09DEIM_ROM.C

Introduction to tutorial 9

In this tutorial we implement a test where we use the Discrete Empirical Interpolation Method for a case where we have a non-linear dependency with respect to the input parameters.

The following image illustrates the computational domain which is the same as the previous example

The physical problem is given by a heat transfer problem which is described by the Poisson equation:

\[ \nabla \cdot (\nu \nabla T) = S \]

The parametric diffusivity is described by a parametric Gaussian function:

\[ \nu(\mathbf{x},\mathbf{\mu}) = e^{-2(x-\mu_x-1)^2 - 2(y-\mu_y-0.5)^2}, \]

The problem is then discretized as:

\[ A(\mu)T = b \]

In this case, even if the problem is linear, due to non-linearity with respect to the input parameter of the conductivity constant it is not possible to have an affine decomposition of the discretized differential operator.

We seek therefore an approximate affine expansion of the differential operator of this type:

\[ A(\mu) = \sum_{i = 1}^{N_D} \theta_i(\mu) A_i \]

using the Discrete Empirical Interpolation Method

The plain program

Here there's the plain code

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 * In real Time Highly Advanced Computational Applications for Finite Volumes
 * Copyright (C) 2017 by the ITHACA-FV authors
-------------------------------------------------------------------------------
License
    This file is part of ITHACA-FV
    ITHACA-FV is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
    ITHACA-FV is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    GNU Lesser General Public License for more details.
    You should have received a copy of the GNU Lesser General Public License
    along with ITHACA-FV. If not, see <http://www.gnu.org/licenses/>.
Description
    Example of model reduction problem using the DEIM for a Heat Transfer Problem
SourceFiles
    09DEIM_ROM.C
\*---------------------------------------------------------------------------*/
 
#include "fvCFD.H"
#include "IOmanip.H"
#include "fvOptions.H"
#include "simpleControl.H"
#include "ITHACAutilities.H"
#include "Foam2Eigen.H"
#include <chrono>
#include "fvMeshSubset.H"
#include "ITHACAstream.H"
#include "ITHACAPOD.H"
#include "EigenFunctions.H"
#include "DEIM.H"
#include <chrono>
#include <Eigen/SVD>
#include <Eigen/SparseLU>
#include "laplacianProblem.H"
 
class DEIM_function : public DEIM<fvScalarMatrix>
{
        using DEIM::DEIM;
    public:
        static fvScalarMatrix evaluate_expression(volScalarField& T, Eigen::MatrixXd mu)
        {
            volScalarField yPos = T.mesh().C().component(vector::Y).ref();
            volScalarField xPos = T.mesh().C().component(vector::X).ref();
            volScalarField nu(T);
 
            for (auto i = 0; i < nu.size(); i++)
            {
                nu[i] = std::exp( - 2 * std::pow(xPos[i] - mu(0) - 1,
                                                 2) - 2 * std::pow(yPos[i] - mu(1) - 0.5, 2)) + 1;
            }
 
            nu.correctBoundaryConditions();
            fvScalarMatrix TiEqn22
            (
                fvm::laplacian(nu, T, "Gauss linear")
            );
            return TiEqn22;
        }
 
        Eigen::MatrixXd onlineCoeffsA(Eigen::MatrixXd mu)
        {
            Eigen::MatrixXd theta(magicPointsAcol().size(), 1);
            fvScalarMatrix Aof = evaluate_expression(fieldA(), mu);
            Eigen::SparseMatrix<double> Mr;
            Eigen::VectorXd br;
            Foam2Eigen::fvMatrix2Eigen(Aof, Mr, br);
 
            for (int i = 0; i < magicPointsAcol().size(); i++)
            {
                int ind_row = localMagicPointsArow[i] + xyz_Arow()[i] *
                              fieldA().size();
                int ind_col = localMagicPointsAcol[i] + xyz_Acol()[i] *
                              fieldA().size();
                theta(i) = Mr.coeffRef(ind_row, ind_col);
            }
 
            return theta;
        }
 
        Eigen::MatrixXd onlineCoeffsB(Eigen::MatrixXd mu)
        {
            Eigen::MatrixXd theta(magicPointsB().size(), 1);
            fvScalarMatrix Aof = evaluate_expression(fieldB(), mu);
            Eigen::SparseMatrix<double> Mr;
            Eigen::VectorXd br;
            Foam2Eigen::fvMatrix2Eigen(Aof, Mr, br);
 
            for (int i = 0; i < magicPointsB().size(); i++)
            {
                int ind_row = localMagicPointsB[i] + xyz_B()[i] * fieldB().size();
                theta(i) = br(ind_row);
            }
 
            return theta;
        }
 
        PtrList<volScalarField> fieldsA;
        autoPtr<volScalarField> fieldA;
        autoPtr<volScalarField> fieldB;
        PtrList<volScalarField> fieldsB;
};
 
class DEIMLaplacian: public laplacianProblem
{
    public:
        explicit DEIMLaplacian(int argc, char* argv[])
            :
            laplacianProblem(argc, argv),
            nu(_nu()),
            S(_S()),
            T(_T())
        {
            fvMesh& mesh = _mesh();
            ITHACAdict = new IOdictionary
            (
                IOobject
                (
                    "ITHACAdict",
                    "./system",
                    mesh,
                    IOobject::MUST_READ,
                    IOobject::NO_WRITE
                )
            );
            NTmodes = readInt(ITHACAdict->lookup("N_modes_T"));
            NmodesDEIMA = readInt(ITHACAdict->lookup("N_modes_DEIM_A"));
            NmodesDEIMB = readInt(ITHACAdict->lookup("N_modes_DEIM_B"));
        }
 
        volScalarField& nu;
        volScalarField& S;
        volScalarField& T;
 
 
        DEIM_function* DEIMmatrice;
        PtrList<fvScalarMatrix> Mlist;
        Eigen::MatrixXd ModesTEig;
        std::vector<Eigen::MatrixXd> ReducedMatricesA;
        std::vector<Eigen::MatrixXd> ReducedVectorsB;
 
        int NTmodes;
        int NmodesDEIMA;
        int NmodesDEIMB;
 
        double time_full;
        double time_rom;
 
        void OfflineSolve(Eigen::MatrixXd par, word Folder)
        {
            if (offline)
            {
                ITHACAstream::read_fields(Tfield, T, "./ITHACAoutput/Offline/");
            }
            else
            {
                for (int i = 0; i < par.rows(); i++)
                {
                    fvScalarMatrix Teqn = DEIMmatrice->evaluate_expression(T, par.row(i));
                    Teqn.solve();
                    Mlist.append((Teqn).clone());
                    ITHACAstream::exportSolution(T, name(i + 1), "./ITHACAoutput/" + Folder);
                    Tfield.append((T).clone());
                }
            }
        };
 
        void OnlineSolveFull(Eigen::MatrixXd par, word Folder)
        {
            auto t1 = std::chrono::high_resolution_clock::now();
            auto t2 = std::chrono::high_resolution_clock::now();
            auto time_span = std::chrono::duration_cast<std::chrono::duration<double>>
                             (t2 - t1);
            time_full = 0;
 
            for (int i = 0; i < par.rows(); i++)
            {
                fvScalarMatrix Teqn = DEIMmatrice->evaluate_expression(T, par.row(i));
                t1 = std::chrono::high_resolution_clock::now();
                Teqn.solve();
                t2 = std::chrono::high_resolution_clock::now();
                time_span = std::chrono::duration_cast<std::chrono::duration<double>>(t2 - t1);
                time_full += time_span.count();
                ITHACAstream::exportSolution(T, name(i + 1), "./ITHACAoutput/" + Folder);
                Tfield.append((T).clone());
            }
        };
 
        void PODDEIM()
        {
            PODDEIM(NTmodes, NmodesDEIMA, NmodesDEIMB);
        }
 
        void PODDEIM(int NmodesT, int NmodesDEIMA, int NmodesDEIMB)
        {
            DEIMmatrice = new DEIM_function(Mlist, NmodesDEIMA, NmodesDEIMB, "T_matrix");
            fvMesh& mesh  =  const_cast<fvMesh&>(T.mesh());
            // Differential Operator
            DEIMmatrice->fieldA = autoPtr<volScalarField>(new volScalarField(
                                      DEIMmatrice->generateSubmeshMatrix(2, mesh, T)));
            DEIMmatrice->fieldB = autoPtr<volScalarField>(new volScalarField(
                                      DEIMmatrice->generateSubmeshVector(2, mesh, T)));
            // Source Terms
            ModesTEig = Foam2Eigen::PtrList2Eigen(Tmodes);
            ModesTEig.conservativeResize(ModesTEig.rows(), NmodesT);
            ReducedMatricesA.resize(NmodesDEIMA);
            ReducedVectorsB.resize(NmodesDEIMB);
 
            for (int i = 0; i < NmodesDEIMA; i++)
            {
                ReducedMatricesA[i] = ModesTEig.transpose() * DEIMmatrice->MatrixOnlineA[i] *
                                      ModesTEig;
            }
 
            for (int i = 0; i < NmodesDEIMB; i++)
            {
                ReducedVectorsB[i] = ModesTEig.transpose() * DEIMmatrice->MatrixOnlineB;
            }
        };
 
        void OnlineSolve(Eigen::MatrixXd par_new, word Folder)
        {
            auto t1 = std::chrono::high_resolution_clock::now();
            auto t2 = std::chrono::high_resolution_clock::now();
            auto time_span = std::chrono::duration_cast<std::chrono::duration<double>>
                             (t2 - t1);
            time_rom = 0;
 
            for (int i = 0; i < par_new.rows(); i++)
            {
                // solve
                t1 = std::chrono::high_resolution_clock::now();
                Eigen::MatrixXd thetaonA = DEIMmatrice->onlineCoeffsA(par_new.row(i));
                Eigen::MatrixXd thetaonB = DEIMmatrice->onlineCoeffsB(par_new.row(i));
                Eigen::MatrixXd A = EigenFunctions::MVproduct(ReducedMatricesA, thetaonA);
                Eigen::VectorXd B = EigenFunctions::MVproduct(ReducedVectorsB, thetaonB);
                Eigen::VectorXd x = A.fullPivLu().solve(B);
                Eigen::VectorXd full = ModesTEig * x;
                t2 = std::chrono::high_resolution_clock::now();
                time_span = std::chrono::duration_cast<std::chrono::duration<double>>(t2 - t1);
                time_rom += time_span.count();
                // Export
                volScalarField Tred("Tred", T);
                Tred = Foam2Eigen::Eigen2field(Tred, full);
                ITHACAstream::exportSolution(Tred, name(i + 1), "./ITHACAoutput/" + Folder);
                Tonline.append((Tred).clone());
            }
        }
};
 
int main(int argc, char* argv[])
{
    // Construct the case
    DEIMLaplacian example(argc, argv);
    // Read some parameters from file
    ITHACAparameters* para = ITHACAparameters::getInstance(example._mesh(),
                             example._runTime());
    // Create the offline parameters for the solve
    example.mu = ITHACAutilities::rand(100, 2, -0.5, 0.5);
    // Solve the offline problem to compute the snapshots for the projections
    example.OfflineSolve(example.mu, "Offline");
    // Compute the POD modes
    ITHACAPOD::getModes(example.Tfield, example.Tmodes, example._T().name(),
                        example.podex, 0, 0, 20);
    // Compute the offline part of the DEIM procedure
    example.PODDEIM();
    // Construct a new set of parameters
    Eigen::MatrixXd par_new1 = ITHACAutilities::rand(100, 2, -0.5, 0.5);
    // Solve the online problem with the new parameters
    example.OnlineSolve(par_new1, "Online_red");
    // Solve a new full problem with the new parameters (necessary to compute speed up and error)
    DEIMLaplacian example_new(argc, argv);
    example_new.OnlineSolveFull(par_new1, "Online_full");
    // Output some infos
    std::cout << std::endl << "The FOM Solve took: " << example_new.time_full  <<
              " seconds." << std::endl;
    std::cout << std::endl << "The ROM Solve took: " << example.time_rom  <<
              " seconds." << std::endl;
    std::cout << std::endl << "The Speed-up is: " << example_new.time_full /
              example.time_rom  << std::endl << std::endl;
    Eigen::MatrixXd error = ITHACAutilities::errorL2Abs(example_new.Tfield,
                            example.Tonline);
    std::cout << "The mean L2 error is: " << error.mean() << std::endl;
    exit(0);
}