FluidSim/Components/Pipe1D.cs

using System;
using FluidSim.Interfaces;

namespace FluidSim.Components
{
    public class Pipe1D
    {
        public Port PortA { get; }
        public Port PortB { get; }
        public double Area => _area;

        private int _n;
        private double _dx, _dt, _gamma = 1.4, _area;
        private double[] _rho, _rhou, _E;

        // Boundary fluxes (set by solver before each step)
        private double _fxL_mass, _fxL_mom, _fxL_ener;
        private double _fxR_mass, _fxR_mom, _fxR_ener;
        private bool _leftSet, _rightSet;

        public double FrictionFactor { get; set; } = 0.02;

        public int GetCellCount() => _n;
        public double GetCellDensity(int i) => _rho[i];
        public double GetCellPressure(int i) => Pressure(i);
        public double GetCellVelocity(int i) => _rhou[i] / Math.Max(_rho[i], 1e-12);

        public Pipe1D(double length, double area, int nCells, int sampleRate)
        {
            _n = nCells;
            _dx = length / nCells;
            _dt = 1.0 / sampleRate;
            _area = area;

            _rho = new double[_n];
            _rhou = new double[_n];
            _E = new double[_n];

            PortA = new Port();
            PortB = new Port();
        }

        public void SetUniformState(double rho, double u, double p)
        {
            double e = p / ((_gamma - 1) * rho);
            double Etot = rho * e + 0.5 * rho * u * u;
            for (int i = 0; i < _n; i++)
            {
                _rho[i] = rho;
                _rhou[i] = rho * u;
                _E[i] = Etot;
            }
        }

        public double GetLeftPressure() => Pressure(0);
        public double GetRightPressure() => Pressure(_n - 1);
        public double GetLeftDensity() => _rho[0];
        public double GetRightDensity() => _rho[_n - 1];

        public void SetLeftBoundaryFlux(double m, double p, double e)
        {
            _fxL_mass = m; _fxL_mom = p; _fxL_ener = e; _leftSet = true;
        }

        public void SetRightBoundaryFlux(double m, double p, double e)
        {
            _fxR_mass = m; _fxR_mom = p; _fxR_ener = e; _rightSet = true;
        }

        public void Simulate()
        {
            int n = _n;
            double[] Fm = new double[n + 1], Fp = new double[n + 1], Fe = new double[n + 1];

            // Left face
            if (_leftSet) { Fm[0] = _fxL_mass; Fp[0] = _fxL_mom; Fe[0] = _fxL_ener; }
            else { Fm[0] = 0; Fp[0] = Pressure(0); Fe[0] = 0; }   // reflective wall

            // Internal faces (HLLC)
            for (int i = 0; i < n - 1; i++)
            {
                double uL = _rhou[i] / Math.Max(_rho[i], 1e-12);
                double uR = _rhou[i + 1] / Math.Max(_rho[i + 1], 1e-12);
                HLLCFlux(_rho[i], uL, Pressure(i), _rho[i + 1], uR, Pressure(i + 1),
                         out Fm[i + 1], out Fp[i + 1], out Fe[i + 1]);
            }

            // Right face
            if (_rightSet) { Fm[n] = _fxR_mass; Fp[n] = _fxR_mom; Fe[n] = _fxR_ener; }
            else { Fm[n] = 0; Fp[n] = Pressure(n - 1); Fe[n] = 0; }

            // Update cells
            for (int i = 0; i < n; i++)
            {
                double dM = (Fm[i + 1] - Fm[i]) / _dx;
                double dP = (Fp[i + 1] - Fp[i]) / _dx;
                double dE = (Fe[i + 1] - Fe[i]) / _dx;
                _rho[i] -= _dt * dM;
                _rhou[i] -= _dt * dP;
                _E[i] -= _dt * dE;

                // Clamp to physical
                if (_rho[i] < 1e-12) _rho[i] = 1e-12;
                double u = _rhou[i] / _rho[i];
                double kinetic = 0.5 * _rho[i] * u * u;
                if (_E[i] < kinetic) _E[i] = kinetic;
            }

            // Friction
            if (FrictionFactor > 0)
            {
                double D = 2.0 * Math.Sqrt(_area / Math.PI);
                for (int i = 0; i < n; i++)
                {
                    double u = _rhou[i] / Math.Max(_rho[i], 1e-12);
                    double f = FrictionFactor / (2.0 * D) * _rho[i] * u * Math.Abs(u);
                    _rhou[i] -= _dt * f;
                    if (_E[i] > _dt * f * u) _E[i] -= _dt * f * u;
                }
            }

            // Write port flows for the solver
            PortA.MassFlowRate = _leftSet ? _fxL_mass * _area : 0.0;
            PortB.MassFlowRate = _rightSet ? -_fxR_mass * _area : 0.0;

            // Enthalpy for upwinding
            PortA.SpecificEnthalpy = _gamma / (_gamma - 1.0) * Pressure(0) / Math.Max(_rho[0], 1e-12);
            PortB.SpecificEnthalpy = _gamma / (_gamma - 1.0) * Pressure(_n - 1) / Math.Max(_rho[_n - 1], 1e-12);

            // Reset for next step
            _leftSet = _rightSet = false;
        }

        double Pressure(int i) =>
            (_gamma - 1.0) * (_E[i] - 0.5 * _rhou[i] * _rhou[i] / Math.Max(_rho[i], 1e-12));

        void HLLCFlux(double rL, double uL, double pL, double rR, double uR, double pR,
                      out double fm, out double fp, out double fe)
        {
            double cL = Math.Sqrt(_gamma * pL / Math.Max(rL, 1e-12));
            double cR = Math.Sqrt(_gamma * pR / Math.Max(rR, 1e-12));
            double EL = pL / ((_gamma - 1) * rL) + 0.5 * uL * uL;
            double ER = pR / ((_gamma - 1) * rR) + 0.5 * uR * uR;
            double SL = Math.Min(uL - cL, uR - cR);
            double SR = Math.Max(uL + cL, uR + cR);
            double Ss = (pR - pL + rL * uL * (SL - uL) - rR * uR * (SR - uR))
                      / (rL * (SL - uL) - rR * (SR - uR));

            double FrL_m = rL * uL, FrL_p = rL * uL * uL + pL, FrL_e = (rL * EL + pL) * uL;
            double FrR_m = rR * uR, FrR_p = rR * uR * uR + pR, FrR_e = (rR * ER + pR) * uR;

            if (SL >= 0) { fm = FrL_m; fp = FrL_p; fe = FrL_e; }
            else if (SR <= 0) { fm = FrR_m; fp = FrR_p; fe = FrR_e; }
            else if (Ss >= 0)
            {
                double rsL = rL * (SL - uL) / (SL - Ss);
                double ps = pL + rL * (SL - uL) * (Ss - uL);
                double EsL = EL + (Ss - uL) * (Ss + pL / (rL * (SL - uL)));
                fm = rsL * Ss; fp = rsL * Ss * Ss + ps; fe = (rsL * EsL + ps) * Ss;
            }
            else
            {
                double rsR = rR * (SR - uR) / (SR - Ss);
                double ps = pL + rL * (SL - uL) * (Ss - uL);
                double EsR = ER + (Ss - uR) * (Ss + pR / (rR * (SR - uR)));
                fm = rsR * Ss; fp = rsR * Ss * Ss + ps; fe = (rsR * EsR + ps) * Ss;
            }
        }
    }
}