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;

        // Volume states at boundaries
        private double _rhoLeft, _pLeft, _rhoRight, _pRight;
        private bool _leftBCSet, _rightBCSet;

        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];

        // ★ New: pass both density and pressure from the volume
        public void SetLeftVolumeState(double rhoVol, double pVol)
        {
            _rhoLeft = rhoVol;
            _pLeft = pVol;
            _leftBCSet = true;
        }

        public void SetRightVolumeState(double rhoVol, double pVol)
        {
            _rhoRight = rhoVol;
            _pRight = pVol;
            _rightBCSet = true;
        }

        private double GetCellTotalSpecificEnthalpy(int i)
        {
            double rho = Math.Max(_rho[i], 1e-12);
            double u = _rhou[i] / rho;
            double p = Pressure(i);
            double h = _gamma / (_gamma - 1.0) * p / rho;
            return h + 0.5 * u * u;
        }

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

            // --- Left boundary (face 0) ---
            if (_leftBCSet)
            {
                // Ghost = actual volume state (ρ_vol, u=0, p_vol)
                double rhoL = _rhoLeft;
                double uL = 0.0;
                double pL = _pLeft;

                double rhoR = _rho[0];
                double uR = _rhou[0] / Math.Max(rhoR, 1e-12);
                double pR = Pressure(0);

                HLLCFlux(rhoL, uL, pL, rhoR, uR, pR, out Fm[0], out Fp[0], out Fe[0]);
            }
            else
            {
                Fm[0] = 0;
                Fp[0] = Pressure(0);
                Fe[0] = 0;
            }

            // --- Internal faces ---
            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 boundary (face n) ---
            if (_rightBCSet)
            {
                double rhoL = _rho[n - 1];
                double uL = _rhou[n - 1] / Math.Max(rhoL, 1e-12);
                double pL = Pressure(n - 1);

                // Ghost = actual volume state (ρ_vol, u=0, p_vol)
                double rhoR = _rhoRight;
                double uR = 0.0;
                double pR = _pRight;

                HLLCFlux(rhoL, uL, pL, rhoR, uR, pR, out Fm[n], out Fp[n], out Fe[n]);
            }
            else
            {
                Fm[n] = 0;
                Fp[n] = Pressure(n - 1);
                Fe[n] = 0;
            }

            // --- Cell update ---
            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;

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

            // --- Friction disabled ---
            // if (FrictionFactor > 0) { … }

            // --- Port flows ---
            PortA.MassFlowRate = _leftBCSet ? Fm[0] * _area : 0.0;
            PortB.MassFlowRate = _rightBCSet ? -Fm[n] * _area : 0.0;

            PortA.SpecificEnthalpy = GetCellTotalSpecificEnthalpy(0);
            PortB.SpecificEnthalpy = GetCellTotalSpecificEnthalpy(_n - 1);

            _leftBCSet = _rightBCSet = 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;
            }
        }
    }
}