[GerhardHolzinger]

I am running an incompressible simulation using PIMPLE. I noticed something with the p residuals which I can't explain to myself:

The initial residual for p decreases over the PIMPLE iterations. However, the initial residual of the last iteration is of the same order of magnitude as the initial residual at the beginning of the PIMPLE iteration.

Why is this the case?
From the code I see nothing that is done differently at the last iteration (except solving for turbulence, but that happens after solving for p anyway. So that might not be the reason.)

PIMPLE: iteration 1
DILUPBiCG:  Solving for Ux, Initial residual = 3.60231e-05, Final residual = 1.66706e-09, No Iterations 2
...
GAMG:  Solving for p, Initial residual = 0.342611, Final residual = 3.12937e-05, No Iterations 17
GAMG:  Solving for p, Initial residual = 0.00752642, Final residual = 6.27839e-07, No Iterations 16
time step continuity errors : sum local = 1.37565e-12, global = -2.03189e-13, cumulative = -1.55453e-09
PIMPLE: iteration 2
DILUPBiCG:  Solving for Ux, Initial residual = 0.000179105, Final residual = 4.61726e-08, No Iterations 1
...
GAMG:  Solving for p, Initial residual = 0.0170125, Final residual = 1.33717e-06, No Iterations 22
GAMG:  Solving for p, Initial residual = 0.0023444, Final residual = 2.1267e-07, No Iterations 16
time step continuity errors : sum local = 4.58019e-13, global = -6.99882e-14, cumulative = -1.5546e-09
PIMPLE: iteration 3
DILUPBiCG:  Solving for Ux, Initial residual = 5.00896e-05, Final residual = 4.90796e-08, No Iterations 1
...
GAMG:  Solving for p, Initial residual = 0.0047698, Final residual = 3.81808e-07, No Iterations 20
GAMG:  Solving for p, Initial residual = 0.000571794, Final residual = 4.58333e-08, No Iterations 16
time step continuity errors : sum local = 9.82129e-14, global = -1.57202e-14, cumulative = -1.55461e-09
PIMPLE: iteration 4
DILUPBiCG:  Solving for Ux, Initial residual = 1.71822e-05, Final residual = 2.14214e-10, No Iterations 2
...
GAMG:  Solving for p, Initial residual = 0.00145923, Final residual = 1.36203e-07, No Iterations 18
GAMG:  Solving for p, Initial residual = 0.000143427, Final residual = 1.27477e-08, No Iterations 18
time step continuity errors : sum local = 2.72637e-14, global = -5.41738e-15, cumulative = -1.55462e-09
PIMPLE: iteration 5
DILUPBiCG:  Solving for Ux, Initial residual = 6.04597e-06, Final residual = 7.98942e-10, No Iterations 2
...
GAMG:  Solving for p, Initial residual = 0.000479671, Final residual = 4.53569e-08, No Iterations 17
GAMG:  Solving for p, Initial residual = 4.41661e-05, Final residual = 8.06229e-09, No Iterations 15
time step continuity errors : sum local = 1.7232e-14, global = -3.20335e-15, cumulative = -1.55462e-09
PIMPLE: iteration 6
DILUPBiCG:  Solving for Ux, Initial residual = 3.72214e-06, Final residual = 1.18203e-09, No Iterations 2
...
GAMG:  Solving for p, Initial residual = 0.327238, Final residual = 3.2432e-05, No Iterations 16
GAMG:  Solving for p, Initial residual = 0.0132261, Final residual = 8.92727e-09, No Iterations 41
time step continuity errors : sum local = 1.61865e-14, global = 2.78977e-15, cumulative = -1.55462e-09
DILUPBiCG:  Solving for epsilon, Initial residual = 3.86725e-06, Final residual = 4.98789e-10, No Iterations 1
DILUPBiCG:  Solving for k, Initial residual = 2.39725e-06, Final residual = 3.44058e-10, No Iterations 1
PIMPLE: converged in 6 iterations

Note: I removed some output from solving for U (the dots).