CFD Online Discussion Forums - pressure eq. "converges" after few time steps

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- OpenFOAM Running, Solving & CFD (https://www.cfd-online.com/Forums/openfoam-solving/)

- - pressure eq. "converges" after few time steps (https://www.cfd-online.com/Forums/openfoam-solving/81461-pressure-eq-converges-after-few-time-steps.html)

pressure eq. "converges" after few time steps

Hi everybody,
weird simpleFoam convergence over here, need your help!
I have a complex pipes geometry, similar to what sketched in the geom.png file. The two main pipes are connected by fans to the outside, which is represented by a spherical domain. Reynolds is around 21000 on the smallest pipe, thus a launderSharmaKE model is applied, using wallfunction to keep low the cell number. In any case, the mesh is not really fine since I first want to evaluate my setup. BC are standard:

external domain:
- U, epsilon, k inletOutlet;
- p fixedValue 0;
pipes:
- U: fixedValue;
- epsilon, k wallFunction;
- p zeroGradient;

fvSchemes is as follow:

Code:

grad         faceMDLimited Gauss linear 0.5;

div         Gauss linearUpwind cellLimited Gauss linear 1;

laplacian   Gauss linear limited 0.5;

while I tried different combinations for fvSolution:

Code:

this is the first one:

    p

    {

        solver          GAMG;

        tolerance       1e-06;

        relTol          0;

        smoother        GaussSeidel;

        nPreSweeps      0;

        nPostSweeps     2;

        cacheAgglomeration true;

        nCellsInCoarsestLevel 10;

        agglomerator    faceAreaPair;

        mergeLevels     1;

    }



    U epsilon k

    {

        solver          smoothSolver;

        smoother        GaussSeidel;

        tolerance       1e-04;

        relTol          0;

    }

and the second one:

Code:

   p

    {

        solver          GAMG;

        tolerance       1e-6;

        relTol          1e-3;

        smoother        GaussSeidel;

        nPreSweeps      0;

        nPostSweeps     2;

        cacheAgglomeration true;

        nCellsInCoarsestLevel 10;

        agglomerator    faceAreaPair;

        mergeLevels     1;

    }



    U

    {

        solver          PBiCG;

        preconditioner  DILU;

        tolerance       1e-4;

        relTol          1e-3;

    }

    

    k epsilon

    {

        solver          smoothSolver;

        smoother    GaussSeidel;

        tolerance       1e-4;

        relTol          1e-3;

    }

I ended up with this log file:

Code:

Time = 1



smoothSolver:  Solving for Ux, Initial residual = 0, Final residual = 0, No Iterations 0    //correct, I applied two fans!

smoothSolver:  Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0

smoothSolver:  Solving for Uz, Initial residual = 0, Final residual = 0, No Iterations 0

GAMG:  Solving for p, Initial residual = 1, Final residual = 9.59218e-07, No Iterations 393

GAMG:  Solving for p, Initial residual = 9.29457e-08, Final residual = 9.29457e-08, No Iterations 0

GAMG:  Solving for p, Initial residual = 9.29457e-08, Final residual = 9.29457e-08, No Iterations 0

time step continuity errors : sum local = 0.00122424, global = -1.32307e-12, cumulative = -1.32307e-12

smoothSolver:  Solving for epsilon, Initial residual = 0.454767, Final residual = 9.2459e-06, No Iterations 1

smoothSolver:  Solving for k, Initial residual = 1, Final residual = 7.38315e-05, No Iterations 1

ExecutionTime = 174.05 s  ClockTime = 175 s



Time = 2



smoothSolver:  Solving for Ux, Initial residual = 0.141374, Final residual = 5.00021e-05, No Iterations 5

smoothSolver:  Solving for Uy, Initial residual = 0.272753, Final residual = 7.82825e-05, No Iterations 5

smoothSolver:  Solving for Uz, Initial residual = 0.202637, Final residual = 8.15781e-05, No Iterations 5

GAMG:  Solving for p, Initial residual = 8.8497e-08, Final residual = 8.8497e-08, No Iterations 0

GAMG:  Solving for p, Initial residual = 8.8497e-08, Final residual = 8.8497e-08, No Iterations 0

GAMG:  Solving for p, Initial residual = 8.8497e-08, Final residual = 8.8497e-08, No Iterations 0

time step continuity errors : sum local = 0.000634968, global = 7.45987e-07, cumulative = 7.45986e-07

smoothSolver:  Solving for epsilon, Initial residual = 0.164053, Final residual = 2.52305e-06, No Iterations 1

smoothSolver:  Solving for k, Initial residual = 0.24006, Final residual = 1.351e-05, No Iterations 1

ExecutionTime = 192.39 s  ClockTime = 194 s

Does this implies that the pressure is already converged at the second time step??? :eek:
This does not change when:
- reducing the nNonOrthogonalCorrectors;
- using the second fvSchemes files;
- switching off the turbulence.

Any explanation on that? Really no ideas...

thank you!

maddalena

Hi,

try lowering the tolerances of the linear solvers. Something in the order of 1e-12 might be appropriate.

Remember, your simulation is not converged if the residuals of just one equation fall below a certain tolerance. The pressure field depends on the velocity field and vice versa - since your velocity field clearly hasn't converged after the second iteration your whole problem hasn't (would be very surprising if it had) and the pressure equation might and probably will be re-solved at some point.

Greetings,
Felix.

Dear Maddalena,

I would try also changing the initial conditions: I am not aware of your problem but maybe you can try to force the raising of pressure gradients by assigning non zero values for the velocity in the pipes and zero outside.

Regards,

Franco

Thanks Felix and Francesco for your suggestions.
Some observations after some more try:

Quote:

Originally Posted by FelixL (Post 281036)

try lowering the tolerances of the linear solvers. Something in the order of 1e-12 might be appropriate.

That helped a lot. Now I have p and U equations that are solved at every time step.

Quote:

Originally Posted by FelixL (Post 281036)

The pressure field depends on the velocity field and vice versa - since your velocity field clearly hasn't converged after the second iteration your whole problem hasn't (would be very surprising if it had) and the pressure equation might and probably will be re-solved at some point..

Yes, of course I have a feeling of this. But is it correct to first solve U equation and than, after some time, solve p? Is this not longer than solve all the equations at the same time?

Quote:

Originally Posted by francescomarra (Post 281159)

I would try also changing the initial conditions: I am not aware of your problem but maybe you can try to force the raising of pressure gradients by assigning non zero values for the velocity in the pipes and zero outside.

About the boundary conditions I am almost sure that everything is fine, since I have already used this setup in the past. However, you suggest to initialize the pipe field with different velocity values, is this correct? If so, how can I do that?

What I have got up to now is a converging but unstable solution: I have bounding epsilon and k warnings, but a convergent solution till time step 295. After that, the max epsilon and k raise suddently and the solution blows away:

Code:

Time = 295



DILUPBiCG:  Solving for Ux, Initial residual = 0.11505, Final residual = 8.32225e-11, No Iterations 13

DILUPBiCG:  Solving for Uy, Initial residual = 0.934182, Final residual = 7.36365e-11, No Iterations 14

DILUPBiCG:  Solving for Uz, Initial residual = 0.713126, Final residual = 4.13259e-11, No Iterations 13

GAMG:  Solving for p, Initial residual = 1.86515e-09, Final residual = 9.94925e-13, No Iterations 206

GAMG:  Solving for p, Initial residual = 6.88289e-10, Final residual = 7.66889e-13, No Iterations 9

time step continuity errors : sum local = 4.67002e-09, global = 1.51568e-10, cumulative = -2.05894e-08

smoothSolver:  Solving for epsilon, Initial residual = 0.00158922, Final residual = 7.46827e-11, No Iterations 24

bounding epsilon, min: 1.30954e-18 max: 5812.76 average: 93.9024

smoothSolver:  Solving for k, Initial residual = 7.82377e-09, Final residual = 4.92151e-11, No Iterations 5

bounding k, min: 6.82659e-17 max: 14.436 average: 0.424139

ExecutionTime = 65389.8 s  ClockTime = 65515 s



Time = 296



DILUPBiCG:  Solving for Ux, Initial residual = 0.328206, Final residual = 5.57292e-11, No Iterations 33

DILUPBiCG:  Solving for Uy, Initial residual = 0.904646, Final residual = 3.81614e-11, No Iterations 30

DILUPBiCG:  Solving for Uz, Initial residual = 0.678265, Final residual = 3.27509e-11, No Iterations 29

GAMG:  Solving for p, Initial residual = 0.000198723, Final residual = 9.87374e-13, No Iterations 723

GAMG:  Solving for p, Initial residual = 5.88726e-07, Final residual = 9.89924e-13, No Iterations 273

time step continuity errors : sum local = 2.72317e-07, global = -1.44975e-09, cumulative = -2.20392e-08

smoothSolver:  Solving for epsilon, Initial residual = 0.998411, Final residual = 5.92556e-11, No Iterations 35

bounding epsilon, min: 1.30845e-18 max: 7.92429e+07 average: 1496.69

smoothSolver:  Solving for k, Initial residual = 1.39098e-07, Final residual = 5.71729e-11, No Iterations 5

bounding k, min: -2.01645e-14 max: 449863 average: 2.44943

ExecutionTime = 65807.2 s  ClockTime = 65932 s



Time = 297



DILUPBiCG:  Solving for Ux, Initial residual = 0.693738, Final residual = 2.49235e-11, No Iterations 28

DILUPBiCG:  Solving for Uy, Initial residual = 0.745261, Final residual = 2.71405e-11, No Iterations 28

DILUPBiCG:  Solving for Uz, Initial residual = 0.715263, Final residual = 2.6154e-11, No Iterations 28

GAMG:  Solving for p, Initial residual = 1.06952e-05, Final residual = 9.67823e-13, No Iterations 738

GAMG:  Solving for p, Initial residual = 3.03894e-09, Final residual = 9.87472e-13, No Iterations 31

time step continuity errors : sum local = 0.00097431, global = 7.21803e-06, cumulative = 7.196e-06

smoothSolver:  Solving for epsilon, Initial residual = 0.587168, Final residual = 7.92945e-11, No Iterations 18

bounding epsilon, min: 1.56953e-17 max: 9.66762e+12 average: 6.49312e+07

smoothSolver:  Solving for k, Initial residual = 0.00708516, Final residual = 1.51465e-11, No Iterations 11

bounding k, min: -3.00985e-15 max: 3.33247e+10 average: 154254

ExecutionTime = 66136 s  ClockTime = 66261 s



Time = 298



DILUPBiCG:  Solving for Ux, Initial residual = 0.652017, Final residual = 5.22231e-11, No Iterations 23

DILUPBiCG:  Solving for Uy, Initial residual = 0.523133, Final residual = 6.68451e-11, No Iterations 21

DILUPBiCG:  Solving for Uz, Initial residual = 0.677302, Final residual = 8.86501e-11, No Iterations 22

GAMG:  Solving for p, Initial residual = 4.62234e-08, Final residual = 9.77244e-13, No Iterations 566

GAMG:  Solving for p, Initial residual = 2.08294e-10, Final residual = 7.37324e-13, No Iterations 4

time step continuity errors : sum local = 0.0110342, global = 0.000335115, cumulative = 0.000342311

smoothSolver:  Solving for epsilon, Initial residual = 0.00963571, Final residual = 9.99631e-11, No Iterations 26

bounding epsilon, min: -2.55095e-15 max: 2.70603e+18 average: 2.26696e+13

smoothSolver:  Solving for k, Initial residual = 0.00019889, Final residual = 6.88818e-11, No Iterations 21

bounding k, min: -2.62403e-13 max: 6.83816e+11 average: 6.84336e+06

ExecutionTime = 66386.5 s  ClockTime = 66512 s

I tried to reduce relaxation (till 0.3 on U, epsilon and k and 0.15 on p) and switch to upwind. No way. The weird thing is that the solution looks good at time 295!
Ideas?

maddalena

You could look at the individual cell residuals? That might help you figure out what's causing the instability.

Quote:

Originally Posted by l_r_mcglashan (Post 281833)

You could look at the individual cell residuals?

That sounds interesting. How can I do that?

I need to do it myself and am just about to look into it :D

You can jump ahead here:
http://www.cfd-online.com/Forums/ope...residuals.html

Quote:

Originally Posted by l_r_mcglashan (Post 281842)

I need to do it myself and am just about to look into it

Ok, so you need a workmate! I will look into it as well.
Stay in touch.

mad

Quote:

Originally Posted by l_r_mcglashan (Post 281833)

You could look at the individual cell residuals

Done. I modified the simpleFoamResidual utility that hrvoje posted in order to match the new turbulence definition. What I get is a plot showing that the uResidual are higher at the corners of my pipe geometry, thus on the points where the flow is more difficult to model. Does this mean that my mesh is not fine enough on those points?

mad

Same. Popped it up on github:

http://github.com/lrm29/OpenFOAM.loc...dualCalculator

OpenFOAM-1.7.x has some errorEstimation libraries which would be nice to use. The residual that simpleFoamResidual calculates needs to be normalised (by the initial state possibly?) so that the changes from step to step can be seen more clearly.

Quote:

Originally Posted by maddalena (Post 281857)

What I get is a plot showing that the uResidual are higher at the corners of my pipe geometry, thus on the points where the flow is more difficult to model. Does this mean that my mesh is not fine enough on those points?

Were the residuals increasing in those areas? It's difficult to say, there are a number of problems it could be related to. Versteeg and Malalasekera's book has a nice chapter on possible causes of problems.

Another thing that may be worth checking is that the mass flow into and out of your domain matches.

Why my solutiion is not converging?

Hello,
still here with the same problem on a different case: pressure & velocity fields look good and their residual's trend is "converging"; however local value of residuals (simpleFoamResidual) does not looks good and I have weird oscillations on the initial residual of every time step.
My mesh (tet mesh with no boundary layer) is nice. I have tried tons of different BC-fvSchemes-fvSolution settings. No way.
May this be due to the high number of cyclic patches I am using? Two fans BC + cyclic sides.
I do not what to think now. Suggestions?

mad

Reversed problem!

Ok, maybe this is not the right place where to post, but I hope to get some answer on a problem that is similar to what posted above...
I started this thread speaking of a pressure equation that converges too soon... And I am writing now for a pressure equation that is never solved within the 1000 iterations of a time step! Geometry and bc are similar to what described above, only the pipe geometry is a little bit more complex than what sketched. Check mesh does not complain about that:

Code:

    Overall domain bounding box (-37.4532 -6.70564 -3.99289e-17) (42.605 6.70578 27.2094)

    Mesh (non-empty, non-wedge) directions (1 1 1)

    Mesh (non-empty) directions (1 1 1)

    Boundary openness (-2.78883e-18 -1.17153e-15 -2.36782e-14) OK.

    Max cell openness = 3.29759e-16 OK.

    Max aspect ratio = 42.4261 OK.

    Minumum face area = 1.27273e-06. Maximum face area = 9.60387.  Face area magnitudes OK.

    Min volume = 1.12921e-09. Max volume = 8.07969.  Total volume = 9723.47.  Cell volumes OK.

    Mesh non-orthogonality Max: 69.699 average: 18.046

    Non-orthogonality check OK.

    Face pyramids OK.

    Max skewness = 0.956692 OK

.
fvSchemes and fvSolution are:

Code:

grad         faceMDLimited Gauss linear 0.5;

div         Gauss linearUpwind cellLimited Gauss linear 1;

laplacian   Gauss linear limited 0.5;

Code:

p

    {

        solver          GAMG;

        tolerance       1e-10;

        relTol          0;

        smoother        GaussSeidel;

        nPreSweeps      0;

        nPostSweeps     2;

        cacheAgglomeration true;

        nCellsInCoarsestLevel 10;

        agglomerator    faceAreaPair;

        mergeLevels     1;

    }



    U

    {

        solver          PBiCG;

        preconditioner  DILU;

        tolerance       1e-08;

        relTol          0;

    }

    

    k epsilon

    {

        solver          smoothSolver;

        smoother    GaussSeidel;

        tolerance       1e-08;

        relTol          0;

    }

nNonOrthogonalCorrectors 3;

relaxationFactors

    p: 0.15;

    U, k, epsilon: 0.3;

and this is my weird log.simpleFoam file:

Code:

DILUPBiCG:  Solving for Ux, Initial residual = 0.0028965, Final residual = 2.16561e-11, No Iterations 7

DILUPBiCG:  Solving for Uy, Initial residual = 0.00286544, Final residual = 2.35329e-11, No Iterations 7

DILUPBiCG:  Solving for Uz, Initial residual = 0.00271231, Final residual = 2.42359e-11, No Iterations 7

GAMG:  Solving for p, Initial residual = 0.127338, Final residual = 7.19827e-06, No Iterations 1000

GAMG:  Solving for p, Initial residual = 0.0408166, Final residual = 2.54205e-06, No Iterations 1000

GAMG:  Solving for p, Initial residual = 0.0144267, Final residual = 1.11529e-06, No Iterations 1000

GAMG:  Solving for p, Initial residual = 0.00831105, Final residual = 1.09388e-07, No Iterations 1000

time step continuity errors : sum local = 8.4358e-08, global = -1.12046e-09, cumulative = 7.57121e-10

smoothSolver:  Solving for epsilon, Initial residual = 0.0201266, Final residual = 4.78163e-11, No Iterations 10

smoothSolver:  Solving for k, Initial residual = 0.00307404, Final residual = 3.2731e-11, No Iterations 10

I am using 4 nonOrthogonalCorrectors in order to try to lower p residuals. However, as you can see, pressure equation does not reaches convergence within the 1000 iterations x 4 of each time step. Of course, velocity, turbulence and pressure solution field are far to be as expected.
What I should do? What to change? I really need a help from you!

mad

maddalena

Are you doing some internal loops?

Hello,
fastest answer ever!

Quote:

Originally Posted by makaveli_lcf (Post 293912)

Are you doing some internal loops?

No, that is standard simpleFoam, thus no internal loop.

mad

:cool:

could U please post more of your log?

Yes, sure. Here it is.
As you can see, something strange happens around time step 17 on Uz. On the contrary, pressure does what explained above.:confused:

mad

... and your fvScheme and fvSolution in the studio please)))

Quote:

Originally Posted by makaveli_lcf (Post 293916)

... and your fvScheme and fvSolution in the studio please)))

Written above. However, I tried different combination of them. The log file posted above refers to these two files.

mad

Maybe it is worth adding that my mesh is tetra mainly. I used prisms in proximity of fans, for the reason explained here. Also, the answer I get here is a bit worrying. Any experience on the subject?

mad