openfoam/applications/solvers/lagrangian/reactingParcelFoam/pEqn.H

rho = thermo.rho();

volScalarField rAU(1.0/UEqn.A());
U = rAU*UEqn.H();

if (transonic)
{
    surfaceScalarField phid
    (
        "phid",
        fvc::interpolate(psi)
       *(
            (fvc::interpolate(U) & mesh.Sf())
          + fvc::ddtPhiCorr(rAU, rho, U, phi)
        )
    );

    for (int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++)
    {
        fvScalarMatrix pEqn
        (
            fvm::ddt(psi, p)
          + fvm::div(phid, p)
          - fvm::laplacian(rho*rAU, p)
         ==
            parcels.Srho()
        );

        pEqn.solve
        (
            mesh.solver
            (
                p.select
                (
                    pimpleCorr.finalIter()
                 && corr == nCorr-1
                 && nonOrth == nNonOrthCorr
                )
            )
        );

        if (nonOrth == nNonOrthCorr)
        {
            phi == pEqn.flux();
        }
    }
}
else
{
    phi =
        fvc::interpolate(rho)
       *(
            (fvc::interpolate(U) & mesh.Sf())
          + fvc::ddtPhiCorr(rAU, rho, U, phi)
        );

    for (int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++)
    {
        fvScalarMatrix pEqn
        (
            fvm::ddt(psi, p)
          + fvc::div(phi)
          - fvm::laplacian(rho*rAU, p)
         ==
            parcels.Srho()
        );

        pEqn.solve
        (
            mesh.solver
            (
                p.select
                (
                    pimpleCorr.finalIter()
                 && corr == nCorr-1
                 && nonOrth == nNonOrthCorr
                )
            )
        );

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

#include "rhoEqn.H"
#include "compressibleContinuityErrs.H"

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


DpDt = fvc::DDt(surfaceScalarField("phiU", phi/fvc::interpolate(rho)), p);