CFD Online Discussion Forums - pimpleFoam adjustable timestep

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pimpleFoam adjustable timestep

Hi guys!

I was running a simulation with pimpleFoam. Everything is ok except the maximum Courant number remains small(~0.1) even if I set the maxco to be 1. The current time step is 0.02(it is stable and doesn't change) and it is also smaller than maxDeltaT that I set to be 0.1.
If I would like to increase my simulation speed, that is, to make the Maxco number close to 1, can anyone tell me what should I do?

Thanks a lot!
Jeremy

Code:

/*--------------------------------*- C++ -*----------------------------------*\

| =========                 |                                                 |

| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |

|  \\    /   O peration     | Version:  3.0.1                                 |

|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |

|    \\/     M anipulation  |                                                 |

\*---------------------------------------------------------------------------*/

FoamFile

{

    version     2.0;

    format      ascii;

    class       dictionary;

    object      controlDict;

}

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //



libs            ("libABLBC.so" "libturbulentABLBCInletVelocity.so");



application     pimpleFoam;



startFrom       latestTime;



startTime       0;



stopAt          endTime;



endTime         500;



deltaT          0.1;



writeControl    adjustableRunTime;



writeInterval   0.1;



purgeWrite      0;



writeFormat     ascii;



writePrecision  6;



writeCompression off;



timeFormat      general;



timePrecision   6;



runTimeModifiable true;



adjustTimeStep yes;



maxCo 1.0;



maxDeltaT 0.1;

Hi Jeremy,
Did you try to increase 'maxdeltaT'?

Hi,

@bling

In general, OpenFOAM wants to make at least 5 time steps between saves, since you writeInterval is 0.1, your maximum time step is 0.02.

So, to increase time step, increase your writeInterval.

Quote:

Originally Posted by alexeym (Post 624892)

Old thread, but I have a problem with this.

I have set maxCo=10, however it starts low (1e-4) and doesn't increase. In fact it flat-lines. Why won't the Courant number increase if the solution isn't in trouble? RH.

The time step will not be larger than write interval.

Quote:

Originally Posted by piu58 (Post 673692)

The time step will not be larger than write interval.

Thanks for your help, I understand now. I had lowered my writeInterval because I wanted to generate data right at the beginning of my simulation, not realising this was affecting my Courant number. Does the rule above apply? That is

deltaT $\leq$ writeInterval/5,

where 5 is the number of saves OpenFOAM wants to make? RH.

I don't know form where you the "5" have. I would say

deltaT <= writeInterval

If you need soma valuses for the first steps you may change the write interval during the simulation. At least if you use only one processor it should change during the run.

I have the same issue, icoFoam runs with constant time step smaller than the maximum time step even though the Courant number is smaller than the max courant number. Here is the controlDict file:

Code:

FoamFile

{

    version     2.0;

    format      ascii;

    class       dictionary;

    location    "system";

    object      controlDict;

}

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //



application     icoFoam;



startFrom       latestTime;



startTime       0;



stopAt          endTime;



endTime         300;



deltaT          2e-03;



maxDeltaT       5e-03;



adjustTimeStep  yes;



maxCo           1.0;



writeControl    adjustableRunTime;



writeInterval   10;



purgeWrite      0;



writeFormat     binary;



writePrecision  6;



writeCompression off;



timeFormat      general;



timePrecision   6;





runTimeModifiable true;



functions



{

    #include  "forceCoeffs"



    fieldAverage1

    {

        type            fieldAverage;

        functionObjectLibs ( "libfieldFunctionObjects.so" );

        enabled         true;

        timeStart       100;

        timeEnd         300;

        outputControl   outputTime;



        fields

        (

            U

            {

                mean        on;

                prime2Mean  on;

                base        time;

            }



            p

            {

                mean        on;

                prime2Mean  on;

                base        time;

            }

        );

    }



    probes

    {

        type            probes;

        libs            ("libsampling.so");

        writeControl    timeStep;

        enable          false;

        probeLocations

        (

            (1 0 0)

            (1 0.6 0)

            (2.4 0.6 0)

            (3 0.6 0)

            (5 0.6 0)

        );

        fixedLocations  true;

        fields

        (

            U

        );

    }



}

Here is the output of the simulation

Code:

Time = 11.628



Courant Number mean: 0.0151234 max: 0.143464

smoothSolver:  Solving for Ux, Initial residual = 6.03065e-05, Final residual = 5.06024e-08, No Iterations 1

smoothSolver:  Solving for Uy, Initial residual = 6.03038e-05, Final residual = 5.03623e-08, No Iterations 1

smoothSolver:  Solving for Uz, Initial residual = 1.11447e-06, Final residual = 1.11447e-06, No Iterations 0

DICPCG:  Solving for p, Initial residual = 0.000156221, Final residual = 7.45208e-06, No Iterations 154

time step continuity errors : sum local = 1.41406e-13, global = -4.9692e-15, cumulative = 5.69836e-12

DICPCG:  Solving for p, Initial residual = 7.45005e-05, Final residual = 9.65905e-07, No Iterations 184

time step continuity errors : sum local = 1.83327e-14, global = 3.18384e-16, cumulative = 5.69868e-12

ExecutionTime = 152.05 s  ClockTime = 155 s





Time = 11.63



Courant Number mean: 0.0151234 max: 0.143464

smoothSolver:  Solving for Ux, Initial residual = 6.02107e-05, Final residual = 5.03845e-08, No Iterations 1

smoothSolver:  Solving for Uy, Initial residual = 6.02125e-05, Final residual = 5.01549e-08, No Iterations 1

smoothSolver:  Solving for Uz, Initial residual = 1.11191e-06, Final residual = 1.11191e-06, No Iterations 0

DICPCG:  Solving for p, Initial residual = 0.000155738, Final residual = 7.40201e-06, No Iterations 74

time step continuity errors : sum local = 1.40469e-13, global = -7.92162e-15, cumulative = 5.69076e-12

DICPCG:  Solving for p, Initial residual = 7.46332e-05, Final residual = 9.66485e-07, No Iterations 183

time step continuity errors : sum local = 1.83421e-14, global = 3.82859e-16, cumulative = 5.69114e-12

ExecutionTime = 152.21 s  ClockTime = 155 s

Time step is constant at 2e-3.

Could there be an issue with the field averaging or other operation?

Can anyone please help me?

icoFoam does not support adjustable time stepping. You can actually check which solver application uses it, by checking its .C file and searching setDeltaT.H in that file. If it exists, the solver supports it.!