FilterReynoldsStresses.cpp

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///////////////////////////////////////////////////////////////////////////////
//
// File: FilterReynoldsStresses.cpp
//
// For more information, please see: http://www.nektar.info
//
// The MIT License
//
// Copyright (c) 2006 Division of Applied Mathematics, Brown University (USA),
// Department of Aeronautics, Imperial College London (UK), and Scientific
// Computing and Imaging Institute, University of Utah (USA).
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
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// Software is furnished to do so, subject to the following conditions:
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// The above copyright notice and this permission notice shall be included
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// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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//
// Description: Append Reynolds stresses to the average fields
//
///////////////////////////////////////////////////////////////////////////////
 
#include <IncNavierStokesSolver/Filters/FilterReynoldsStresses.h>
 
namespace Nektar::SolverUtils
{
std::string FilterReynoldsStresses::className =
    GetFilterFactory().RegisterCreatorFunction("ReynoldsStresses",
                                               FilterReynoldsStresses::create);
 
/**
 * @class FilterReynoldsStresses
 *
 * @brief Append Reynolds stresses to the average fields
 *
 * This class appends the average fields with the Reynolds stresses of the form
 * \f$ \overline{u' v'} \f$.
 *
 * For the default case, this is achieved by calculating
 * \f$ C_{n} = \Sigma_{i=1}^{n} (u_i - \bar{u}_n)(v_i - \bar{v}_n)\f$
 * using the recursive relation:
 *
 * \f[ C_{n} = C_{n-1} + \frac{n}{n-1} (u_n - \bar{u}_n)(v_n - \bar{v}_n) \f]
 *
 * The FilterSampler base class then divides the result by n, leading
 * to the Reynolds stress.
 *
 * It is also possible to perform the averages using an exponential moving
 *  average, in which case either the moving average parameter \f$ \alpha \f$
 * or the time constant \f$ \tau \f$ must be prescribed.
 */
FilterReynoldsStresses::FilterReynoldsStresses(
    const LibUtilities::SessionReaderSharedPtr &pSession,
    const std::shared_ptr<SolverUtils::EquationSystem> &pEquation,
    const std::map<std::string, std::string> &pParams)
    : FilterFieldConvert(pSession, pEquation, pParams)
{
    // Load sampling frequency
    auto it = pParams.find("SampleFrequency");
    if (it == pParams.end())
    {
        m_sampleFrequency = 1;
    }
    else
    {
        LibUtilities::Equation equ(m_session->GetInterpreter(), it->second);
        m_sampleFrequency = round(equ.Evaluate());
    }
 
    // Check if should use moving average
    it = pParams.find("MovingAverage");
    if (it == pParams.end())
    {
        m_movAvg = false;
    }
    else
    {
        std::string sOption = it->second.c_str();
        m_movAvg            = (boost::iequals(sOption, "true")) ||
                   (boost::iequals(sOption, "yes"));
    }
 
    if (!m_movAvg)
    {
        return;
    }
 
    // Load alpha parameter for moving average
    it = pParams.find("alpha");
    if (it == pParams.end())
    {
        it = pParams.find("tau");
        if (it == pParams.end())
        {
            ASSERTL0(false, "MovingAverage needs either alpha or tau.");
        }
        else
        {
            // Load time constant
            LibUtilities::Equation equ(m_session->GetInterpreter(), it->second);
            NekDouble tau = equ.Evaluate();
            // Load delta T between samples
            NekDouble dT;
            m_session->LoadParameter("TimeStep", dT);
            dT = dT * m_sampleFrequency;
            // Calculate alpha
            m_alpha = dT / (tau + dT);
        }
    }
    else
    {
        LibUtilities::Equation equ(m_session->GetInterpreter(), it->second);
        m_alpha = equ.Evaluate();
        // Check if tau was also defined
        it = pParams.find("tau");
        if (it != pParams.end())
        {
            ASSERTL0(false,
                     "Cannot define both alpha and tau in MovingAverage.");
        }
    }
    // Check bounds of m_alpha
    ASSERTL0(m_alpha > 0 && m_alpha < 1, "Alpha out of bounds.");
}
 
FilterReynoldsStresses::~FilterReynoldsStresses()
{
}
 
void FilterReynoldsStresses::v_Initialise(
    const Array<OneD, const MultiRegions::ExpListSharedPtr> &pFields,
    const NekDouble &time)
{
    size_t dim          = pFields.size() - 1;
    size_t nExtraFields = (dim + 1) * dim / 2;
    size_t origFields   = pFields.size();
    size_t nqtot        = pFields[0]->GetTotPoints();
 
    // Allocate storage
    m_fields.resize(origFields + nExtraFields);
    m_delta.resize(dim);
 
    for (size_t n = 0; n < m_fields.size(); ++n)
    {
        m_fields[n] = Array<OneD, NekDouble>(nqtot, 0.0);
    }
    for (size_t n = 0; n < m_delta.size(); ++n)
    {
        m_delta[n] = Array<OneD, NekDouble>(nqtot, 0.0);
    }
 
    // Initialise output arrays
    FilterFieldConvert::v_Initialise(pFields, time);
 
    // Update m_fields if using restart file
    if (m_numSamples)
    {
        for (size_t j = 0; j < m_fields.size(); ++j)
        {
            pFields[0]->BwdTrans(m_outFields[j], m_fields[j]);
            if (pFields[0]->GetWaveSpace())
            {
                pFields[0]->HomogeneousBwdTrans(nqtot, m_fields[j],
                                                m_fields[j]);
            }
        }
    }
}
 
void FilterReynoldsStresses::v_FillVariablesName(
    const Array<OneD, const MultiRegions::ExpListSharedPtr> &pFields)
{
    size_t dim        = pFields.size() - 1;
    size_t origFields = pFields.size();
 
    // Fill name of variables
    for (size_t n = 0; n < origFields; ++n)
    {
        m_variables.push_back(pFields[n]->GetSession()->GetVariable(n));
    }
    for (int i = 0; i < dim; ++i)
    {
        for (int j = i; j < dim; ++j)
        {
            std::string var = pFields[i]->GetSession()->GetVariable(i) +
                              pFields[j]->GetSession()->GetVariable(j);
            m_variables.push_back(var);
        }
    }
}
 
void FilterReynoldsStresses::v_ProcessSample(
    const Array<OneD, const MultiRegions::ExpListSharedPtr> &pFields,
    [[maybe_unused]] std::vector<Array<OneD, NekDouble>> &fieldcoeffs,
    [[maybe_unused]] const NekDouble &time)
{
    size_t i, j, n;
    size_t nq          = pFields[0]->GetTotPoints();
    size_t dim         = pFields.size() - 1;
    bool waveSpace     = pFields[0]->GetWaveSpace();
    NekDouble nSamples = (NekDouble)m_numSamples;
 
    // For moving average, take first sample as initial vector
    NekDouble alpha = m_alpha;
    if (m_numSamples == 1)
    {
        alpha = 1.0;
    }
 
    // Define auxiliary constants for averages
    NekDouble facOld, facAvg, facStress, facDelta;
    if (m_movAvg)
    {
        facOld    = 1.0 - alpha;
        facAvg    = alpha;
        facStress = alpha;
        facDelta  = 1.0;
    }
    else
    {
        facOld    = 1.0;
        facAvg    = 1.0;
        facStress = nSamples / (nSamples - 1);
        facDelta  = 1.0 / nSamples;
    }
 
    Array<OneD, NekDouble> vel(nq);
    Array<OneD, NekDouble> tmp(nq);
 
    // Update original velocities in phys space and calculate (\bar{u} - u_n)
    for (n = 0; n < dim; ++n)
    {
        if (waveSpace)
        {
            pFields[n]->HomogeneousBwdTrans(nq, pFields[n]->GetPhys(), vel);
        }
        else
        {
            vel = pFields[n]->GetPhys();
        }
        Vmath::Svtsvtp(nq, facAvg, vel, 1, facOld, m_fields[n], 1, m_fields[n],
                       1);
        Vmath::Svtvm(nq, facDelta, m_fields[n], 1, vel, 1, m_delta[n], 1);
    }
    // Update pressure (directly to outFields)
    Vmath::Svtsvtp(m_outFields[dim].size(), facAvg, pFields[dim]->GetCoeffs(),
                   1, facOld, m_outFields[dim], 1, m_outFields[dim], 1);
 
    // Ignore Reynolds stress for first sample (its contribution is zero)
    if (m_numSamples == 1)
    {
        return;
    }
 
    // Calculate C_{n} = facOld * C_{n-1} + facStress * deltaI * deltaJ
    for (i = 0, n = dim + 1; i < dim; ++i)
    {
        for (j = i; j < dim; ++j, ++n)
        {
            Vmath::Vmul(nq, m_delta[i], 1, m_delta[j], 1, tmp, 1);
            Vmath::Svtsvtp(nq, facStress, tmp, 1, facOld, m_fields[n], 1,
                           m_fields[n], 1);
        }
    }
}
 
void FilterReynoldsStresses::v_PrepareOutput(
    const Array<OneD, const MultiRegions::ExpListSharedPtr> &pFields,
    [[maybe_unused]] const NekDouble &time)
{
    size_t dim = pFields.size() - 1;
 
    m_fieldMetaData["NumberOfFieldDumps"] = std::to_string(m_numSamples);
 
    // Set wavespace to false, as calculations were performed in physical space
    bool waveSpace = pFields[0]->GetWaveSpace();
    pFields[0]->SetWaveSpace(false);
 
    // Forward transform and put into m_outFields (except pressure)
    for (size_t i = 0; i < m_fields.size(); ++i)
    {
        if (i != dim)
        {
            pFields[0]->FwdTransLocalElmt(m_fields[i], m_outFields[i]);
        }
    }
 
    // Restore waveSpace
    pFields[0]->SetWaveSpace(waveSpace);
}
 
NekDouble FilterReynoldsStresses::v_GetScale()
{
    if (m_movAvg)
    {
        return 1.0;
    }
    else
    {
        return 1.0 / m_numSamples;
    }
}
 
} // namespace Nektar::SolverUtils