Program Listing for File Elasticity_Becker2009.cpp

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#include "Elasticity_Becker2009.h"
#include "SPlisHSPlasH/Simulation.h"
#include "SPlisHSPlasH/Utilities/MathFunctions.h"

using namespace SPH;
using namespace GenParam;

int Elasticity_Becker2009::ALPHA = -1;


Elasticity_Becker2009::Elasticity_Becker2009(FluidModel *model) :
    ElasticityBase(model)
{
    const unsigned int numParticles = model->numActiveParticles();
    m_restVolumes.resize(numParticles);
    m_current_to_initial_index.resize(numParticles);
    m_initial_to_current_index.resize(numParticles);
    m_initialNeighbors.resize(numParticles);
    m_rotations.resize(numParticles, Matrix3r::Identity());
    m_stress.resize(numParticles);
    m_F.resize(numParticles);
    m_alpha = 0.0;

    initValues();

    model->addField({ "rest volume", FieldType::Scalar, [&](const unsigned int i) -> Real* { return &m_restVolumes[i]; }, true });
    model->addField({ "rotation", FieldType::Matrix3, [&](const unsigned int i) -> Real* { return &m_rotations[i](0,0); } });
    model->addField({ "stress", FieldType::Vector6, [&](const unsigned int i) -> Real* { return &m_stress[i][0]; } });
    model->addField({ "deformation gradient", FieldType::Matrix3, [&](const unsigned int i) -> Real* { return &m_F[i](0,0); } });
}

Elasticity_Becker2009::~Elasticity_Becker2009(void)
{
    m_model->removeFieldByName("rest volume");
    m_model->removeFieldByName("rotation");
    m_model->removeFieldByName("stress");
    m_model->removeFieldByName("deformation gradient");
}

void Elasticity_Becker2009::initParameters()
{
    ElasticityBase::initParameters();

    ALPHA = createNumericParameter("alpha", "Zero-energy modes suppression", &m_alpha);
    setGroup(ALPHA, "Fluid Model|Elasticity");
    setDescription(ALPHA, "Coefficent for zero-energy modes suppression method");
    RealParameter *rparam = static_cast<RealParameter*>(getParameter(ALPHA));
    rparam->setMinValue(0.0);
}

void Elasticity_Becker2009::initValues()
{
    Simulation *sim = Simulation::getCurrent();
    sim->getNeighborhoodSearch()->find_neighbors();

    FluidModel *model = m_model;
    const unsigned int numParticles = model->numActiveParticles();
    const unsigned int fluidModelIndex = model->getPointSetIndex();

    // Store the neighbors in the reference configurations and
    // compute the volume of each particle in rest state
    #pragma omp parallel default(shared)
    {
        #pragma omp for schedule(static)
        for (int i = 0; i < (int)numParticles; i++)
        {
            m_current_to_initial_index[i] = i;
            m_initial_to_current_index[i] = i;

            // only neighbors in same phase will influence elasticity
            const unsigned int numNeighbors = sim->numberOfNeighbors(fluidModelIndex, fluidModelIndex, i);
            m_initialNeighbors[i].resize(numNeighbors);
            for (unsigned int j = 0; j < numNeighbors; j++)
                m_initialNeighbors[i][j] = sim->getNeighbor(fluidModelIndex, fluidModelIndex, i, j);

            // compute volume
            Real density = model->getMass(i) * sim->W_zero();
            const Vector3r &xi = model->getPosition(i);
            forall_fluid_neighbors_in_same_phase(
                density += model->getMass(neighborIndex) * sim->W(xi - xj);
            )
            m_restVolumes[i] = model->getMass(i) / density;
        }
    }

    // mark all particles in the bounding box as fixed
    determineFixedParticles();
}

void Elasticity_Becker2009::step()
{
    computeRotations();
    computeStress();
    computeForces();
}


void Elasticity_Becker2009::reset()
{
    initValues();
}

void Elasticity_Becker2009::performNeighborhoodSearchSort()
{
    const unsigned int numPart = m_model->numActiveParticles();
    if (numPart == 0)
        return;

    Simulation *sim = Simulation::getCurrent();
    auto const& d = sim->getNeighborhoodSearch()->point_set(m_model->getPointSetIndex());
    d.sort_field(&m_restVolumes[0]);
    d.sort_field(&m_current_to_initial_index[0]);

    for (unsigned int i = 0; i < numPart; i++)
        m_initial_to_current_index[m_current_to_initial_index[i]] = i;
}

void Elasticity_Becker2009::computeRotations()
{
    Simulation *sim = Simulation::getCurrent();
    const unsigned int numParticles = m_model->numActiveParticles();
    const unsigned int fluidModelIndex = m_model->getPointSetIndex();
    FluidModel *model = m_model;

    #pragma omp parallel default(shared)
    {
        #pragma omp for schedule(static)
        for (int i = 0; i < (int)numParticles; i++)
        {
            const unsigned int i0 = m_current_to_initial_index[i];
            const Vector3r &xi = m_model->getPosition(i);
            const Vector3r &xi0 = m_model->getPosition0(i0);
            Matrix3r Apq;
            Apq.setZero();

            const size_t numNeighbors = m_initialNeighbors[i0].size();

            // Fluid
            for (unsigned int j = 0; j < numNeighbors; j++)
            {
                const unsigned int neighborIndex = m_initial_to_current_index[m_initialNeighbors[i0][j]];
                // get initial neighbor index considering the current particle order
                const unsigned int neighborIndex0 = m_initialNeighbors[i0][j];

                const Vector3r &xj = model->getPosition(neighborIndex);
                const Vector3r &xj0 = m_model->getPosition0(neighborIndex0);
                const Vector3r xj_xi = xj - xi;
                const Vector3r xj_xi_0 = xj0 - xi0;
                Apq += m_model->getMass(neighborIndex) * sim->W(xj_xi_0) * (xj_xi * xj_xi_0.transpose());
            }

//          Vector3r sigma;
//          Matrix3r U, VT;
//          MathFunctions::svdWithInversionHandling(Apq, sigma, U, VT);
//          m_rotations[i] = U * VT;
            Quaternionr q(m_rotations[i]);
            MathFunctions::extractRotation(Apq, q, 10);
            m_rotations[i] = q.matrix();
        }
    }
}

void Elasticity_Becker2009::computeStress()
{
    Simulation *sim = Simulation::getCurrent();
    const unsigned int numParticles = m_model->numActiveParticles();
    const unsigned int fluidModelIndex = m_model->getPointSetIndex();
    FluidModel *model = m_model;

    // Elasticity tensor
    Matrix6r C;
    C.setZero();
    const Real factor = m_youngsModulus / ((static_cast<Real>(1.0) + m_poissonRatio)*(static_cast<Real>(1.0) - static_cast<Real>(2.0) * m_poissonRatio));
    C(0, 0) = C(1, 1) = C(2, 2) = factor * (static_cast<Real>(1.0) - m_poissonRatio);
    C(0, 1) = C(0, 2) = C(1, 0) = C(1, 2) = C(2, 0) = C(2, 1) = factor * (m_poissonRatio);
    C(3, 3) = C(4, 4) = C(5, 5) = factor * static_cast<Real>(0.5)*(static_cast<Real>(1.0) - static_cast<Real>(2.0) * m_poissonRatio);

    #pragma omp parallel default(shared)
    {
        #pragma omp for schedule(static)
        for (int i = 0; i < (int)numParticles; i++)
        {
            if (model->getParticleState(i) == ParticleState::Active)
            {
                const unsigned int i0 = m_current_to_initial_index[i];
                const Vector3r& xi = m_model->getPosition(i);
                const Vector3r& xi0 = m_model->getPosition0(i0);

                Matrix3r nablaU;
                nablaU.setZero();
                const size_t numNeighbors = m_initialNeighbors[i0].size();

                // Fluid
                for (unsigned int j = 0; j < numNeighbors; j++)
                {
                    const unsigned int neighborIndex = m_initial_to_current_index[m_initialNeighbors[i0][j]];
                    // get initial neighbor index considering the current particle order
                    const unsigned int neighborIndex0 = m_initialNeighbors[i0][j];

                    const Vector3r& xj = model->getPosition(neighborIndex);
                    const Vector3r& xj0 = m_model->getPosition0(neighborIndex0);

                    const Vector3r xj_xi = xj - xi;
                    const Vector3r xj_xi_0 = xj0 - xi0;

                    const Vector3r uji = m_rotations[i].transpose() * xj_xi - xj_xi_0;
                    // subtract because kernel gradient is taken in direction of xji0 instead of xij0
                    nablaU -= (m_restVolumes[neighborIndex] * uji) * sim->gradW(xj_xi_0).transpose();
                }
                m_F[i] = nablaU + Matrix3r::Identity();

                // compute Cauchy strain: epsilon = 0.5 (nabla u + nabla u^T)
                Vector6r strain;
                strain[0] = nablaU(0, 0);                       // \epsilon_{00}
                strain[1] = nablaU(1, 1);                       // \epsilon_{11}
                strain[2] = nablaU(2, 2);                       // \epsilon_{22}
                strain[3] = static_cast<Real>(0.5) * (nablaU(0, 1) + nablaU(1, 0)); // \epsilon_{01}
                strain[4] = static_cast<Real>(0.5) * (nablaU(0, 2) + nablaU(2, 0)); // \epsilon_{02}
                strain[5] = static_cast<Real>(0.5) * (nablaU(1, 2) + nablaU(2, 1)); // \epsilon_{12}

                // stress = C * epsilon
                m_stress[i] = C * strain;
            }
            else
                m_stress[i].setZero();
        }
    }
}

void Elasticity_Becker2009::computeForces()
{
    Simulation *sim = Simulation::getCurrent();
    const unsigned int numParticles = m_model->numActiveParticles();
    const unsigned int fluidModelIndex = m_model->getPointSetIndex();
    FluidModel *model = m_model;

    #pragma omp parallel default(shared)
    {
        #pragma omp for schedule(static)
        for (int i = 0; i < (int)numParticles; i++)
        {
            if (model->getParticleState(i) == ParticleState::Active)
            {
                const unsigned int i0 = m_current_to_initial_index[i];
                const Vector3r& xi0 = m_model->getPosition0(i0);

                const size_t numNeighbors = m_initialNeighbors[i0].size();
                Vector3r fi;
                fi.setZero();

                // Fluid
                for (unsigned int j = 0; j < numNeighbors; j++)
                {
                    const unsigned int neighborIndex = m_initial_to_current_index[m_initialNeighbors[i0][j]];
                    // get initial neighbor index considering the current particle order
                    const unsigned int neighborIndex0 = m_initialNeighbors[i0][j];

                    const Vector3r& xj0 = m_model->getPosition0(neighborIndex0);

                    const Vector3r xj_xi_0 = xj0 - xi0;
                    const Vector3r gradW0 = sim->gradW(xj_xi_0);

                    const Vector3r dji = m_restVolumes[i] * gradW0;
                    const Vector3r dij = -m_restVolumes[neighborIndex] * gradW0;

                    Vector3r sdji, sdij;
                    symMatTimesVec(m_stress[neighborIndex], dji, sdji);
                    symMatTimesVec(m_stress[i], dij, sdij);

                    const Vector3r fij = -m_restVolumes[neighborIndex] * sdji;
                    const Vector3r fji = -m_restVolumes[i] * sdij;

                    fi += m_rotations[neighborIndex] * fij - m_rotations[i] * fji;
                }
                fi = 0.5*fi;

                if (m_alpha != 0.0)
                {
                    // Ganzenmüller, G.C. 2015. An hourglass control algorithm for Lagrangian
                    // Smooth Particle Hydrodynamics. Computer Methods in Applied Mechanics and
                    // Engineering 286, 87.106.
                    Vector3r fi_hg;
                    fi_hg.setZero();
                    const Vector3r& xi = m_model->getPosition(i);
                    for (unsigned int j = 0; j < numNeighbors; j++)
                    {
                        const unsigned int neighborIndex = m_initial_to_current_index[m_initialNeighbors[i0][j]];
                        // get initial neighbor index considering the current particle order
                        const unsigned int neighborIndex0 = m_initialNeighbors[i0][j];

                        const Vector3r& xj = model->getPosition(neighborIndex);
                        const Vector3r& xj0 = m_model->getPosition0(neighborIndex0);

                        // Note: Ganzenm�ller defines xij = xj-xi
                        const Vector3r xi_xj = -(xi - xj);
                        const Real xixj_l = xi_xj.norm();
                        if (xixj_l > 1.0e-6)
                        {
                            // Note: Ganzenm�ller defines xij = xj-xi
                            const Vector3r xi_xj_0 = -(xi0 - xj0);
                            const Real xixj0_l2 = xi_xj_0.squaredNorm();
                            const Real W0 = sim->W(xi_xj_0);

                            const Vector3r xij_i = m_F[i] * m_rotations[i] * xi_xj_0;
                            const Vector3r xji_j = -m_F[neighborIndex] * m_rotations[neighborIndex] * xi_xj_0;
                            const Vector3r epsilon_ij_i = xij_i - xi_xj;
                            const Vector3r epsilon_ji_j = xji_j + xi_xj;

                            const Real delta_ij_i = epsilon_ij_i.dot(xi_xj) / xixj_l;
                            const Real delta_ji_j = -epsilon_ji_j.dot(xi_xj) / xixj_l;

                            fi_hg -= m_restVolumes[neighborIndex] * W0 / xixj0_l2 * (delta_ij_i + delta_ji_j) * xi_xj / xixj_l;
                        }
                    }
                    fi_hg *= m_alpha * m_youngsModulus * m_restVolumes[i];
                    model->getAcceleration(i) += fi_hg / model->getMass(i);
                }

                // elastic acceleration
                Vector3r& ai = model->getAcceleration(i);
                ai += fi / model->getMass(i);
            }
        }
    }
}

void SPH::Elasticity_Becker2009::saveState(BinaryFileWriter &binWriter)
{
    binWriter.writeBuffer((char*)m_current_to_initial_index.data(), m_current_to_initial_index.size() * sizeof(unsigned int));
    binWriter.writeBuffer((char*)m_initial_to_current_index.data(), m_initial_to_current_index.size() * sizeof(unsigned int));
}

void SPH::Elasticity_Becker2009::loadState(BinaryFileReader &binReader)
{
    binReader.readBuffer((char*)m_current_to_initial_index.data(), m_current_to_initial_index.size() * sizeof(unsigned int));
    binReader.readBuffer((char*)m_initial_to_current_index.data(), m_initial_to_current_index.size() * sizeof(unsigned int));
}