Fix MLMG::getGradSolution & getFluxes for inhomogeneous Neumann and R…

…obin BC (AMReX-Codes#2984)

Because of the way how inhomogeneous and Robin BC are handled, we must
add the inhomogeneous fluxes back, otherwise they would be zero at those
boundaries.

Src/LinearSolvers/MLMG/AMReX_MLCellABecLap.H

@@ -61,6 +61,10 @@ public:
 
     virtual void applyInhomogNeumannTerm (int amrlev, Any& rhs) const final override;
 
+    virtual void addInhomogNeumannFlux (
+        int amrlev, const Array<MultiFab*,AMREX_SPACEDIM>& grad,
+        MultiFab const& sol, bool mult_bcoef) const final override;
+
     virtual void applyOverset (int amlev, Any& rhs) const override;
 
 #if defined(AMREX_USE_HYPRE) && (AMREX_SPACEDIM > 1)

Src/LinearSolvers/MLMG/AMReX_MLCellABecLap.cpp

@@ -189,6 +189,7 @@ MLCellABecLap::getFluxes (const Vector<Array<MultiFab*,AMREX_SPACEDIM> >& a_flux
                 a_flux[alev][idim]->mult(betainv);
             }
         }
+        addInhomogNeumannFlux(alev, a_flux[alev], *a_sol[alev], true);
     }
 }
 
@@ -416,6 +417,116 @@ MLCellABecLap::applyInhomogNeumannTerm (int amrlev, Any& a_rhs) const
     }
 }
 
+void
+MLCellABecLap::addInhomogNeumannFlux (
+    int amrlev, const Array<MultiFab*,AMREX_SPACEDIM>& grad, MultiFab const& sol,
+    bool mult_bcoef) const
+{
+    /*
+     * if (mult_bcoef == true)
+     *     grad is -bceof*grad phi
+     * else
+     *     grad is grad phi
+     */
+    Real fac = mult_bcoef ? Real(-1.0) : Real(1.0);
+
+    bool has_inhomog_neumann = hasInhomogNeumannBC();
+    bool has_robin = hasRobinBC();
+
+    if (!has_inhomog_neumann && !has_robin) return;
+
+    int ncomp = getNComp();
+    const int mglev = 0;
+
+    const auto dxinv = m_geom[amrlev][mglev].InvCellSize();
+    const Box domain = m_geom[amrlev][mglev].growPeriodicDomain(1);
+
+    Array<MultiFab const*, AMREX_SPACEDIM> bcoef = {AMREX_D_DECL(nullptr,nullptr,nullptr)};
+    if (mult_bcoef) {
+        bcoef = getBCoeffs(amrlev,mglev);
+    }
+
+    const auto& bndry = *m_bndry_sol[amrlev];
+
+    MFItInfo mfi_info;
+    if (Gpu::notInLaunchRegion()) mfi_info.SetDynamic(true);
+
+#ifdef AMREX_USE_OMP
+#pragma omp parallel if (Gpu::notInLaunchRegion())
+#endif
+    for (MFIter mfi(sol, mfi_info); mfi.isValid(); ++mfi)
+    {
+        Box const& vbx = mfi.validbox();
+        for (OrientationIter orit; orit.isValid(); ++orit) {
+            const Orientation ori = orit();
+            const int idim = ori.coordDir();
+            const Box& ccb = amrex::adjCell(vbx, ori);
+            const Dim3 os = IntVect::TheDimensionVector(idim).dim3();
+            const Real dxi = dxinv[idim];
+            if (! domain.contains(ccb)) {
+                for (int icomp = 0; icomp < ncomp; ++icomp) {
+                    auto const& phi = sol.const_array(mfi,icomp);
+                    auto const bv = bndry.bndryValues(ori).multiFab().const_array(mfi,icomp);
+                    auto const bc = bcoef[idim] ? bcoef[idim]->const_array(mfi,icomp)
+                        : Array4<Real const>{};
+                    auto const& f = grad[idim]->array(mfi,icomp);
+                    if (ori.isLow()) {
+                        if (m_lobc_orig[icomp][idim] ==
+                            LinOpBCType::inhomogNeumann) {
+                            AMREX_HOST_DEVICE_FOR_3D(ccb, i, j, k,
+                            {
+                                int ii = i+os.x;
+                                int jj = j+os.y;
+                                int kk = k+os.z;
+                                Real b = bc ? bc(ii,jj,kk) : Real(1.0);
+                                f(ii,jj,kk) = fac*b*bv(i,j,k);
+                            });
+                        } else if (m_lobc_orig[icomp][idim] ==
+                                   LinOpBCType::Robin) {
+                            Array4<Real const> const& rbc = (*m_robin_bcval[amrlev])[mfi].const_array(icomp*3);
+                            AMREX_HOST_DEVICE_FOR_3D(ccb, i, j, k,
+                            {
+                                int ii = i+os.x;
+                                int jj = j+os.y;
+                                int kk = k+os.z;
+                                Real tmp = Real(1.0) /
+                                    (rbc(i,j,k,1)*dxi + rbc(i,j,k,0)*Real(0.5));
+                                Real RA = rbc(i,j,k,2) * tmp;
+                                Real RB = (rbc(i,j,k,1)*dxi - rbc(i,j,k,0)*Real(0.5)) * tmp;
+                                Real b = bc ? bc(ii,jj,kk) : Real(1.0);
+                                f(ii,jj,kk) = fac*b*dxi*((Real(1.0)-RB)*phi(ii,jj,kk)-RA);
+                            });
+                        }
+                    } else {
+                        if (m_hibc_orig[icomp][idim] ==
+                            LinOpBCType::inhomogNeumann) {
+                            AMREX_HOST_DEVICE_FOR_3D(ccb, i, j, k,
+                            {
+                                Real b = bc ? bc(i,j,k) : Real(1.0);
+                                f(i,j,k) = fac*b*bv(i,j,k);
+                            });
+                        } else if (m_hibc_orig[icomp][idim] ==
+                                   LinOpBCType::Robin) {
+                            Array4<Real const> const& rbc = (*m_robin_bcval[amrlev])[mfi].const_array(icomp*3);
+                            AMREX_HOST_DEVICE_FOR_3D(ccb, i, j, k,
+                            {
+                                Real tmp = Real(1.0) /
+                                    (rbc(i,j,k,1)*dxi + rbc(i,j,k,0)*Real(0.5));
+                                Real RA = rbc(i,j,k,2) * tmp;
+                                Real RB = (rbc(i,j,k,1)*dxi - rbc(i,j,k,0)*Real(0.5)) * tmp;
+                                Real b = bc ? bc(i,j,k) : Real(1.0);
+                                f(i,j,k) = fac*b*dxi*(RA+(RB-Real(1.0))*
+                                                      phi(i-os.x,j-os.y,k-os.z));
+                            });
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+
 void
 MLCellABecLap::applyOverset (int amrlev, Any& a_rhs) const
 {

Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.H

@@ -164,6 +164,11 @@ public:
 
     virtual void AnyAverageDownAndSync (Vector<Any>& sol) const override;
 
+    virtual void addInhomogNeumannFlux (int /*amrlev*/,
+                                        const Array<MultiFab*,AMREX_SPACEDIM>& /*grad*/,
+                                        MultiFab const& /*sol*/,
+                                        bool /*mult_bcoef*/) const {}
+
     struct BCTL {
         BoundCond type;
         Real location;

Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.cpp

@@ -938,6 +938,8 @@ MLCellLinOp::compGrad (int amrlev, const Array<MultiFab*,AMREX_SPACEDIM>& grad,
         });
 #endif
     }
+
+    addInhomogNeumannFlux(amrlev, grad, sol, false);
 }
 
 void