No Iterations 1
bounding omega, min: -1.516985427e+30 max: 1.062801912e+39 average: 9.166903976e+33
DILUPBiCG:  Solving for k, Initial residual = 0.07131516516, Final residual = 6.099168081e-20, No Iterations 1
bounding k, min: -0.009392783275 max: 0.772638833 average: 0.02917238148
BiCGStab:  Solving for Ux, Initial residual = 1.01505102e-05, Final residual = 1.30896549e-10, No Iterations 1
BiCGStab:  Solving for Uy, Initial residual = 9.251842454e-05, Final residual = 1.687160026e-09, No Iterations 1
DICPCG:  Solving for p, Initial residual = 0.09103617579, Final residual = 0.0009079158009, No Iterations 78
DICPCG:  Solving for p, Initial residual = 0.02176100366, Final residual = 0.0002142317277, No Iterations 95
time step continuity errors : sum local = 1.713213508e-11, global = -5.551266578e-13, cumulative = -4.645729063e-08
Moving mesh time step continuity errors : sum local = 1.713213508e-11, global = -5.551266578e-13, cumulative = -4.645784575e-08
DILUPBiCG:  Solving for omega, Initial residual = 0.5384702048, Final residual = 1.03626683e-24, No Iterations 1
bounding omega, min: -8.19869406e+27 max: 1.172466292e+42 average: 9.879681126e+36
DILUPBiCG:  Solving for k, Initial residual = 0.007802550999, Final residual = 3.15921722e-21, No Iterations 1
bounding k, min: -0.01736656878 max: 0.7762871421 average: 0.02846047138
BiCGStab:  Solving for Ux, Initial residual = 3.03715729e-06, Final residual = 6.582417299e-11, No Iterations 1
BiCGStab:  Solving for Uy, Initial residual = 2.762881919e-05, Final residual = 8.983379148e-10, No Iterations 1
DICPCG:  Solving for p, Initial residual = 0.06789445479, Final residual = 0.0006646313472, No Iterations 105
DICPCG:  Solving for p, Initial residual = 0.01819497908, Final residual = 0.0001793243817, No Iterations 49
time step continuity errors : sum local = 1.428880237e-11, global = -5.944843051e-13, cumulative = -4.645844024e-08
Moving mesh time step continuity errors : sum local = 1.428880237e-11, global = -5.944843051e-13, cumulative = -4.645903472e-08
DILUPBiCG:  Solving for omega, Initial residual = 0.5384722664, Final residual = 1.053127148e-18, No Iterations 1
bounding omega, min: -1.601887488e+29 max: 4.807690365e+41 average: 5.320023349e+36
DILUPBiCG:  Solving for k, Initial residual = 0.06206841838, Final residual = 1.196518663e-19, No Iterations 1
bounding k, min: -0.02250918779 max: 0.7775090679 average: 0.02792288273
BiCGStab:  Solving for Ux, Initial residual = 9.972561103e-07, Final residual = 9.972561103e-07, No Iterations 0
BiCGStab:  Solving for Uy, Initial residual = 8.755201077e-06, Final residual = 4.79050003e-10, No Iterations 1
DICPCG:  Solving for p, Initial residual = 0.05596834485, Final residual = 0.0005584424212, No Iterations 106
DICPCG:  Solving for p, Initial residual = 0.01482521461, Final residual = 0.0001474662352, No Iterations 59
time step continuity errors : sum local = 1.186660564e-11, global = -6.256943322e-13, cumulative = -4.645966042e-08
Moving mesh time step continuity errors : sum local = 1.186660564e-11, global = -6.256943321e-13, cumulative = -4.646028611e-08
DILUPBiCG:  Solving for omega, Initial residual = 0.5384730951, Final residual = 1.017998611e-16, No Iterations 1
bounding omega, min: -6.509354347e+28 max: 1.442307109e+41 average: 1.596028289e+36
DILUPBiCG:  Solving for k, Initial residual = 0.01758189299, Final residual = 1.020152112e-21, No Iterations 1
bounding k, min: -0.02147217547 max: 0.7778232691 average: 0.02732371026
BiCGStab:  Solving for Ux, Initial residual = 4.328171372e-07, Final residual = 4.328171372e-07, No Iterations 0
BiCGStab:  Solving for Uy, Initial residual = 3.432014852e-06, Final residual = 2.469458324e-10, No Iterations 1
DICPCG:  Solving for p, Initial residual = 0.04010861452, Final residual = 0.0003913547452, No Iterations 115
DICPCG:  Solving for p, Initial residual = 0.01167541556, Final residual = 0.0001142343015, No Iterations 63
time step continuity errors : sum local = 9.196848067e-12, global = -6.411126025e-13, cumulative = -4.646092722e-08
Moving mesh time step continuity errors : sum local = 9.196848067e-12, global = -6.411126025e-13, cumulative = -4.646156834e-08
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mpirun noticed that process rank 1 with PID 313 on node hpb0213 exited on signal 8 (Floating point exception).

Thanks in advance!