Exemple #1
0
void iniLattice( MultiBlockLattice3D<T,DESCRIPTOR>& lattice,
                 VoxelizedDomain3D<T>& voxelizedDomain )
{
    // Switch all remaining outer cells to no-dynamics, except the outer
    //   boundary layer, and keep the rest as BGKdynamics.
    defineDynamics(lattice, voxelizedDomain.getVoxelMatrix(), lattice.getBoundingBox(),
                   new NoDynamics<T,DESCRIPTOR>, voxelFlag::outside);
    initializeAtEquilibrium(lattice, lattice.getBoundingBox(), (T) 1., Array<T,3>((T)0.,(T)0.,(T)0.));
    lattice.initialize();
}
Exemple #2
0
void cavitySetup( MultiBlockLattice3D<T,DESCRIPTOR>& lattice,
                  IncomprFlowParam<T> const& parameters,
                  OnLatticeBoundaryCondition3D<T,DESCRIPTOR>& boundaryCondition )
{
    const plint nx = parameters.getNx();
    const plint ny = parameters.getNy();
    const plint nz = parameters.getNz();
    Box3D topLid = Box3D(0, nx-1, ny-1, ny-1, 0, nz-1);
    Box3D bottomLid = Box3D(0, nx-1, 0, 0, 0, nz-1);
    Box3D everythingButTopLid = Box3D(0, nx-1, 0, ny-2, 0, nz-1);

    /*
    instantiateOuterNLDboundary(lattice, lattice.getBoundingBox());
    setOuterNLDboundaryDynamics(lattice, lattice.getBackgroundDynamics().clone(),
                                lattice.getBoundingBox(), boundary::dirichlet);
    setOuterNLDboundaryDynamics(lattice, lattice.getBackgroundDynamics().clone(), bottomLid, boundary::neumann);
    defineDynamics(lattice, bottomLid, lattice.getBackgroundDynamics().clone());
    */

    boundaryCondition.setVelocityConditionOnBlockBoundaries(lattice, lattice.getBoundingBox(), boundary::dirichlet);

    T u = sqrt((T)2)/(T)2 * parameters.getLatticeU();
    initializeAtEquilibrium(lattice, everythingButTopLid, (T) 1., Array<T,3>(0.,0.,0.) );
    initializeAtEquilibrium(lattice, topLid, (T) 1., Array<T,3>(u,0.,u) );
    setBoundaryVelocity(lattice, topLid, Array<T,3>(u,0.,u) );

    lattice.initialize();
}
void writeVTK(MultiBlockLattice3D<T,DESCRIPTOR>& lattice,
              IncomprFlowParam<T> const& parameters,
              PhysUnits3D<T> const& units, plint iter)
{

    T p_fact = units.getPhysForce(1)/pow(units.getPhysLength(1),2)/3.;

    std::string fname(createFileName("vtk", iter, 6));

    VtkImageOutput3D<T> vtkOut(fname, units.getPhysLength(1));
    vtkOut.writeData<3,float>(*computeVelocity(lattice), "velocity", units.getPhysVel(1));
    vtkOut.writeData<float>(*computeDensity(lattice), "density",units.getPhysDensity(1));

    MultiScalarField3D<T> p(*computeDensity(lattice));
    subtractInPlace(p,1.);
    vtkOut.writeData<float>(p,"pressure",p_fact );

    IBscalarQuantity sf = SolidFraction;
    applyProcessingFunctional(new GetScalarQuantityFromDynamicsFunctional<T,DESCRIPTOR,T>(sf),
                              lattice.getBoundingBox(),lattice,p);

    vtkOut.writeData<float>(p,"solidfraction",1. );


    pcout << "wrote " << fname << std::endl;
}
void channelSetup( MultiBlockLattice3D<T,DESCRIPTOR>& lattice,
                   IncomprFlowParam<T> const& parameters,
                   OnLatticeBoundaryCondition3D<T,DESCRIPTOR>& boundaryCondition )
{
    // Create Velocity boundary conditions
    boundaryCondition.setVelocityConditionOnBlockBoundaries(lattice);

    setBoundaryVelocity (
        lattice, lattice.getBoundingBox(),
        PoiseuilleVelocity<T>(parameters) );

    lattice.initialize();
}
Exemple #5
0
int main(int argc, char* argv[]) {

    plbInit(&argc, &argv);
    //defaultMultiBlockPolicy3D().toggleBlockingCommunication(true);

    plint N;
    try {
        global::argv(1).read(N);
    }
    catch(...)
    {
        pcout << "Wrong parameters. The syntax is " << std::endl;
        pcout << argv[0] << " N" << std::endl;
        pcout << "where N is the resolution. The benchmark cases published " << std::endl;
        pcout << "on the Palabos Wiki use N=100, N=400, N=1000, or N=4000." << std::endl;
        exit(1);
    }

    pcout << "Starting benchmark with " << N+1 << "x" << N+1 << "x" << N+1 << " grid points "
          << "(approx. 2 minutes on modern processors)." << std::endl;


    IncomprFlowParam<T> parameters(
            (T) 1e-2,  // uMax
            (T) 1.,    // Re
            N,         // N
            1.,        // lx
            1.,        // ly
            1.         // lz
    );


    MultiBlockLattice3D<T, DESCRIPTOR> lattice (
            parameters.getNx(), parameters.getNy(), parameters.getNz(),
            new BGKdynamics<T,DESCRIPTOR>(parameters.getOmega()) );

    plint numCores = global::mpi().getSize();
    pcout << "Number of MPI threads: " << numCores << std::endl;
    // Current cores run approximately at 5 Mega Sus.
    T estimateSus= 5.e6*numCores;
    // The benchmark should run for approximately two minutes
    // (2*60 seconds).
    T wishNumSeconds = 60.;
    plint numCells = lattice.getBoundingBox().nCells();

    // Run at least three iterations.
    plint numIter = std::max( (plint)3,
                              (plint)(estimateSus*wishNumSeconds/numCells+0.5));

    OnLatticeBoundaryCondition3D<T,DESCRIPTOR>* boundaryCondition
        = createLocalBoundaryCondition3D<T,DESCRIPTOR>();

    cavitySetup(lattice, parameters, *boundaryCondition);

    // Run the benchmark once "to warm up the machine".
    for (plint iT=0; iT<numIter; ++iT) {
        lattice.collideAndStream();
    }

    // Run the benchmark for good.
    global::timer("benchmark").start();
    global::profiler().turnOn();
    for (plint iT=0; iT<numIter; ++iT) {
        lattice.collideAndStream();
    }

    pcout << "After " << numIter << " iterations: "
          << (T) (numCells*numIter) /
             global::timer("benchmark").getTime() / 1.e6
          << " Mega site updates per second." << std::endl << std::endl;

    global::profiler().writeReport();

    delete boundaryCondition;
}
Exemple #6
0
int main(int argc, char* argv[]) {

    plbInit(&argc, &argv);
    global::directories().setOutputDir("./tmp/");

    IncomprFlowParam<T> parameters(
            (T) 1e-2,  // uMax
            (T) 10.,   // Re
            30,        // N
            1.,        // lx
            1.,        // ly
            1.         // lz
    );
    const T logT     = (T)1/(T)100;
    const T imSave   = (T)1/(T)10;
    const T vtkSave  = (T)1;
    const T maxT     = (T)10.1;

    pcout << "omega= " << parameters.getOmega() << std::endl;
    writeLogFile(parameters, "3D diagonal cavity");

    T omega = parameters.getOmega();

#ifdef USE_MRT
    plint mrtId = 0;
    mrtParam<T,DESCRIPTOR>().set(mrtId,MRTparam<T,DESCRIPTOR>(omega));
#endif


    MultiBlockLattice3D<T, DESCRIPTOR> lattice (
            parameters.getNx(), parameters.getNy(), parameters.getNz(), DYNAMICS );

#ifdef USE_ASINARI
        integrateProcessingFunctional(new AsinariPostCollide3D<T,DESCRIPTOR>, lattice.getBoundingBox(), lattice, 0);
#endif

    OnLatticeBoundaryCondition3D<T,DESCRIPTOR>* boundaryCondition
        //= createInterpBoundaryCondition3D<T,DESCRIPTOR>();
        = createLocalBoundaryCondition3D<T,DESCRIPTOR>();

    cavitySetup(lattice, parameters, *boundaryCondition);

    T previousIterationTime = T();
    // Loop over main time iteration.
    for (plint iT=0; iT<parameters.nStep(maxT); ++iT) {
        global::timer("mainLoop").restart();

        if (iT%parameters.nStep(imSave)==0) {
            pcout << "Writing Gif ..." << endl;
            writeGifs(lattice, parameters, iT);
        }

        if (iT%parameters.nStep(vtkSave)==0 && iT>0) {
            pcout << "Saving VTK file ..." << endl;
            writeVTK(lattice, parameters, iT);
        }

        if (iT%parameters.nStep(logT)==0) {
            pcout << "step " << iT
                  << "; t=" << iT*parameters.getDeltaT();
        }

        // Execute a time iteration.
        lattice.collideAndStream();

        // Access averages from internal statistics ( their value is defined
        //   only after the call to lattice.collideAndStream() )
        if (iT%parameters.nStep(logT)==0) {
            pcout << "; av energy="
                  << setprecision(10) << getStoredAverageEnergy<T>(lattice)
                  << "; av rho="
                  << setprecision(10) << getStoredAverageDensity<T>(lattice) << endl;
            pcout << "Time spent during previous iteration: "
                  << previousIterationTime << endl;
        }

        previousIterationTime = global::timer("mainLoop").stop();
    }

    delete boundaryCondition;
}
void writeVTK(MultiBlockLattice3D<T,DESCRIPTOR>& lattice, plint iter){
        VtkImageOutput3D<T> vtkOut(createFileName("vtk", iter, 6), 1.);
        vtkOut.writeData<float>(*computeDensity(lattice), "density", 1.);
        std::auto_ptr<MultiScalarField3D<T> > velocity(plb::computeVelocityComponent(lattice, lattice.getBoundingBox(), 2));
        
        vtkOut.writeData<T>(*velocity, "velocity", 1.);
}
int main(int argc, char* argv[]) {

    plbInit(&argc, &argv);

    T uMax;

    plint N;

    T nu_f,d_part,v_frac, v_inf;

    std::string outDir;

    try {
        global::argv(1).read(d_part);
        global::argv(2).read(N);
        global::argv(3).read(v_frac);
        global::argv(4).read(nu_f);
        global::argv(5).read(v_inf);
        global::argv(6).read(uMax);
        global::argv(7).read(outDir);
    } catch(PlbIOException& exception) {
        pcout << exception.what() << endl;
        pcout << "Command line arguments:\n";
        pcout << "1 : d_part\n";
        pcout << "2 : N per particle diameter\n";
        pcout << "3 : particle volume fraction\n";
        pcout << "4 : nu_fluid\n";
        pcout << "5 : estimated v_inf\n";
        pcout << "6 : uMax\n";
        pcout << "7 : outDir\n";
        exit(1);
    }

    std::string lbOutDir(outDir), demOutDir(outDir);
    lbOutDir.append("tmp/");
    demOutDir.append("post/");
    global::directories().setOutputDir(lbOutDir);

    const T rho_f = 1000;

    LiggghtsCouplingWrapper wrapper(argv,global::mpi().getGlobalCommunicator());

    // particle size and volume fraction are handed over to LIGGGHTS
    // as variables (see LIGGGHTS docu for details)
    wrapper.setVariable("r_part",d_part/2);
    wrapper.setVariable("v_frac",v_frac);

    wrapper.execFile("in.lbdem");


    T g = 9.81;

    const T lx = 1., ly = 1., lz = 2.;


    T r_ = d_part/2.;
    T rho_s = 1100.;
    T m = r_*r_*r_*4./3.*3.14*rho_s;

    PhysUnits3D<T> units(2.*r_,v_inf,nu_f,lx,ly,lz,N,uMax,rho_f);

    IncomprFlowParam<T> parameters(units.getLbParam());

    plint nx = parameters.getNx(), ny = parameters.getNy(), nz = parameters.getNz();

    // get lattice decomposition from LIGGGHTS and create lattice according to parallelization
    // given in the LIGGGHTS input script
    LatticeDecomposition lDec(parameters.getNx(),parameters.getNy(),parameters.getNz(),
                              wrapper.lmp);
    SparseBlockStructure3D blockStructure = lDec.getBlockDistribution();
    ExplicitThreadAttribution* threadAttribution = lDec.getThreadAttribution();
    plint envelopeWidth = 1;

    MultiBlockLattice3D<T, DESCRIPTOR>
    lattice (MultiBlockManagement3D (blockStructure, threadAttribution, envelopeWidth ),
             defaultMultiBlockPolicy3D().getBlockCommunicator(),
             defaultMultiBlockPolicy3D().getCombinedStatistics(),
             defaultMultiBlockPolicy3D().getMultiCellAccess<T,DESCRIPTOR>(),
             new DYNAMICS );

    defineDynamics(lattice,lattice.getBoundingBox(),new DYNAMICS);


    const T maxT = ceil(3.*lz/v_inf);
    const T vtkT = 0.1;
    const T logT = 0.0000001;

    const plint maxSteps = units.getLbSteps(maxT);
    const plint vtkSteps = max<plint>(units.getLbSteps(vtkT),1);
    const plint logSteps = max<plint>(units.getLbSteps(logT),1);

    writeLogFile(parameters, "sedimenting spheres benchmark");


    lattice.initialize();
    T dt_phys = units.getPhysTime(1);
    plint demSubsteps = 10;
    T dt_dem = dt_phys/(T)demSubsteps;


    pcout << "------------------------------\n"
          << "omega: " << parameters.getOmega() << "\n"
          << "dt_phys: " << dt_phys << "\n"
          << "maxT: " << maxT << " | maxSteps: " << maxSteps << "\n"
          << "v_inf: " << v_inf << "\n"
          << "Re : " << parameters.getRe() << "\n"
          << "vtkT: " << vtkT << " | vtkSteps: " << vtkSteps << "\n"
          << "grid size: " << nx << " " << ny << " " << nz << "\n"
          << "------------------------------" << std::endl;
    // set timestep and output directory
    wrapper.setVariable("t_step",dt_dem);
    wrapper.setVariable("dmp_stp",vtkSteps*demSubsteps);
    wrapper.setVariable("dmp_dir",demOutDir);


    wrapper.execFile("in2.lbdem");
    wrapper.runUpto(demSubsteps-1);

    clock_t start = clock();
    clock_t loop = clock();
    clock_t end = clock();
    // Loop over main time iteration.
    for (plint iT=0; iT<=maxSteps; ++iT) {

        bool initWithVel = false;
        setSpheresOnLattice(lattice,wrapper,units,initWithVel);


        if(iT%vtkSteps == 0 && iT > 0) // LIGGGHTS does not write at timestep 0
            writeVTK(lattice,parameters,units,iT);

        lattice.collideAndStream();

        getForcesFromLattice(lattice,wrapper,units);

        wrapper.run(demSubsteps);


        if(iT%logSteps == 0) {
            end = clock();
            T time = difftime(end,loop)/((T)CLOCKS_PER_SEC);
            T totaltime = difftime(end,start)/((T)CLOCKS_PER_SEC);
            T mlups = ((T) (lattice.getNx()*lattice.getNy()*lattice.getNz()*logSteps))/time/1e6;
            pcout << "time: " << time << " " ;
            pcout << "calculating at " << mlups << " MLU/s"
                  << " | total time running: " << totaltime << std::endl;
            loop = clock();
        }
    }
    T totaltime = difftime(end,start)/((T)CLOCKS_PER_SEC);
    T totalmlups = ((T) (lattice.getNx()*lattice.getNy()*lattice.getNz()*(maxSteps+1)))/totaltime/1e6;
    pcout << " ********************** \n"
          << "total time: " << totaltime
          << " calculating at " << totalmlups << " MLU/s" << std::endl;

}
void runProgram()
{
    /*
     * Read the obstacle geometry.
     */

    pcout << std::endl << "Reading STL data for the obstacle geometry." << std::endl;
    Array<T,3> center(param.cx, param.cy, param.cz);
    Array<T,3> centerLB(param.cxLB, param.cyLB, param.czLB);
    // The triangle-set defines the surface of the geometry.
    TriangleSet<T> triangleSet(param.geometry_fname, DBL);

    // Place the obstacle in the correct place in the simulation domain.
    // Here the "geometric center" of the obstacle is computed manually,
    // by computing first its bounding cuboid. In cases that the STL
    // file with the geometry of the obstacle contains its center as
    // the point, say (0, 0, 0), then the following variable
    // "obstacleCenter" must be set to (0, 0, 0) manually.
    Cuboid<T> bCuboid = triangleSet.getBoundingCuboid();
    Array<T,3> obstacleCenter = 0.5 * (bCuboid.lowerLeftCorner + bCuboid.upperRightCorner);
    triangleSet.translate(-obstacleCenter);
    triangleSet.scale(1.0/param.dx); // In lattice units from now on...
    triangleSet.translate(centerLB);
    triangleSet.writeBinarySTL(outputDir+"obstacle_LB.stl");

    // The DEFscaledMesh, and the triangle-boundary are more sophisticated data
    // structures used internally by Palabos to treat the boundary.
    plint xDirection = 0;
    plint borderWidth = 1;      // Because Guo acts in a one-cell layer.
                                // Requirement: margin>=borderWidth.
    plint margin = 1;           // Extra margin of allocated cells around the obstacle, for the case of moving walls.
    plint blockSize = 0;        // Size of blocks in the sparse/parallel representation.
                                // Zero means: don't use sparse representation.
    DEFscaledMesh<T> defMesh(triangleSet, 0, xDirection, margin, Dot3D(0, 0, 0));
    TriangleBoundary3D<T> boundary(defMesh);
    //boundary.getMesh().inflate();

    pcout << "tau = " << 1.0/param.omega << std::endl;
    pcout << "dx = " << param.dx << std::endl;
    pcout << "dt = " << param.dt << std::endl;
    pcout << "Number of iterations in an integral time scale: " << (plint) (1.0/param.dt) << std::endl;

    /*
     * Voxelize the domain.
     */

    // Voxelize the domain means: decide which lattice nodes are inside the obstacle and which are outside.
    pcout << std::endl << "Voxelizing the domain." << std::endl;
    plint extendedEnvelopeWidth = 2;   // Extrapolated off-lattice BCs.
    const int flowType = voxelFlag::outside;
    VoxelizedDomain3D<T> voxelizedDomain (
            boundary, flowType, param.boundingBox(), borderWidth, extendedEnvelopeWidth, blockSize );
    pcout << getMultiBlockInfo(voxelizedDomain.getVoxelMatrix()) << std::endl;

    /*
     * Generate the lattice, the density and momentum blocks.
     */

    pcout << "Generating the lattice, the rhoBar and j fields." << std::endl;
    MultiBlockLattice3D<T,DESCRIPTOR> *lattice = new MultiBlockLattice3D<T,DESCRIPTOR>(voxelizedDomain.getVoxelMatrix());
    if (param.useSmago) {
        defineDynamics(*lattice, lattice->getBoundingBox(),
                new SmagorinskyBGKdynamics<T,DESCRIPTOR>(param.omega, param.cSmago));
        pcout << "Using Smagorinsky BGK dynamics." << std::endl;
    } else {
        defineDynamics(*lattice, lattice->getBoundingBox(),
                new BGKdynamics<T,DESCRIPTOR>(param.omega));
        pcout << "Using BGK dynamics." << std::endl;
    }
    bool velIsJ = false;
    defineDynamics(*lattice, voxelizedDomain.getVoxelMatrix(), lattice->getBoundingBox(),
            new NoDynamics<T,DESCRIPTOR>(), voxelFlag::inside);
    lattice->toggleInternalStatistics(false);

    MultiBlockManagement3D sparseBlockManagement(lattice->getMultiBlockManagement());

    // The rhoBar and j fields are used at both the collision and at the implementation of the
    // outflow boundary condition.
    plint envelopeWidth = 1;
    MultiScalarField3D<T> *rhoBar = new MultiScalarField3D<T> (
            MultiBlockManagement3D (
                sparseBlockManagement.getSparseBlockStructure(),
                sparseBlockManagement.getThreadAttribution().clone(),
                envelopeWidth ),
            defaultMultiBlockPolicy3D().getBlockCommunicator(),
            defaultMultiBlockPolicy3D().getCombinedStatistics(),
            defaultMultiBlockPolicy3D().getMultiScalarAccess<T>() );
    rhoBar->toggleInternalStatistics(false);

    MultiTensorField3D<T,3> *j = new MultiTensorField3D<T,3> (
            MultiBlockManagement3D (
                sparseBlockManagement.getSparseBlockStructure(),
                sparseBlockManagement.getThreadAttribution().clone(),
                envelopeWidth ),
            defaultMultiBlockPolicy3D().getBlockCommunicator(),
            defaultMultiBlockPolicy3D().getCombinedStatistics(),
            defaultMultiBlockPolicy3D().getMultiTensorAccess<T,3>() );
    j->toggleInternalStatistics(false);

    std::vector<MultiBlock3D*> lattice_rho_bar_j_arg;
    lattice_rho_bar_j_arg.push_back(lattice);
    lattice_rho_bar_j_arg.push_back(rhoBar);
    lattice_rho_bar_j_arg.push_back(j);
    integrateProcessingFunctional(
            new ExternalRhoJcollideAndStream3D<T,DESCRIPTOR>(),
            lattice->getBoundingBox(), lattice_rho_bar_j_arg, 0);
    integrateProcessingFunctional(
            new BoxRhoBarJfunctional3D<T,DESCRIPTOR>(),
            lattice->getBoundingBox(), lattice_rho_bar_j_arg, 3); // rhoBar and j are computed at level 3 because
                                                                  // the boundary conditions are on levels 1 and 2.

    /*
     * Generate the off-lattice boundary condition on the obstacle and the outer-domain boundary conditions.
     */

    pcout << "Generating boundary conditions." << std::endl;

    OffLatticeBoundaryCondition3D<T,DESCRIPTOR,Velocity> *boundaryCondition;
    
    BoundaryProfiles3D<T,Velocity> profiles;
    bool useAllDirections=true;
    OffLatticeModel3D<T,Velocity>* offLatticeModel=0;
    if (param.freeSlipWall) {
        profiles.setWallProfile(new FreeSlipProfile3D<T>);
    }
    else {
        profiles.setWallProfile(new NoSlipProfile3D<T>);
    }
    offLatticeModel =
         new GuoOffLatticeModel3D<T,DESCRIPTOR> (
            new TriangleFlowShape3D<T,Array<T,3> >(voxelizedDomain.getBoundary(), profiles),
            flowType, useAllDirections );
    offLatticeModel->setVelIsJ(velIsJ);
    boundaryCondition = new OffLatticeBoundaryCondition3D<T,DESCRIPTOR,Velocity>(
            offLatticeModel, voxelizedDomain, *lattice);
    
    boundaryCondition->insert();

    // The boundary condition algorithm or the outer domain.
    OnLatticeBoundaryCondition3D<T,DESCRIPTOR>* outerBoundaryCondition
        = createLocalBoundaryCondition3D<T,DESCRIPTOR>();
    outerDomainBoundaries(lattice, rhoBar, j, outerBoundaryCondition);

    /*
     * Implement the outlet sponge zone.
     */

    if (param.numOutletSpongeCells > 0) {
        T bulkValue;
        Array<plint,6> numSpongeCells;

        if (param.outletSpongeZoneType == 0) {
            pcout << "Generating an outlet viscosity sponge zone." << std::endl;
            bulkValue = param.omega;
        } else if (param.outletSpongeZoneType == 1) {
            pcout << "Generating an outlet Smagorinsky sponge zone." << std::endl;
            bulkValue = param.cSmago;
        } else {
            pcout << "Error: unknown type of sponge zone." << std::endl;
            exit(-1);
        }

        // Number of sponge zone lattice nodes at all the outer domain boundaries.
        // So: 0 means the boundary at x = 0
        //     1 means the boundary at x = nx-1
        //     2 means the boundary at y = 0
        //     and so on...
        numSpongeCells[0] = 0;
        numSpongeCells[1] = param.numOutletSpongeCells;
        numSpongeCells[2] = 0;
        numSpongeCells[3] = 0;
        numSpongeCells[4] = 0;
        numSpongeCells[5] = 0;

        std::vector<MultiBlock3D*> args;
        args.push_back(lattice);

        if (param.outletSpongeZoneType == 0) {
            applyProcessingFunctional(new ViscositySpongeZone<T,DESCRIPTOR>(
                        param.nx, param.ny, param.nz, bulkValue, numSpongeCells),
                    lattice->getBoundingBox(), args);
        } else {
            applyProcessingFunctional(new SmagorinskySpongeZone<T,DESCRIPTOR>(
                        param.nx, param.ny, param.nz, bulkValue, param.targetSpongeCSmago, numSpongeCells),
                    lattice->getBoundingBox(), args);
        }
    }

    /*
     * Setting the initial conditions.
     */

    // Initial condition: Constant pressure and velocity-at-infinity everywhere.
    Array<T,3> uBoundary(param.getInletVelocity(0), 0.0, 0.0);
    initializeAtEquilibrium(*lattice, lattice->getBoundingBox(), 1.0, uBoundary);
    applyProcessingFunctional(
            new BoxRhoBarJfunctional3D<T,DESCRIPTOR>(),
            lattice->getBoundingBox(), lattice_rho_bar_j_arg); // Compute rhoBar and j before VirtualOutlet is executed.
    lattice->executeInternalProcessors(1); // Execute all processors except the ones at level 0.
    lattice->executeInternalProcessors(2);
    lattice->executeInternalProcessors(3);

    /*
     * Particles (streamlines).
     */

    // This part of the code that relates to particles, is purely for visualization
    // purposes. Particles are used to compute streamlines essentially.

    // Definition of a particle field.
    MultiParticleField3D<ParticleFieldT>* particles = 0;

    if (param.useParticles) {
        particles = new MultiParticleField3D<ParticleFieldT> (
            lattice->getMultiBlockManagement(),
            defaultMultiBlockPolicy3D().getCombinedStatistics() );

        std::vector<MultiBlock3D*> particleArg;
        particleArg.push_back(particles);

        std::vector<MultiBlock3D*> particleFluidArg;
        particleFluidArg.push_back(particles);
        particleFluidArg.push_back(lattice);

        // Functional that advances the particles to their new position at each predefined time step.
        integrateProcessingFunctional (
                new AdvanceParticlesEveryWhereFunctional3D<T,DESCRIPTOR>(param.cutOffSpeedSqr),
                lattice->getBoundingBox(), particleArg, 0);
        // Functional that assigns the particle velocity according to the particle's position in the fluid.
        integrateProcessingFunctional (
                new FluidToParticleCoupling3D<T,DESCRIPTOR>((T) param.particleTimeFactor),
                lattice->getBoundingBox(), particleFluidArg, 1 );

        // Definition of a domain from which particles will be injected in the flow field.
        Box3D injectionDomain(0, 0, centerLB[1]-0.25*param.ny, centerLB[1]+0.25*param.ny,
                centerLB[2]-0.25*param.nz, centerLB[2]+0.25*param.nz);

        // Definition of simple mass-less particles.
        Particle3D<T,DESCRIPTOR>* particleTemplate=0;
        particleTemplate = new PointParticle3D<T,DESCRIPTOR>(0, Array<T,3>(0.,0.,0.), Array<T,3>(0.,0.,0.));

        // Functional which injects particles with predefined probability from the specified injection domain.
        std::vector<MultiBlock3D*> particleInjectionArg;
        particleInjectionArg.push_back(particles);

        integrateProcessingFunctional (
                new InjectRandomParticlesFunctional3D<T,DESCRIPTOR>(particleTemplate, param.particleProbabilityPerCell),
                injectionDomain, particleInjectionArg, 0 );

        // Definition of an absorbtion domain for the particles.
        Box3D absorbtionDomain(param.outlet);

        // Functional which absorbs the particles which reach the specified absorbtion domain.
        integrateProcessingFunctional (
                new AbsorbParticlesFunctional3D<T,DESCRIPTOR>, absorbtionDomain, particleArg, 0 );

        particles->executeInternalProcessors();
    }

    /*
     * Starting the simulation.
     */

    plb_ofstream energyFile((outputDir+"average_energy.dat").c_str());

    pcout << std::endl;
    pcout << "Starting simulation." << std::endl;
    for (plint i = 0; i < param.maxIter; ++i) {
        if (i <= param.initialIter) {
            Array<T,3> uBoundary(param.getInletVelocity(i), 0.0, 0.0);
            setBoundaryVelocity(*lattice, param.inlet, uBoundary);
        }

        if (i % param.statIter == 0) {
             pcout << "At iteration " << i << ", t = " << i*param.dt << std::endl;
             Array<T,3> force(boundaryCondition->getForceOnObject());
             T factor = util::sqr(util::sqr(param.dx)) / util::sqr(param.dt);
             pcout << "Force on object over fluid density: F[x] = " << force[0]*factor << ", F[y] = "
                   << force[1]*factor << ", F[z] = " << force[2]*factor << std::endl;
             T avEnergy = boundaryCondition->computeAverageEnergy() * util::sqr(param.dx) / util::sqr(param.dt);
             pcout << "Average kinetic energy over fluid density: E = " << avEnergy << std::endl;
             energyFile << i*param.dt << "  " << avEnergy << std::endl;
             pcout << std::endl;
        }

        if (i % param.vtkIter == 0) {
            pcout << "Writing VTK at time t = " << i*param.dt << endl;
            writeVTK(*boundaryCondition, i);
            if (param.useParticles) {
                writeParticleVtk<T,DESCRIPTOR> (
                        *particles, createFileName(outputDir+"particles_", i, PADDING) + ".vtk",
                        param.maxNumParticlesToWrite );
            }
        }

        if (i % param.imageIter == 0) {
            pcout << "Writing PPM image at time t = " << i*param.dt << endl;
            writePPM(*boundaryCondition, i);
        }

        lattice->executeInternalProcessors();
        lattice->incrementTime();
        if (param.useParticles && i % param.particleTimeFactor == 0) {
            particles->executeInternalProcessors();
        }
    }
    
    energyFile.close();
    delete outerBoundaryCondition;
    delete boundaryCondition;
    if (param.useParticles) {
        delete particles;
    }
    delete j;
    delete rhoBar;
    delete lattice;
}