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negative min.(alpha1) value interFoam

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Old   October 19, 2014, 14:38
Default negative min.(alpha1) value interFoam
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Arjun
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Hi...
I am new to openFoam, for the past week I am stuck with my problem. I am trying to simulate water falling on a circular tube surface. interFoam was selected as the base solver with dambreak as the base case.
slight modification were made with files of dambreak.

internalField was set as uniform 0. alpha.water for inlet as type fixedValue and value uniform 1. Wall B.C's as zeroGradient.

p_rgh B.C's
internalField uniform 0
walls as type fixedFluxPressure; value uniform 0;
inlet as type fixedValue; value uniform 0;

U B.C's
internalField uniform 0
wall as type fixedValue; value uniform (0 0 0);
inlet as type flowRateInletVelocity; massFlowRate constant 0.1; rhoInlet 1;

setFields as
defaultFieldValues
(
volScalarFieldValue alpha.water 0
);

After some time steps min(alpha1) deviates to too negative value. the terminal shows

Time = 0.000215

smoothSolver: Solving for alpha.water, Initial residual = 0.135945, Final residual = 4.85033e-09, No Iterations 6
Phase-1 volume fraction = -3.89404e-14 Min(alpha1) = -0.494288 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -3.89404e-14 Min(alpha1) = -0.494288 Max(alpha1) = 1
DICPCG: Solving for p_rgh, Initial residual = 0.421282, Final residual = 0.0204154, No Iterations 6
time step continuity errors : sum local = 9.04479e-07, global = -1.89978e-07, cumulative = -5.23877e-07
DICPCG: Solving for p_rgh, Initial residual = 0.304169, Final residual = 0.0149129, No Iterations 5
time step continuity errors : sum local = 8.80687e-07, global = -1.71627e-07, cumulative = -6.95504e-07
DICPCG: Solving for p_rgh, Initial residual = 0.244908, Final residual = 9.59156e-08, No Iterations 462
time step continuity errors : sum local = 6.22699e-12, global = -1.30827e-14, cumulative = -6.95504e-07
ExecutionTime = 3316.69 s ClockTime = 4119 s

Courant Number mean: 5.63527e-05 max: 5.31579
Interface Courant Number mean: 2.13444e-06 max: 2.08533
Time = 0.00022

smoothSolver: Solving for alpha.water, Initial residual = 0.173116, Final residual = 5.69664e-09, No Iterations 14
Phase-1 volume fraction = -5.2177e-14 Min(alpha1) = -0.70481 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -5.2177e-14 Min(alpha1) = -0.70481 Max(alpha1) = 1
DICPCG: Solving for p_rgh, Initial residual = 0.381394, Final residual = 0.0167599, No Iterations 10
time step continuity errors : sum local = 1.58839e-06, global = -2.25771e-07, cumulative = -9.21275e-07
DICPCG: Solving for p_rgh, Initial residual = 0.271973, Final residual = 0.0121764, No Iterations 5
time step continuity errors : sum local = 1.32254e-06, global = -6.32625e-08, cumulative = -9.84538e-07
DICPCG: Solving for p_rgh, Initial residual = 0.191749, Final residual = 9.99155e-08, No Iterations 469
time step continuity errors : sum local = 1.11654e-11, global = 9.08119e-14, cumulative = -9.84538e-07
ExecutionTime = 3398.03 s ClockTime = 4207 s

Courant Number mean: 8.40514e-05 max: 8.08132
Interface Courant Number mean: 4.53269e-06 max: 4.24642
Time = 0.000225

smoothSolver: Solving for alpha.water, Initial residual = 0.170608, Final residual = 3.34717e-09, No Iterations 10
Phase-1 volume fraction = -1.10859e-13 Min(alpha1) = -0.56912 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -1.10859e-13 Min(alpha1) = -0.56912 Max(alpha1) = 1
DICPCG: Solving for p_rgh, Initial residual = 0.418703, Final residual = 0.0192012, No Iterations 5
time step continuity errors : sum local = 2.23831e-06, global = -6.45109e-08, cumulative = -1.04905e-06
DICPCG: Solving for p_rgh, Initial residual = 0.264399, Final residual = 0.0117227, No Iterations 5
time step continuity errors : sum local = 1.56011e-06, global = 2.27227e-07, cumulative = -8.21821e-07
DICPCG: Solving for p_rgh, Initial residual = 0.161567, Final residual = 9.91333e-08, No Iterations 465
time step continuity errors : sum local = 1.33674e-11, global = -9.29279e-14, cumulative = -8.21821e-07
ExecutionTime = 3476.99 s ClockTime = 4294 s

Courant Number mean: 0.000106596 max: 9.31549
Interface Courant Number mean: 5.9444e-06 max: 3.21124
Time = 0.00023

smoothSolver: Solving for alpha.water, Initial residual = 0.174622, Final residual = 8.06469e-09, No Iterations 8
Phase-1 volume fraction = -1.36574e-13 Min(alpha1) = -1.08339 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -1.36574e-13 Min(alpha1) = -1.08339 Max(alpha1) = 1
DICPCG: Solving for p_rgh, Initial residual = 0.563562, Final residual = 0.0271184, No Iterations 4
time step continuity errors : sum local = 3.22933e-06, global = -3.09184e-07, cumulative = -1.13101e-06
DICPCG: Solving for p_rgh, Initial residual = 0.325179, Final residual = 0.0159427, No Iterations 5
time step continuity errors : sum local = 2.12216e-06, global = -2.7194e-07, cumulative = -1.40295e-06
DICPCG: Solving for p_rgh, Initial residual = 0.218524, Final residual = 9.34715e-08, No Iterations 480
time step continuity errors : sum local = 1.2633e-11, global = 8.28896e-14, cumulative = -1.40295e-06
ExecutionTime = 3557.34 s ClockTime = 4382 s

Courant Number mean: 0.000124486 max: 9.14801
Interface Courant Number mean: 5.45008e-06 max: 1.54779
Time = 0.000235

smoothSolver: Solving for alpha.water, Initial residual = 0.203736, Final residual = 9.52402e-09, No Iterations 17
Phase-1 volume fraction = -3.94801e-13 Min(alpha1) = -0.606969 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -3.94801e-13 Min(alpha1) = -0.606969 Max(alpha1) = 1
DICPCG: Solving for p_rgh, Initial residual = 0.110379, Final residual = 0.00435067, No Iterations 2
time step continuity errors : sum local = 9.18565e-06, global = -8.93057e-08, cumulative = -1.49225e-06
DICPCG: Solving for p_rgh, Initial residual = 0.0295617, Final residual = 0.00142974, No Iterations 5
time step continuity errors : sum local = 3.09834e-06, global = 3.46288e-07, cumulative = -1.14596e-06
DICPCG: Solving for p_rgh, Initial residual = 0.0228198, Final residual = 9.75055e-08, No Iterations 453
time step continuity errors : sum local = 2.164e-10, global = -9.17758e-13, cumulative = -1.14596e-06
ExecutionTime = 3635.92 s ClockTime = 4469 s

Courant Number mean: 0.000156587 max: 22.8963
Interface Courant Number mean: 7.49421e-06 max: 2.03706
Time = 0.00024

smoothSolver: Solving for alpha.water, Initial residual = 0.20387, Final residual = 3.9583e-09, No Iterations 13
Phase-1 volume fraction = 6.13117e-13 Min(alpha1) = -4.09164 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 6.13117e-13 Min(alpha1) = -4.09164 Max(alpha1) = 1
DICPCG: Solving for p_rgh, Initial residual = 0.749905, Final residual = 0.0333727, No Iterations 9
time step continuity errors : sum local = 9.46227e-06, global = -3.61406e-06, cumulative = -4.76002e-06
DICPCG: Solving for p_rgh, Initial residual = 0.307627, Final residual = 0.01456, No Iterations 6
time step continuity errors : sum local = 6.64208e-06, global = -1.50695e-06, cumulative = -6.26697e-06
DICPCG: Solving for p_rgh, Initial residual = 0.160218, Final residual = 9.90744e-08, No Iterations 489
time step continuity errors : sum local = 4.83966e-11, global = 2.85499e-13, cumulative = -6.26697e-06
ExecutionTime = 3718.94 s ClockTime = 4560 s

Courant Number mean: 0.000414007 max: 145.401
Interface Courant Number mean: 9.92862e-06 max: 5.30545
Time = 0.000245

smoothSolver: Solving for alpha.water, Initial residual = 0.426234, Final residual = 8.67918e-09, No Iterations 50
Phase-1 volume fraction = -6.7424e-14 Min(alpha1) = -5.62255 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -6.7424e-14 Min(alpha1) = -5.54044 Max(alpha1) = 1.9061
DICPCG: Solving for p_rgh, Initial residual = 0.801868, Final residual = 0.0396942, No Iterations 4
time step continuity errors : sum local = 2.10043e-05, global = -7.37201e-06, cumulative = -1.3639e-05
DICPCG: Solving for p_rgh, Initial residual = 0.166809, Final residual = 0.00802116, No Iterations 6
time step continuity errors : sum local = 2.94754e-05, global = -1.06916e-05, cumulative = -2.43305e-05
exit
DICPCG: Solving for p_rgh, Initial residual = 0.330884, Final residual = 9.89421e-08, No Iterations 501
time step continuity errors : sum local = 3.14305e-10, global = 6.02658e-13, cumulative = -2.43305e-05
ExecutionTime = 3811.48 s ClockTime = 4667 s

Courant Number mean: 0.00181847 max: 479.069
Interface Courant Number mean: 0.000605703 max: 479.069
Time = 0.00025

smoothSolver: Solving for alpha.water, Initial residual = 0.702312, Final residual = 9.28023e-09, No Iterations 66
Phase-1 volume fraction = -3.41965e-12 Min(alpha1) = -18.9554 Max(alpha1) = 1
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = -3.41965e-12 Min(alpha1) = -15.4267 Max(alpha1) = 6.79324
DICPCG: Solving for p_rgh, Initial residual = 0.563373, Final residual = 0.0229745, No Iterations 5
time step continuity errors : sum local = 4.96062e-05, global = -1.45167e-05, cumulative = -3.88473e-05

really confused with what to do?
Thanks in advance.
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Old   October 20, 2014, 02:54
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Fabian Roesler
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From the first glimpse I'd say you miss an outlet boundary. But I can't tell for sure until you post your p_rgh an U files. You let water flow into a domain. As for interFoam, all phases including gas/air are incompressible, some material has to leave the domain to fulfill continuity. Add an atmosphere at the top or an outlet boundary at the bottom of your domain.

Cheers

Fabian
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Old   October 20, 2014, 03:38
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Dear Fabian,
Thanks for your time....
My p_rgh...
internalField uniform 0;

boundaryField
{
WALL
{
type fixedFluxPressure;
value uniform 0;
}

TUBEWALL
{
type fixedFluxPressure;
value uniform 0;
}
INLET
{
type fixedFluxPressure;
value uniform 0;
}
OUTLET
{
type totalPressure;
value uniform 0;
p0 uniform 0;
U U;
phi phi;
gamma 1;
value uniform 1;
}

FRONTANDBACK
{

type fixedFluxPressure;
value uniform 0;
}
}

U file...
internalField uniform (0 0 0);

boundaryField
{
WALL
{
type fixedValue;
value uniform (0 0 0);
}
TUBEWALL
{
type fixedValue;
value uniform (0 0 0);
}
INLET
{
type fixedValue;
value uniform (0 0.03 0);

}
OUTLET
{
type pressureInletOutletVelocity;
value uniform (0 0.03 0);
}
FRONTANDBACK
{
type fixedValue;
value uniform (0 0 0);
}

}

thanks in advance
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Old   October 20, 2014, 04:38
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Fabian Roesler
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So now I see, that you have an atmosphere boundary condition. So my first guess was wrong. But I also see, that your Courant number is quite big for multi phase simulations with interFoam. Could you post a picture of your simulation domain and the mesh you use? You could try to increase the nonOrthCorrectors and do a checkMesh to see if your mesh is sufficient for simulation.
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Old   October 20, 2014, 04:42
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An one more question! Do you use fixed time steps? This could be the reason for the high Courant number. Change adjustTimeSteps to true in controlDict.
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Old   October 20, 2014, 05:23
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Dear Fabian,

checkMesh will give....

Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
points: 161101
faces: 1658928
internal faces: 1571476
cells: 807601
faces per cell: 4
boundary patches: 7
point zones: 0
face zones: 0
cell zones: 0

Overall number of cells of each type:
hexahedra: 0
prisms: 0
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 807601
polyhedra: 0

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 Points Surface topology
INLET 2995 1595 ok (non-closed singly connected)
WALL 9201 4856 ok (non-closed singly connected)
SPLITWALL 16917 8651 ok (non-closed singly connected)
FRONTANDBACK 28636 15114 ok (non-closed singly connected)
TUBEWALL 14408 7572 ok (non-closed singly connected)
SYMMETRY 9446 4942 ok (non-closed singly connected)
OUTLET 5849 3101 ok (non-closed singly connected)

Checking geometry...
Overall domain bounding box (0 0 0) (20 35.4 10)
Mesh (non-empty, non-wedge) directions (1 1 1)
Mesh (non-empty) directions (1 1 1)
Boundary openness (1.87929e-15 -7.48659e-16 -1.06194e-16) OK.
Max cell openness = 2.68207e-16 OK.
Max aspect ratio = 5.77614 OK.
Minimum face area = 0.00152086. Maximum face area = 0.682333. Face area magnitudes OK.
Min volume = 3.19342e-05. Max volume = 0.158068. Total volume = 4547.61. Cell volumes OK.
Mesh non-orthogonality Max: 56.2501 average: 14.9404
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.646766 OK.
Coupled point location match (average 0) OK.

Mesh OK.
1.jpg

2.jpg

3.jpg
End

Snapshots of mesh

currently i am working with fixed time step. a dynamic time step is leading to very small time step.

thanks
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Old   October 20, 2014, 05:58
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That's the point. you have to keep Co below 1 even if this leads to small dT. I see you have a tet-Mesh. So I'd increase the nonOrth correctors to 2 and use adjustTimeStep. This should solve your problems.

Cheers

Fabian
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Old   October 20, 2014, 06:17
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Dear Fabian,

Currently working with adjusttimestep. time step in the order of e-6....will update the results soon,

Thanks
Arjun
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Old   October 21, 2014, 06:35
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Dear Fabian,

Carried out simulations for 0.008 seconds. The results obtained are confusing. It looks like no flow have happened. velocity and pressure ranges are so high after 0.008 seconds.
4.jpg alpha

5.jpg pressure

6.jpg velocity

I doubt some thing have gone wrong with the boundary condition. Please have a look at the B.C's posted in earlier comment. only variation made from above is that of outlet velocity where I have used
OUTLET
{
type fluxCorrectedVelocity;
phi phi;
rho rho;
}
Since I am unsure of the velocity at outlet.

Thanks in advance
Arjun
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Old   October 21, 2014, 07:33
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Have you increased the nonOrth correctos? As you have a tet mesh you should adapt your fvSchemes and fvSolution. And moreover, as you have a small inlet velocity of 0.03 m/s, the phase will only travel 0.00024 m in the first 0.008 s. I guess your mesh is not that fine.
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Old   October 21, 2014, 07:42
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Dear Fabian,
Yes I have increased the nNonOrthogonalCorrectors to 2. I agree that the time for which simulation have done is too low but why i am i getting high pressure and velocity range, also alpha is still appreciably negativeand becomes more negative with time step.

Thanks in advance
Arjun

Last edited by Arjun Jayakumar; October 21, 2014 at 13:48.
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