并行计算更新边界的问题？

[Samuel-Tu]

在网上找到一段讲解并行计算的代码

 while (runTime.loop())
{
    Info<< "Time = " << runTime.timeName() << nl << endl;

    // assign values to the field;
    // sin function expects a dimensionless argument, hence need to convert
    // current time using .value().
    // r has dimensions of length, hence the small value being added to it
    // needs to match that.
    // Finally, the result has to match dimensions of pressure, which are
    // m^2 / s^-2/
    p = Foam::sin(2.*constant::mathematical::pi*f*runTime.time().value())
            / (r/rFarCell + dimensionedScalar("small", dimLength, 1e-12))
            * dimensionedScalar("tmp", dimensionSet(0, 3, -2, 0, 0), 1.);

  // NOTE: this is needed to update the values on the processor boundaries.
  // If this is not done, the gradient operator will get confused around the
  // processor patches.
   p.correctBoundaryConditions();

       // calculate velocity from gradient of pressure
	U = fvc::grad(p)*dimensionedScalar("tmp", dimTime, 1.);
	runTime.write();
}

疑惑的是在p.correctBoundaryConditions();上一行有NOTE提醒需要执行p.correctBoundaryConditions();，否则计算p的梯度时会有问题。但是我在icoFoam中没有找到
p.correctBoundaryConditions();，只有U.correctBoundaryConditions();，而实际上在预测步里有用到fvc::grad(p).这是什么原因呢？？

int main(int argc, char *argv[])
{
    #include "addCheckCaseOptions.H"
    #include "setRootCase.H"
    #include "createTime.H"
    #include "createMesh.H"

    pisoControl piso(mesh);

    #include "createFields.H"
    #include "initContinuityErrs.H"

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

    Info<< "\nStarting time loop\n" << endl;

    while (runTime.loop())
    {
        Info<< "Time = " << runTime.timeName() << nl << endl;

        #include "CourantNo.H"

        // Momentum predictor

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

        if (piso.momentumPredictor())
        {
            solve(UEqn == -fvc::grad(p));
        }

        // --- PISO loop
        while (piso.correct())
        {
            volScalarField rAU(1.0/UEqn.A());
            volVectorField HbyA(constrainHbyA(rAU*UEqn.H(), U, p));
            surfaceScalarField phiHbyA
            (
                "phiHbyA",
                fvc::flux(HbyA)
              + fvc::interpolate(rAU)*fvc::ddtCorr(U, phi)
            );

            adjustPhi(phiHbyA, U, p);

            // Update the pressure BCs to ensure flux consistency
            constrainPressure(p, U, phiHbyA, rAU);

            // Non-orthogonal pressure corrector loop
            while (piso.correctNonOrthogonal())
            {
                // Pressure corrector

                fvScalarMatrix pEqn
                (
                    fvm::laplacian(rAU, p) == fvc::div(phiHbyA)
                );

                pEqn.setReference(pRefCell, pRefValue);

                pEqn.solve(mesh.solver(p.select(piso.finalInnerIter())));

                if (piso.finalNonOrthogonalIter())
                {
                    phi = phiHbyA - pEqn.flux();
                }
            }

            #include "continuityErrs.H"

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

        runTime.write();

        runTime.printExecutionTime(Info);
    }

    Info<< "End\n" << endl;

    return 0;
}

[李东岳]

=等号类似的操作都需要correctBoundaryConditions()，solve()出来的都不需要

[Samuel-Tu]

@东岳 谢谢

[Samuel-Tu]

@东岳 东岳老师，现在我有点拿不准，我先建立了一个体积力f的体心场，然后利用速度U计算出体积力f，最后需要用体积力f插值成面心场phiF

f=U*系数;
surfaceScalarField phiF
(
    "pihF",
    fvc::flux(rAU*f)
);

计算f=U*系数后，是否还需要更新f的边界呢

[李东岳]

如果f就是f=U*系数，就不需要。
如果f是你需要更新边界的场，就需要，否则U*系数会抹掉你的边界信息

[Samuel-Tu]

@东岳 谢谢老师，我想了下应该要更新边界。因为计算surfaceScalarField的时候，并行计算切割区域的边界面上的surfaceScalarField计算需要f的边界场，不像内部面可以通过fvc::interpolate把体心场插值成面心场

[Micro]

@samuel-tu =等号类似的操作都需要correctBoundaryConditions()
@李东岳 请问岳哥，为啥 = 号等类似的操作都需要显式更新下边条呢？能不能稍微解释下，感谢！