[trailer]

Hello to all,


I am new to OpenFOAM and CFD in general.


I am currently working with pimpleFoam, and would like to now if I can run pimpleFoam as a steady-state solver to compare it with simpleFoam (the one commonly used for steady-state analysis of incompressible flows).


The case I am using has, the same geometry, boundary conditions, material parameters and discretization schemes.


The fvSolutions for simpleFoam is:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2012                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
    p
    {
        solver           GAMG;
        smoother         DICGaussSeidel;
        tolerance        1e-12;
        relTol           0.01;
     }

    U
    {
        solver          smoothSolver;
        smoother        symGaussSeidel;
        tolerance       1e-12;
        relTol          0.1;
    }
}

SIMPLE
{
    momentumPredictor     yes;
    nNonOrthogonalCorrectors 2;

    residualControl
    {
        p               1e-7;
        U               1e-7;
    }
}

relaxationFactors
{
    fields
    {
        p         0.2;
    }
    equations
    {
        U         0.8;
    }
}
// ************************************************************************* //

And the fvSolution for pimpleFoam is:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2012                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
    p
    {
        solver           GAMG;
        smoother         DICGaussSeidel;
        tolerance        1e-12;
        relTol           0.01;
    }

    pFinal
    {
        $p;
        relTol          0;
    }

    U
    {
        solver          smoothSolver;
        smoother        symGaussSeidel;
        tolerance       1e-12;
        relTol          0.1;
    }

    UFinal
    {
        $U;
        relTol          0;
    }
}

PIMPLE
{
    momentumPredictor     yes;
    nNonOrthogonalCorrectors 0;
    nCorrectors          3;
    nOuterCorrectors    5000; 

    residualControl
    {
        p
        {
            tolerance 1e-7;
            relTol 0;
        }  

        U
        {
            tolerance 1e-7;
            relTol 0;
        }               
    }
}

relaxationFactors
{
    fields
    {
        "p.*"         0.2;
    }
    equations
    {
        "U.*"         0.8;
    }
}
// ************************************************************************* //

Since I am considering the simulation to be in steady-state, I set a big number for the nOuterCorrectors to be able to reach the value of convergence (1e-7). Since I will be doing 3 corrections to the pressure value in simpleFoam, I placed the same number in pimpleFoam.


Now, if I relax p field and the U equation but not the pFinal and UFinal (relaxation value of 1), the residuals will shoot up in the last iteration.

PIMPLE: iteration 1349
smoothSolver:  Solving for Ux, Initial residual = 8.19246e-08, Final residual = 5.7518e-10, No Iterations 1000
smoothSolver:  Solving for Uy, Initial residual = 8.17515e-08, Final residual = 7.9777e-10, No Iterations 1000
GAMG:  Solving for p, Initial residual = 0.00157036, Final residual = 1.10094e-05, No Iterations 3
time step continuity errors : sum local = 2.48645e-07, global = -2.4261e-07, cumulative = -0.000346783
GAMG:  Solving for p, Initial residual = 0.00052824, Final residual = 1.99901e-06, No Iterations 4
time step continuity errors : sum local = 4.51441e-08, global = 2.00348e-08, cumulative = -0.000346763
GAMG:  Solving for p, Initial residual = 0.000149263, Final residual = 9.10383e-13, No Iterations 16
time step continuity errors : sum local = 2.58752e-14, global = 1.97548e-14, cumulative = -0.000346763
PIMPLE: converged in 1349 iterations
ExecutionTime = 253.46 s  ClockTime = 262 s

I checked the maximum difference between the calculated pressure field between simpleFoam and pimpleValue, whos value is: 0.00222 Pa.




I redid the pimpleFoam simulation, but this time with pFinal and UFinal with the same relaxation factor:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2012                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
    p
    {
        solver           GAMG;
        smoother         DICGaussSeidel;
        tolerance        1e-12;
        relTol           0.01;
    }

    pFinal
    {
        $p;
        relTol          0;
    }

    U
    {
        solver          smoothSolver;
        smoother        symGaussSeidel;
        tolerance       1e-12;
        relTol          0.1;
    }

    UFinal
    {
        $U;
        relTol          0;
    }
}

PIMPLE
{
    momentumPredictor     yes;
    nNonOrthogonalCorrectors 0;
    nCorrectors          3;
    nOuterCorrectors    5000; 

    residualControl
    {
        p
        {
            tolerance 1e-7;
            relTol 0;
        }  

        U
        {
            tolerance 1e-7;
            relTol 0;
        }               
    }
}

relaxationFactors
{
    fields
    {
        "p.*"         0.2;
    }
    equations
    {
        "U.*"         0.8;
    }
}
// ************************************************************************* //

Now the residuals, do not shoot up (as would be expected)

PIMPLE: iteration 1348
smoothSolver:  Solving for Ux, Initial residual = 1.09084e-08, Final residual = 1.03204e-09, No Iterations 3
smoothSolver:  Solving for Uy, Initial residual = 1.08885e-08, Final residual = 1.03056e-09, No Iterations 3
GAMG:  Solving for p, Initial residual = 1.65431e-07, Final residual = 8.08242e-10, No Iterations 3
time step continuity errors : sum local = 1.46057e-11, global = 8.30149e-13, cumulative = -0.000346541
GAMG:  Solving for p, Initial residual = 1.02425e-07, Final residual = 7.95361e-10, No Iterations 2
time step continuity errors : sum local = 1.43732e-11, global = 1.05386e-12, cumulative = -0.000346541
GAMG:  Solving for p, Initial residual = 9.97459e-08, Final residual = 5.08921e-13, No Iterations 8
time step continuity errors : sum local = 1.6224e-14, global = 8.66275e-15, cumulative = -0.000346541

PIMPLE: iteration 1349
smoothSolver:  Solving for Ux, Initial residual = 1.08227e-08, Final residual = 6.33477e-13, No Iterations 13
smoothSolver:  Solving for Uy, Initial residual = 1.0803e-08, Final residual = 6.2313e-13, No Iterations 13
GAMG:  Solving for p, Initial residual = 1.57387e-07, Final residual = 9.22866e-10, No Iterations 3
time step continuity errors : sum local = 1.66775e-11, global = 1.47003e-12, cumulative = -0.000346541
GAMG:  Solving for p, Initial residual = 1.2202e-07, Final residual = 6.1291e-10, No Iterations 3
time step continuity errors : sum local = 1.10757e-11, global = 1.14102e-12, cumulative = -0.000346541
GAMG:  Solving for p, Initial residual = 1.20591e-07, Final residual = 3.98623e-13, No Iterations 10
time step continuity errors : sum local = 1.48153e-14, global = 5.29874e-15, cumulative = -0.000346541
PIMPLE: converged in 1349 iterations
ExecutionTime = 244.99 s  ClockTime = 254 s

Now the maximum difference between simpleFoam and pimpleFoam is 0.00001 Pa.


Is it Ok for me to assume that, if I want to want to run steady-state simulations with pimpleFoam I should relax both fields and equations (p, pFinal and U, UFinal)?


Additionally, is there any keyword to stop the pimple solver if the difference between the initial residual of consecutive time steps is less than a tolerance?


Kind Regards.

[klausb]

I recommend to read through this: https://openfoamwiki.net/index.php/O...hm_in_OpenFOAM
to better understand what's the intended use-case of PIMPLE.

[Shibi]

Originally Posted by klausb

I recommend to read through this: https://openfoamwiki.net/index.php/O...hm_in_OpenFOAM
to better understand what's the intended use-case of PIMPLE.

From what I could understand, Pimple is using the PISO with SIMPLE advantages (under-relaxation within one time step) so that we can surpass the limitation of Co < 1.


With this in mind, if I want to use pimpleFoam to run a steady-state analysis I can do the above approach. Correct?

[klausb]

What is the purpose of your comparison?

PIMPLE uses SIMPLE at each timestep and can be configured to run in PISO mode, too. It's not designed for steady-state-simulations. It can be an option to speed-up certain transient flow simulations compared to PISO.

pimpleFoam

Transient solver for incompressible, turbulent flow of Newtonian fluids, with optional mesh motion and mesh topology changes.


simpleFoam

Steady-state solver for incompressible, turbulent flow, using the SIMPLE algorithm.

[Shibi]

Hi,



I understand that pimpleFoam is designed for transient simulations. But the steadyState option is available in the discretization schemes. As such, I was just checking how to setup the case to make both solvers get the same results.

[Bazinga]

I guess klausb question is why you would be interested in this comparison. PIMPLE is the transient version of SIMPLE, hence, why bother comparing if you already have the steady state algorithm.

You do you, but you could maybe consider that this comparison might be a bit tedious.

[bsiyahhan]

One advantage of using pimpleFoam as a steady state solver could be to do adaptive mesh refinement for a steady state case without having to modify the code of simpleFoam.