Exemple #1
0
        /// <summary>
        /// Constructor for XDG solvers
        /// </summary>
        public OpAnalysisBase(LevelSetTracker LsTrk, BlockMsrMatrix Mtx, double[] RHS, UnsetteledCoordinateMapping Mapping, MultiphaseCellAgglomerator CurrentAgglomeration, BlockMsrMatrix _mass, IEnumerable <MultigridOperator.ChangeOfBasisConfig[]> OpConfig, ISpatialOperator abstractOperator)
        {
            int RHSlen = Mapping.TotalLength;

            m_map    = Mapping;                              // mapping
            VarGroup = Mapping.BasisS.Count.ForLoop(i => i); //default: all dependent variables are included in operator matrix

            m_LsTrk = LsTrk;

            m_OpMtx  = Mtx.CloneAs();
            localRHS = RHS.CloneAs();

            // create the Dummy XDG aggregation basis
            var baseGrid = Mapping.GridDat;
            var mgSeq    = Foundation.Grid.Aggregation.CoarseningAlgorithms.CreateSequence(baseGrid, 1);

            AggregationGridBasis[][] XAggB = AggregationGridBasis.CreateSequence(mgSeq, Mapping.BasisS);

            //
            XAggB.UpdateXdgAggregationBasis(CurrentAgglomeration);

            // create multigrid operator
            m_MultigridOp = new MultigridOperator(XAggB, Mapping,
                                                  m_OpMtx,
                                                  _mass,
                                                  OpConfig,
                                                  abstractOperator.DomainVar.Select(varName => abstractOperator.FreeMeanValue[varName]).ToArray());
        }
        /// <summary>
        /// Matrix/Affine assembly in the case of an implicit RK stage.
        /// </summary>
        protected override void AssembleMatrixCallback(out BlockMsrMatrix System, out double[] Affine, out BlockMsrMatrix PcMassMatrix, DGField[] argCurSt, bool Linearization)
        {
            if (Linearization == false)
            {
                throw new NotImplementedException("todo");
            }

            int Ndof = m_CurrentState.Count;

            // copy data from 'argCurSt' to 'CurrentStateMapping', if necessary
            // -----------------------------------------------------------
            DGField[] locCurSt = CurrentStateMapping.Fields.ToArray();
            if (locCurSt.Length != argCurSt.Length)
            {
                throw new ApplicationException();
            }
            int NF = locCurSt.Length;

            for (int iF = 0; iF < NF; iF++)
            {
                if (object.ReferenceEquals(locCurSt[iF], argCurSt[iF]))
                {
                    // nothing to do
                }
                else
                {
                    locCurSt[iF].Clear();
                    locCurSt[iF].Acc(1.0, argCurSt[iF]);
                }
            }

            // update of level-set
            // ----------------------

            bool updateAgglom = false;

            if (this.Config_LevelSetHandling == LevelSetHandling.Coupled_Once && m_ImplStParams.m_IterationCounter == 0 ||
                this.Config_LevelSetHandling == LevelSetHandling.Coupled_Iterative)
            {
                //MoveLevelSetAndRelatedStuff(locCurSt, m_CurrentPhystime, m_CurrentDt, 1.0);
                if (Math.Abs(m_ImplStParams.m_ActualLevSetRelTime - m_ImplStParams.m_RelTime) > 1.0e-14)
                {
                    if (m_ImplStParams.m_IterationCounter <= 0)// only push tracker in the first iter
                    {
                        m_LsTrk.PushStacks();
                    }
                    MoveLevelSetAndRelatedStuff(locCurSt,
                                                m_ImplStParams.m_CurrentPhystime, m_ImplStParams.m_CurrentDt * m_ImplStParams.m_RelTime, IterUnderrelax,
                                                m_ImplStParams.m_Mass, m_ImplStParams.m_k);

                    // note that we need to update the agglomeration
                    updateAgglom = true;
                }
            }

            // update agglomeration
            // --------------------
#if DEBUG
            if (this.Config_LevelSetHandling == LevelSetHandling.LieSplitting || this.Config_LevelSetHandling == LevelSetHandling.StrangSplitting)
            {
                Debug.Assert(m_CurrentAgglomeration != null);
                Debug.Assert(m_PrecondMassMatrix != null);
                // ensure, that, when splitting is used we update the agglomerator in the very first iteration.
            }
#endif
            if (updateAgglom || m_CurrentAgglomeration == null)
            {
                this.UpdateAgglom(m_ImplStParams.m_IterationCounter > 0);



                // update Multigrid-XDG basis
                base.MultigridBasis.UpdateXdgAggregationBasis(m_CurrentAgglomeration);
            }


            // mass matrix update
            // ------------------

            if (this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsIdentity)
            {
                // may occur e.g. if one runs the FSI solver as a pure single-phase solver,
                // i.e. if the Level-Set is outside the domain.

                PcMassMatrix = null;
            }
            else
            {
                // checks:
                Debug.Assert(this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsNonIdentity ||
                             this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsTimeDependent ||
                             this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsTimeAndSolutionDependent,
                             "Something is not implemented here.");
                if (this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsNonIdentity &&
                    Config_LevelSetHandling != LevelSetHandling.None)
                {
                    throw new NotSupportedException("Illegal configuration;");
                }

                if (this.Config_LevelSetHandling == LevelSetHandling.Coupled_Once || this.Config_LevelSetHandling == LevelSetHandling.Coupled_Iterative)
                {
                    if ((this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsNonIdentity && m_PrecondMassMatrix == null) || // compute mass matrix (only once in application lifetime)
                        (this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsTimeDependent && m_ImplStParams.m_IterationCounter == 0) || // compute mass matrix once per timestep
                        (this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsTimeAndSolutionDependent) // re-compute mass matrix in every iteration
                        )
                    {
                        BlockMsrMatrix PM, SM;
                        UpdateMassMatrix(out PM, out SM, m_ImplStParams.m_CurrentPhystime + m_ImplStParams.m_CurrentDt * m_ImplStParams.m_RelTime);
                        m_ImplStParams.m_Mass[1] = SM;
                        m_PrecondMassMatrix      = PM;
                    }
                }

                PcMassMatrix = m_PrecondMassMatrix;
            }

            // operator matrix update
            // ----------------------

            // we perform the extrapolation to have valid parameters if
            // - the operator matrix depends on these values
            this.m_CurrentAgglomeration.Extrapolate(CurrentStateMapping);

            var OpMatrix = new BlockMsrMatrix(CurrentStateMapping);
            var OpAffine = new double[Ndof];

            this.ComputeOperatorMatrix(OpMatrix, OpAffine, CurrentStateMapping, locCurSt, base.GetAgglomeratedLengthScales(), m_ImplStParams.m_CurrentPhystime + m_ImplStParams.m_CurrentDt * m_ImplStParams.m_RelTime);


            // assemble system
            // ---------------

            double dt = m_ImplStParams.m_CurrentDt;

            // select mass matrix (and some checks)
            double[]       RHS = new double[Ndof];
            BlockMsrMatrix MamaRHS, MamaLHS;
            if (this.Config_LevelSetHandling == LevelSetHandling.Coupled_Once ||
                this.Config_LevelSetHandling == LevelSetHandling.Coupled_Iterative)
            {
                Debug.Assert(m_ImplStParams.m_Mass.Length == 2);
                MamaRHS = m_ImplStParams.m_Mass[0];
                MamaLHS = m_ImplStParams.m_Mass[1];
            }
            else if (this.Config_LevelSetHandling == LevelSetHandling.LieSplitting ||
                     this.Config_LevelSetHandling == LevelSetHandling.StrangSplitting ||
                     this.Config_LevelSetHandling == LevelSetHandling.None)
            {
                Debug.Assert(m_ImplStParams.m_Mass.Length == 1);
                MamaRHS = m_ImplStParams.m_Mass[0];
                MamaLHS = m_ImplStParams.m_Mass[0];
            }
            else
            {
                throw new NotImplementedException();
            }

            // right-hand-side, resp. affine vector
            if (MamaRHS != null)
            {
                MamaRHS.SpMV(1.0 / dt, m_ImplStParams.m_u0, 0.0, RHS);
            }
            else
            {
                Debug.Assert(this.Config_MassMatrixShapeandDependence == MassMatrixShapeandDependence.IsIdentity);
                RHS.SetV(m_ImplStParams.m_u0, 1.0 / dt);
            }
            for (int l = 0; l < m_ImplStParams.m_s; l++)
            {
                if (m_ImplStParams.m_RK_as[l] != 0.0)
                {
                    RHS.AccV(-m_ImplStParams.m_RK_as[l], m_ImplStParams.m_k[l]);
                }
            }

            Affine = RHS;
            Affine.ScaleV(-1.0);
            Affine.AccV(m_ImplStParams.m_RK_as[m_ImplStParams.m_s], OpAffine);

            // left-hand-side
            System = OpMatrix.CloneAs();
            System.Scale(m_ImplStParams.m_RK_as[m_ImplStParams.m_s]);
            if (MamaLHS != null)
            {
                System.Acc(1.0 / dt, MamaLHS);
            }
            else
            {
                System.AccEyeSp(1.0 / dt);
            }

            // perform agglomeration
            // ---------------------
            Debug.Assert(object.ReferenceEquals(m_CurrentAgglomeration.Tracker, m_LsTrk));
            m_CurrentAgglomeration.ManipulateMatrixAndRHS(System, Affine, CurrentStateMapping, CurrentStateMapping);

            // increase iteration counter
            // --------------------------
            m_ImplStParams.m_IterationCounter++;
        }
Exemple #3
0
        /// <summary>
        /// computes derivatives in various ways and compares them against known values.
        /// </summary>
        protected override double RunSolverOneStep(int TimestepNo, double phystime, double dt)
        {
            base.EndTime       = 0.0;
            base.NoOfTimesteps = 0;

            int D = this.GridData.SpatialDimension;
            int J = this.GridData.iLogicalCells.NoOfLocalUpdatedCells;

            Console.WriteLine("DerivativeTest.exe, test case #" + GRID_CASE + " ******************************");

            //var Fix = this.GridData.iGeomEdges.FaceIndices;
            //for(int iEdge = 0; iEdge < Fix.GetLength(0); iEdge++) {
            //    Debug.Assert(Fix[iEdge, 0] >= 0);
            //    Debug.Assert(Fix[iEdge, 1] >= 0);
            //}

            // sealing test
            // =================

            if (this.GridData is Foundation.Grid.Classic.GridData)
            {
                TestSealing(this.GridData);
            }

            // cell volume and edge area check, if possible
            // ===============================================


            if (this.CellVolume > 0)
            {
                double err      = 0;
                double Treshold = 1.0e-10;


                for (int j = 0; j < J; j++)
                {
                    err += Math.Abs(this.GridData.iLogicalCells.GetCellVolume(j) - this.CellVolume);
                }

                bool passed = (err < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine("Cell volume error: " + err + " passed? " + passed);
                Console.WriteLine("--------------------------------------------");
            }

            if (this.EdgeArea > 0)
            {
                double err      = 0;
                double Treshold = 1.0e-10;

                int E = this.GridData.iLogicalEdges.Count;

                for (int e = 0; e < E; e++)
                {
                    err += Math.Abs(this.GridData.iLogicalEdges.GetEdgeArea(e) - this.EdgeArea);
                }

                bool passed = (err < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine("Edge area error: " + err + " passed? " + passed);
                Console.WriteLine("--------------------------------------------");
            }

            // Orthonormality of basis in physical coords
            // ==========================================

            {
                Basis Bs      = this.f1.Basis;
                int   N       = Bs.Length;
                int   degQuad = this.GridData.iLogicalCells.GetInterpolationDegree(0) * D + Bs.Degree + 3;
                int[] jG2jL   = this.GridData.iGeomCells.GeomCell2LogicalCell;


                // mass matrix: should be identity!
                MultidimensionalArray MassMatrix = MultidimensionalArray.Create(J, N, N);

                // compute mass matrix by quadrature.
                var quad = CellQuadrature.GetQuadrature(new int[] { N, N }, base.GridData,
                                                        (new CellQuadratureScheme()).Compile(base.GridData, degQuad),
                                                        delegate(int i0, int Length, QuadRule QR, MultidimensionalArray EvalResult) {
                    NodeSet QuadNodes = QR.Nodes;
                    MultidimensionalArray BasisVals = Bs.CellEval(QuadNodes, i0, Length);
                    EvalResult.Multiply(1.0, BasisVals, BasisVals, 0.0, "jknm", "jkn", "jkm");
                },
                                                        delegate(int i0, int Length, MultidimensionalArray ResultsOfIntegration) {
                    if (jG2jL != null)
                    {
                        for (int i = 0; i < Length; i++)
                        {
                            int jG = i + i0;
                            MassMatrix.ExtractSubArrayShallow(jG2jL[jG], -1, -1)
                            .Acc(1.0, ResultsOfIntegration.ExtractSubArrayShallow(i, -1, -1));
                        }
                    }
                    else
                    {
                        MassMatrix.SetSubArray(ResultsOfIntegration, new int[] { i0, 0, 0 }, new int[] { i0 + Length - 1, N - 1, N - 1 });
                    }
                },
                                                        cs: CoordinateSystem.Physical);
                quad.Execute();

                // check that mass matrix is Id.
                int    MaxErrorCell = -1;
                double MaxError     = -1;
                for (int j = 0; j < J; j++)
                {
                    MultidimensionalArray MassMatrix_j = MassMatrix.ExtractSubArrayShallow(j, -1, -1);
                    MassMatrix_j.AccEye(-1.0);

                    double Norm_j = MassMatrix_j.InfNorm();
                    if (Norm_j > MaxError)
                    {
                        MaxError     = Norm_j;
                        MaxErrorCell = j;
                    }
                }

                bool passed = (MaxError < 1.0e-8);
                m_passed = m_passed && passed;
                Console.WriteLine("Mass Matrix, maximum error in Cell #" + MaxErrorCell + ", mass matrix error norm: " + MaxError + " passed? " + passed);
            }

            // Broken Derivatives
            // =================

            double totalVolume = (new SubGrid(CellMask.GetFullMask(this.GridData))).Volume;

            for (int d = 0; d < D; d++)
            {
                // compute
                f1Gradient_Numerical[d].Clear();
                f1Gradient_Numerical[d].Derivative(1.0, f1, d);
                f2Gradient_Numerical[d].Clear();
                f2Gradient_Numerical[d].Derivative(1.0, f2, d);

                // subtract analytical
                var Errfield1 = f1Gradient_Numerical[d].CloneAs();
                Errfield1.Acc(-1, f1Gradient_Analytical[d]);

                var Errfield2 = f2Gradient_Numerical[d].CloneAs();
                Errfield2.Acc(-1, f2Gradient_Analytical[d]);

                Console.WriteLine("Broken Derivatives: ");

                double Treshold = 1.0e-10;
                if (AltRefSol)
                {
                    Treshold = 1.0e-4; // not exactly polynomial, therefore a higher threshold
                }
                double err1_dx = Errfield1.L2Norm() / totalVolume;
                bool   passed  = (err1_dx < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| df1/dx{0}_Numerical - df1/dx{0}_Analytical ||_2 = {1}, passed? {2}", d, err1_dx, passed));

                double err2_dx = Errfield2.L2Norm() / totalVolume;
                passed   = (err2_dx < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| df2/dx{0}_Numerical - df2/dx{0}_Analytical ||_2 = {1}, passed? {2}", d, err2_dx, passed));

                Console.WriteLine("--------------------------------------------");
            }

            // Flux Derivatives
            // =================
            for (int d = 0; d < D; d++)
            {
                // compute
                f1Gradient_Numerical[d].Clear();
                f1Gradient_Numerical[d].DerivativeByFlux(1.0, f1, d);
                f2Gradient_Numerical[d].Clear();
                f2Gradient_Numerical[d].DerivativeByFlux(1.0, f2, d);

                f1Gradient_Numerical[d].CheckForNanOrInf(true, true, true);
                f2Gradient_Numerical[d].CheckForNanOrInf(true, true, true);

                // subtract analytical
                var Errfield1 = f1Gradient_Numerical[d].CloneAs();
                Errfield1.Acc(-1, f1Gradient_Analytical[d]);

                var Errfield2 = f2Gradient_Numerical[d].CloneAs();
                Errfield2.Acc(-1, f2Gradient_Analytical[d]);

                Console.WriteLine("Flux Derivatives: ");

                double Treshold = 1.0e-10;
                if (AltRefSol)
                {
                    Treshold = 1.0e-4; // not exactly polynomial, therefore a higher threshold
                }
                double err1_dx = Errfield1.L2Norm() / totalVolume;
                bool   passed  = (err1_dx < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| df1/dx{0}_Numerical - df1/dx{0}_Analytical ||_2 = {1}, passed? {2}", d, err1_dx, passed));

                double err2_dx = Errfield2.L2Norm() / totalVolume;
                passed   = (err2_dx < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| df2/dx{0}_Numerical - df2/dx{0}_Analytical ||_2 = {1}, passed? {2}", d, err2_dx, passed));

                Console.WriteLine("--------------------------------------------");
            }


            // Linear flux Derivatives
            // =======================
            for (int d = 0; d < D; d++)
            {
                double[] korrekto = f1Gradient_Numerical[d].CoordinateVector.ToArray();

                // compute
                DerivativeByFluxLinear(f1, f1Gradient_Numerical[d], d, f1);
                DerivativeByFluxLinear(f2, f2Gradient_Numerical[d], d, f2);

                // subtract analytical
                var Errfield1 = f1Gradient_Numerical[d].CloneAs();
                Errfield1.Acc(-1, f1Gradient_Analytical[d]);

                var Errfield2 = f2Gradient_Numerical[d].CloneAs();
                Errfield2.Acc(-1, f2Gradient_Analytical[d]);

                Console.WriteLine("Linear Flux Derivatives: ");

                double Treshold = 1.0e-10;
                if (AltRefSol)
                {
                    Treshold = 1.0e-4; // not exactly polynomial, therefore a higher threshold
                }
                double err1_dx = Errfield1.L2Norm() / totalVolume;
                bool   passed  = (err1_dx < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| df1/dx{0}_Numerical - df1/dx{0}_Analytical ||_2 = {1}, passed? {2}", d, err1_dx, passed));

                double err2_dx = Errfield2.L2Norm() / totalVolume;
                passed   = (err2_dx < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| df2/dx{0}_Numerical - df2/dx{0}_Analytical ||_2 = {1}, passed? {2}", d, err2_dx, passed));

                Console.WriteLine("--------------------------------------------");
            }

            // Laplacian, nonlinear
            // ====================

            if (!AltRefSol)
            {
                var Laplace = (new ipLaplace()).Operator(1);

                Laplace.Evaluate(new DGField[] { this.f1 }, new DGField[] { this.Laplace_f1_Numerical });
                Laplace.Evaluate(new DGField[] { this.f2 }, new DGField[] { this.Laplace_f2_Numerical });

                double Treshold = 1.0e-8;

                // subtract analytical
                var Errfield1 = Laplace_f1_Numerical.CloneAs();
                Errfield1.Acc(-1, Laplace_f1_Analytical);

                var Errfield2 = Laplace_f2_Numerical.CloneAs();
                Errfield2.Acc(-1, Laplace_f2_Analytical);

                double err_Lf1 = Errfield1.L2Norm() / totalVolume;
                bool   passed  = (err_Lf1 < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| /\\f1 Numerical - /\\f1 Analytical ||_2 = {0} (nonlinear evaluation), passed? {1}", err_Lf1, passed));

                double err_Lf2 = Errfield2.L2Norm() / totalVolume;
                passed   = (err_Lf2 < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| /\\f2 Numerical - /\\f2 Analytical ||_2 = {0} (nonlinear evaluation), passed? {1}", err_Lf2, passed));

                Console.WriteLine("--------------------------------------------");
            }


            // Laplacian, linear
            // ====================

            if (!AltRefSol)
            {
                var Laplace = (new ipLaplace()).Operator(1);

                var LaplaceMtx    = new BlockMsrMatrix(this.f1.Mapping, this.Laplace_f1_Numerical.Mapping);
                var LaplaceAffine = new double[LaplaceMtx.RowPartitioning.LocalLength];

                Laplace.ComputeMatrix(this.f1.Mapping, null, this.Laplace_f1_Numerical.Mapping,
                                      LaplaceMtx, LaplaceAffine, false);

                this.Laplace_f1_Numerical.CoordinateVector.SetV(LaplaceAffine);
                LaplaceMtx.SpMV(1.0, this.f1.CoordinateVector, 1.0, this.Laplace_f1_Numerical.CoordinateVector);

                this.Laplace_f2_Numerical.CoordinateVector.SetV(LaplaceAffine);
                LaplaceMtx.SpMV(1.0, this.f2.CoordinateVector, 1.0, this.Laplace_f2_Numerical.CoordinateVector);

                // subtract analytical
                var Errfield1 = Laplace_f1_Numerical.CloneAs();
                Errfield1.Acc(-1, Laplace_f1_Analytical);

                var Errfield2 = Laplace_f2_Numerical.CloneAs();
                Errfield2.Acc(-1, Laplace_f2_Analytical);


                double Treshold = 1.0e-8;

                double err_Lf1 = Errfield1.L2Norm() / totalVolume;
                bool   passed  = (err_Lf1 < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| /\\f1 Numerical - /\\f1 Analytical ||_2 = {0} (linear evaluation), passed? {1}", err_Lf1, passed));

                double err_Lf2 = Errfield2.L2Norm() / totalVolume;
                passed   = (err_Lf2 < Treshold);
                m_passed = m_passed && passed;
                Console.WriteLine(string.Format("|| /\\f2 Numerical - /\\f2 Analytical ||_2 = {0} (linear evaluation), passed? {1}", err_Lf2, passed));


                // comparison of finite difference Jacobian and Operator matrix
                if (TestFDJacobian)
                {
                    this.f1.Clear();
                    var FDJbuilder = Laplace.GetFDJacobianBuilder(this.f1.Mapping.Fields, null, this.f1.Mapping,
                                                                  delegate(IEnumerable <DGField> U0, IEnumerable <DGField> Params) {
                        return;
                    });
                    var CheckMatrix = new BlockMsrMatrix(FDJbuilder.CodomainMapping, FDJbuilder.DomainMapping);
                    var CheckAffine = new double[FDJbuilder.CodomainMapping.LocalLength];
                    FDJbuilder.ComputeMatrix(CheckMatrix, CheckAffine);

                    var ErrMatrix = LaplaceMtx.CloneAs();
                    var ErrAffine = LaplaceAffine.CloneAs();
                    ErrMatrix.Acc(-1.0, CheckMatrix);
                    ErrAffine.AccV(-1.0, CheckAffine);
                    double LinfMtx = ErrMatrix.InfNorm();
                    double L2Aff   = ErrAffine.L2NormPow2().MPISum().Sqrt();
                    bool   passed1 = (LinfMtx < 1.0e-3);
                    bool   passed2 = (L2Aff < Treshold);
                    Console.WriteLine("Finite Difference Jacobian: Matrix/Affine delta norm {0} {1}, passed? {2} {3}", LinfMtx, L2Aff, passed1, passed2);
                    m_passed = m_passed && passed1;
                    m_passed = m_passed && passed2;
                }
                Console.WriteLine("--------------------------------------------");
            }



            // finally...
            // =================

            if (m_passed)
            {
                Console.WriteLine("All tests passed. *****************************");
            }
            else
            {
                Console.WriteLine("Some error above threshold. *******************");
            }

            return(0.0); // return some artificial timestep
        }