OpenFOAM很多求解器时间步长都是恒定的，这使得我们在CFD计算的时候不得不一个一个的试。求解器时间步长的自动调整会节省我们很多时间。下面以icoFoam为例谈谈如何动态设定时间步长。其实OpenFOAM已经为我们定制了自动调节时间步长的功能，如果要使用这个功能，需要以下几个头文件。

//读入动态设定步长相关参数
#   include "readTimeControls.H"

//计算CourantNo
#   include "CourantNo.H"

//初始化设定时间步长
#   include "setInitialDeltaT.H"

//根据CourantNo重新设定时间步长
#       include "setDeltaT.H"

要让程序自动调节时间步长，主要分4个步骤，下面在icoFoam中实例

#include "fvCFD.H"

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

int main(int argc, char *argv[])
{

#   include "setRootCase.H"

#   include "createTime.H"
#   include "createMesh.H"
#   include "createFields.H"
#   include "initContinuityErrs.H"

//步骤1：将前3个头文件加到这里。

//读入动态设定步长相关参数
#   include "readTimeControls.H"

//计算CourantNo
#   include "CourantNo.H"

//初始化设定时间步长
#   include "setInitialDeltaT.H" 


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

    Info<< "nStarting time loopn" << endl;
//步骤2将for循环改成while循环

//    for (runTime++; !runTime.end(); runTime++)

    while(runTime.run())
    {

//        Info<< "Time = " << runTime.timeName() << nl << endl;

#       include "readPISOControls.H"
#       include "CourantNo.H"

//步骤3：将重新设定步长的头文件写到这，及其推移时间，将上面的Info挪下来。
#       include "setDeltaT.H"
        runTime++;
        Info<< "Time = " << runTime.timeName() << nl << endl;


        fvVectorMatrix UEqn
        (
            fvm::ddt(U)
          + fvm::div(phi, U)
          - fvm::laplacian(nu, U)
        );

        solve(UEqn == -fvc::grad(p));

        // --- PISO loop

        for (int corr=0; corr<nCorr; corr++)
        {
            volScalarField rUA = 1.0/UEqn.A();

            U = rUA*UEqn.H();
            phi = (fvc::interpolate(U) & mesh.Sf())
                + fvc::ddtPhiCorr(rUA, U, phi);

            adjustPhi(phi, U, p);

            for (int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++)
            {
                fvScalarMatrix pEqn
                (
                    fvm::laplacian(rUA, p) == fvc::div(phi)
                );

                pEqn.setReference(pRefCell, pRefValue);
                pEqn.solve();

                if (nonOrth == nNonOrthCorr)
                {
                    phi -= pEqn.flux();
                }
            }

#           include "continuityErrs.H"

            U -= rUA*fvc::grad(p);
            U.correctBoundaryConditions();
        }

        runTime.write();

        Info<< "ExecutionTime = " << runTime.elapsedCpuTime() << " s"
            << "  ClockTime = " << runTime.elapsedClockTime() << " s"
            << nl << endl;
    }

    Info<< "Endn" << endl;

    return(0);
}
//步骤4： 进入控制台重新编译程序
打开控制台，进入/applications/solvers/incompressible/icoFoam/
wmake

如何使用自动调节步长功能呢？

打开case文件夹下system/controDict
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application icoFoam;

startFrom       startTime;

startTime       0;

stopAt          endTime;

endTime         2;

deltaT          0.1;

writeControl    runTime;

writeInterval   0.1;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression uncompressed;

timeFormat      general;

timePrecision   6;

runTimeModifiable yes;

//加入下面的就行了

adjustTimeStep  yes;    //想变成静态步长，no即可

maxCo           0.5;  //如果Co 数大于 0.5则减小步长

maxDeltaT      1;    //最大时间步长

运行程序测试一下。

如何让程序自动调节步长的介绍已经完毕，是不是很简单？