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Simulation Unstable in buoyant simulation

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Old   December 1, 2020, 09:38
Post Simulation Unstable in buoyant simulation
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Jamie MacLeod
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I have a simulation of an ONERA M6 wing which I've added a jettison pipe modification to, that I'm ultimately trying to simulate liquid coming out of during flight conditions.

I've run the simulation in rhoPimpleFoam, and got a stable solution out, but if I try to run the same case in a buoyant simulation such as compressibleInterFoam (without any alpha phase) or buoyantPimpleFoam, the simulation immediately becomes unstable. I've been trying to figure out what it is for weeks, and it could well be my boundary conditions, but i'm obviously unaware how. My primary confusion is the fact that it's stable when in a non-buoyant simulation, but highly unstable in a buoyant one.

I've zipped the case up and attached it, minus the mesh since it was generated with pointwise and not easily reproduceable. I have provided images of the surface and the pipe geometry, to provide context.

There is also the output from checkMesh, which indicates a large amount of non-orthogonality. These cells are primarily where the hex boundary layer transitions to the tetrahedrons, not near the surface. The y+ is 10 for the first cell height.
I'm also aware that there are several regions that have small cells sizes, which may not cooperate well with the floating precision. I do intend to look at upping the scale of the wing to help with that, which should also help with the compressible effects somewhat, but I don't think it should be causing such great problems.

Any help would be greatly appreciated!
Attached Images
File Type: png Pipe_Geom.png (70.5 KB, 6 views)
File Type: png M6_Geom.png (32.5 KB, 5 views)
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File Type: zip M6_Pipe.zip (18.9 KB, 1 views)
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Old   December 2, 2020, 04:39
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Hello Jamie,


two quick thoughts: It is sometimes suggested to turn off the momentum predictor. Additionally, have you tried setting all p-BCs to calculated?
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Old   December 2, 2020, 07:26
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Jamie MacLeod
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Hi Oswald,

Thanks for having a look. I have tried before with all calculated for the p file, and had no difference.

Today, I've scaled the mesh by 5 times, making Uinf ~40m/s to keep the same Reynolds, and increased y+ to 30 since I am using wall functions. So not quite the same problem, but still unstable. The instability now takes a bit longer to arise, but still consistently does.

Turning the momentum predictor off slows the instability down a tiny bit, but not significantly compared to being on. The transonic flag does similarly little to prevent it.
My main concern is omega consistently showing a large negative value, and that something in the turbulence model is not appropriate.

Code:
Starting time loop

Courant Number mean: 2.8829042e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 3.94826e-09

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 0.023131562, Final residual = 7.1347095e-09, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 1.0183002e-10, Final residual = 3.3822749e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 9.5831609e-15, global = -1.1065013e-16, cumulative = -1.1065013e-16
GAMG:  Solving for p_rgh, Initial residual = 4.2921739e-10, Final residual = 3.3684196e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 9.5888176e-15, global = -1.7729167e-16, cumulative = -2.879418e-16
DILUPBiCG:  Solving for omega, Initial residual = 8.3237803e-11, Final residual = 3.4218495e-13, No Iterations 1
bounding omega, min: -804650.5 max: 1.2524555e+08 average: 2126.5542
DILUPBiCG:  Solving for k, Initial residual = 1, Final residual = 2.0333098e-09, No Iterations 6
ExecutionTime = 26.48 s  ClockTime = 28 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 39.949151 in cell 3253496 at location (5.2392603 4.2801903 -0.0082252047)

Courant Number mean: 2.8828259e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 7.89652e-09

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 0.44498867, Final residual = 1.2772306e-09, No Iterations 3
GAMG:  Solving for p_rgh, Initial residual = 6.8971399e-10, Final residual = 3.7490253e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 5.1103629e-14, global = 4.0855221e-14, cumulative = 4.0567279e-14
GAMG:  Solving for p_rgh, Initial residual = 3.4822518e-12, Final residual = 3.775191e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.0484957e-13, global = 9.4236059e-14, cumulative = 1.3480334e-13
DILUPBiCG:  Solving for omega, Initial residual = 0.0015394324, Final residual = 1.1891344e-09, No Iterations 4
bounding omega, min: -811.64631 max: 2.7853173e+09 average: 7125.0224
DILUPBiCG:  Solving for k, Initial residual = 0.0068909976, Final residual = 1.0992279e-09, No Iterations 5
ExecutionTime = 48.72 s  ClockTime = 51 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 92.682749 in cell 3248383 at location (5.2390948 4.2799881 -0.0095646269)

Courant Number mean: 2.8828294e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 1.18448e-08

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 9.7257454e-06, Final residual = 8.434842e-16, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 6.8916607e-10, Final residual = 3.8090523e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 3.5935134e-14, global = -2.7031823e-14, cumulative = 1.0777152e-13
GAMG:  Solving for p_rgh, Initial residual = 2.4328413e-12, Final residual = 3.7691474e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.0506423e-13, global = 9.4441246e-14, cumulative = 2.0221276e-13
DILUPBiCG:  Solving for omega, Initial residual = 2.5335567e-12, Final residual = 1.1009214e-17, No Iterations 1
bounding omega, min: -34223738 max: 1.4575342e+09 average: 5271.0181
DILUPBiCG:  Solving for k, Initial residual = 0.00025081573, Final residual = 2.9421568e-12, No Iterations 1
ExecutionTime = 67.35 s  ClockTime = 70 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 152.33365 in cell 3248383 at location (5.2390948 4.2799881 -0.0095646269)

Courant Number mean: 2.8828294e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 1.5793e-08

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 6.2550572e-06, Final residual = 3.346361e-16, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 6.8916495e-10, Final residual = 3.4537457e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 3.6302365e-14, global = -2.7397395e-14, cumulative = 1.7481537e-13
GAMG:  Solving for p_rgh, Initial residual = 2.4574775e-12, Final residual = 3.6720688e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.0506881e-13, global = 9.4456031e-14, cumulative = 2.692714e-13
DILUPBiCG:  Solving for omega, Initial residual = 9.2616105e-06, Final residual = 5.1952765e-09, No Iterations 2
bounding omega, min: -3341.0684 max: 6.1965267e+17 average: 2.2906381e+11
DILUPBiCG:  Solving for k, Initial residual = 0.00017013508, Final residual = 6.2902545e-14, No Iterations 1
ExecutionTime = 87.54 s  ClockTime = 91 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 222.04675 in cell 3248383 at location (5.2390948 4.2799881 -0.0095646269)

Courant Number mean: 2.8828294e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 1.97413e-08

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 4.5676607e-06, Final residual = 7.8779657e-16, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 6.8915801e-10, Final residual = 3.6588109e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 3.6311189e-14, global = -2.7406608e-14, cumulative = 2.4186479e-13
GAMG:  Solving for p_rgh, Initial residual = 2.4581123e-12, Final residual = 3.5092387e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.0506953e-13, global = 9.4453226e-14, cumulative = 3.3631802e-13
DILUPBiCG:  Solving for omega, Initial residual = 3.5156543e-05, Final residual = 7.7106694e-21, No Iterations 1
bounding omega, min: -2.0576951e+11 max: 2.9245612e+11 average: 35650.964
DILUPBiCG:  Solving for k, Initial residual = 0.00039326169, Final residual = 1.0313711e-11, No Iterations 1
bounding k, min: -5.4867494e-06 max: 4.4611422 average: 1.1115964
ExecutionTime = 108.95 s  ClockTime = 113 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 294.59223 in cell 3248383 at location (5.2390948 4.2799881 -0.0095646269)

Courant Number mean: 2.8828294e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 2.36896e-08

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 3.563638e-06, Final residual = 1.1181637e-15, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 6.891581e-10, Final residual = 3.1975533e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 3.6310467e-14, global = -2.7404093e-14, cumulative = 3.0891392e-13
GAMG:  Solving for p_rgh, Initial residual = 2.4581601e-12, Final residual = 3.5894277e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.0506911e-13, global = 9.4452608e-14, cumulative = 4.0336653e-13
DILUPBiCG:  Solving for omega, Initial residual = 4.2516325e-11, Final residual = 4.1016135e-13, No Iterations 1
bounding omega, min: -722000.77 max: 6.461223e+14 average: 2.1719337e+08
DILUPBiCG:  Solving for k, Initial residual = 0.00046801599, Final residual = 6.4458864e-13, No Iterations 1
ExecutionTime = 127.23 s  ClockTime = 131 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 364.51449 in cell 3248383 at location (5.2390948 4.2799881 -0.0095646269)

Courant Number mean: 2.8828294e-08 max: 0.80000252
deltaT = 3.948262e-09
Time = 2.76378e-08

diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
PIMPLE: iteration 1
DILUPBiCG:  Solving for e, Initial residual = 2.8935767e-06, Final residual = 1.2628636e-15, No Iterations 1
GAMG:  Solving for p_rgh, Initial residual = 6.8915642e-10, Final residual = 3.0947747e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 3.6315208e-14, global = -2.7401581e-14, cumulative = 3.7596495e-13
GAMG:  Solving for p_rgh, Initial residual = 2.4581833e-12, Final residual = 3.6000695e-16, No Iterations 1
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.0506567e-13, global = 9.4453834e-14, cumulative = 4.7041878e-13
DILUPBiCG:  Solving for omega, Initial residual = 3.854097e-09, Final residual = 1.1162769e-18, No Iterations 1
bounding omega, min: -3.7312489e+12 max: 5.5103376e+11 average: -940223.91
DILUPBiCG:  Solving for k, Initial residual = 0.00053724916, Final residual = 4.9795953e-10, No Iterations 1
ExecutionTime = 146.81 s  ClockTime = 151 s

fieldMinMax minMax write:
    min(mag(U)) = 0 in cell 3165841 at location (5.2325182 4.2573145 -0.0019748748)
    max(mag(U)) = 426.735 in cell 3248383 at location (5.2390948 4.2799881 -0.0095646269)
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buoyant flow, buoyant solver, buoyantpimplefoam, compressibleinterfoam

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