CFD Online Discussion Forums - delta T decreases suddenly in pimplefoam

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delta T decreases suddenly in pimplefoam

hi
I have simulated tiny pipe with newtoniam incompressible flow which is laminar by pimplefoam but the time steps are getting smaller and smaller during the solution process(it start from 0.003 and after 10 time steps gets to 10e-23 and more)and finally it is divrged .courant was set as 1 and nothing has changed by increasing the amounts of nOuterCorrectors ,nCorrectors and nNonOrthogonalCorrector.what should I do ?any suggestion?

Code:

Create time



Create polyMesh for time = 0



Time = 0



Mesh stats

    points:           9078

    faces:            90732

    internal faces:   83908

    cells:            43660

    faces per cell:   4

    boundary patches: 3

    point zones:      0

    face zones:       1

    cell zones:       1



Overall number of cells of each type:

    hexahedra:     0

    prisms:        0

    wedges:        0

    pyramids:      0

    tet wedges:    0

    tetrahedra:    43660

    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                  

    body                6496     3278     ok (non-closed singly connected)  

    outlet              164      98       ok (non-closed singly connected)  

    inlet               164      98       ok (non-closed singly connected)  



Checking geometry...

    Overall domain bounding box (-0.0215399 -0.0115756 -0.002365) (0.0198627 0.00851412 0.002365)

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

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

    Boundary openness (3.2476e-18 2.81028e-17 -1.76588e-17) OK.

    Max cell openness = 1.42371e-16 OK.

    Max aspect ratio = 6.62732 OK.

    Minimum face area = 4.49238e-08. Maximum face area = 2.90028e-07.  Face area magnitudes OK.

    Min volume = 3.78117e-12. Max volume = 4.92374e-11.  Total volume = 8.03586e-07.  Cell volumes OK.

    Mesh non-orthogonality Max: 56.6451 average: 19.2493

    Non-orthogonality check OK.

    Face pyramids OK.

    Max skewness = 0.702453 OK.

    Coupled point location match (average 0) OK.



Mesh OK.



End

thanks

By uploading more information about your case, you can get quick replies.

-
Best Regards!

thanks for your answer
these are fv solution and scheme

Code:

solvers

{

     p

    {

        solver          PCG;

        preconditioner  DIC;

        tolerance       1e-06;

        relTol          1e-05;

    }



    pFinal

    {

        solver          PCG;

        preconditioner  DIC;

        tolerance       1e-08;

        relTol          0;

    }



    "(U|k|epsilon)"

    {

        solver          PBiCG;

        preconditioner  DILU;

        tolerance       1e-05;

        relTol          0.1;

    }



    "(U|k|epsilon)Final"

    {

        $U;

        tolerance       1e-05;

        relTol          0;

    }

}



PIMPLE

{

    nOuterCorrectors 4;

    nCorrectors     2;

    nNonOrthogonalCorrectors 3;

    pRefCell        0;

    pRefValue       0;

residualControl

    {

        U

        {

            tolerance 1e-2;

            relTol 0;

        }



        p

        {

            tolerance 1e-2;

            relTol 0;

        }

    }

}



relaxationFactors

{

    fields

    {

    }

    equations

    {

        "p.*"           0.3;

        "U.*"           0.7;

        "k.*"           0.4;

        "epsilon.*"     0.5;

    }

}

Code:

ddtSchemes

{

    default         Euler;

}



gradSchemes

{

    default         Gauss linear;

    grad(p)         Gauss linear;

    grad(U)         Gauss linear;

}



divSchemes

{

    default         none;

    div(phi,U)      Gauss limitedLinearV 1;

    div(phi,k)      Gauss limitedLinear 1;

    div(phi,epsilon) Gauss limitedLinear 1;

    div(phi,R)      Gauss limitedLinear 1;

    div(R)          Gauss linear;

    div(phi,nuTilda) Gauss limitedLinear 1;

    div((nuEff*dev(T(grad(U))))) Gauss linear;

}



laplacianSchemes

{

    default         none;

    laplacian(nuEff,U) Gauss linear corrected;

    laplacian((1|A(U)),p) Gauss linear corrected;

    laplacian(DkEff,k) Gauss linear corrected;

    laplacian(DepsilonEff,epsilon) Gauss linear corrected;

    laplacian(DREff,R) Gauss linear corrected;

    laplacian(DnuTildaEff,nuTilda) Gauss linear corrected;

}



interpolationSchemes

{

    default         linear;

    interpolate(U)  linear;

}



snGradSchemes

{

    default         corrected;

}



fluxRequired

{

    default         no;

    p               ;

}

You're welcome!

In fvscheme file try these instead of mentioned ones. Best is to try with 1st order and check your results.

divSchemes
{
default none;
div(phi,U) Gauss upwind;
div(phi,k) Gauss upwind;
div(phi,epsilon) Gauss upwind;
div(phi,R) Gauss upwind;
div(R) Gauss linear;
div(phi,nuTilda) Gauss upwind;
div((nuEff*dev(T(grad(U))))) Gauss linear;
}

Let us know about your results. post your reply here

-
Best Regards!

that works and everyting is ok
thank you
but may I ask you to explain how did you find that I mean what the big differnces are there between these two div schemes?
thanks

Good!

Actually, there are various ways to avoid such situations. One such problem could be due to the above situation.

You can find the reference for these in any of the CFD books.

These "div schemes" differ in order of accuracy and many other facts.

Quick Hint: Upwind scheme is unconditionally stable of all.

thanks
As you know yhis is a first order scheme, thus too diffusive and I will not get a good solution,am I right?and what is the solution

Please, excuse for late reply. I was away from the desk.

Yes you're correct. With increased mesh resolution you can obtain very close solutions.

I don't know the details of your problem case. It's always wise to compare the CFD results with the available resources (experimental or analytical).

Anyways, with experience and literature survey you can gain grip over many of these CFD techniques.

Best of Luck for your future endeavour.

-
Best Regards!

thanks for protect and excuse me for lots of question (I am so curious ) but I promise that this is the last one :o
I think one way to decrease inaccuracy in using upwind linear in starting the algorithm with upwind and then changing it to more accurate schemes but I dont know when is the right time to do that,I mean is there any clue in the terminal that can aware us?
thanks alot

Actually, you can get comparable solution with upwind scheme too depending upon your problem case. Sometime, results obtained are comparable with the available resources. You can find many literature in which 1st order scheme give very near comparable solutions.

This is because CFD solution doesn't depend on these factors only. There are many other factors which are equally important for CFD solutions.

It's always best to monitor residuals. Generally, convergence check tell us when to stop our iterations.

I hope I tried to answer your queries. :)

-
Best Regards!