Introduced automatic sub stepping and pipe cell count

Components/Pipe1D.cs

@@ -9,14 +9,19 @@ namespace FluidSim.Components
         public Port PortB { get; }
         public double Area => _area;
 
-        private int _n;
+        private int _n;                       // number of cells
-        private double _dx, _dt, _gamma = 1.4, _area;
+        private double _dx, _dt, _gamma, _area;
         private double[] _rho, _rhou, _E;
 
-        // Volume states at boundaries
+        // Volume boundary states, constant during sub‑steps
-        private double _rhoLeft, _pLeft, _rhoRight, _pRight;
+        private double _rhoLeft, _pLeft;
+        private double _rhoRight, _pRight;
         private bool _leftBCSet, _rightBCSet;
 
+        // CFL control
+        private const double CflTarget = 0.8;
+        private const double ReferenceSoundSpeed = 340.0;  // m/s, standard air
+
         public double FrictionFactor { get; set; } = 0.02;
 
         public int GetCellCount() => _n;
@@ -24,12 +29,30 @@ namespace FluidSim.Components
         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)
+        /// <summary>
+        /// Creates a 1D pipe.
+        /// Cell count is automatically determined to satisfy CFL in still air.
+        /// </summary>
+        /// <param name="length">Pipe length in metres.</param>
+        /// <param name="area">Cross‑sectional area in m².</param>
+        /// <param name="sampleRate">Global simulation sample rate (Hz).</param>
+        public Pipe1D(double length, double area, int sampleRate)
         {
+            // Desired spatial step to keep CFL ≤ target for still air
+            double dtGlobal = 1.0 / sampleRate;
+            double dxTarget = ReferenceSoundSpeed * dtGlobal * CflTarget;
+
+            // Number of cells must be at least 2; try to hit dxTarget
+            int nCells = Math.Max(2, (int)Math.Round(length / dxTarget, MidpointRounding.AwayFromZero));
+            // Ensure we don't accidentally overshoot dxTarget by more than a factor
+            while (length / nCells > dxTarget * 1.01 && nCells < int.MaxValue - 1)
+                nCells++;
+
             _n = nCells;
-            _dx = length / nCells;
+            _dx = length / _n;
-            _dt = 1.0 / sampleRate;
+            _dt = dtGlobal;              // global (audio) time step
             _area = area;
+            _gamma = 1.4;
 
             _rho = new double[_n];
             _rhou = new double[_n];
@@ -56,7 +79,6 @@ namespace FluidSim.Components
         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;
@@ -80,95 +102,152 @@ namespace FluidSim.Components
             return h + 0.5 * u * u;
         }
 
+        /// <summary>
+        /// Advance the pipe over one global time step using sub‑stepping.
+        /// Must be called once per global simulation cycle.
+        /// </summary>
         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) ---
+            // --- Determine maximum wave speed in the pipe ---
-            if (_leftBCSet)
+            double maxWaveSpeed = 0.0;
-            {
-                // 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 rho = Math.Max(_rho[i], 1e-12);
-                double dP = (Fp[i + 1] - Fp[i]) / _dx;
+                double u = Math.Abs(_rhou[i] / rho);
-                double dE = (Fe[i + 1] - Fe[i]) / _dx;
+                double c = Math.Sqrt(_gamma * Pressure(i) / rho);
-                _rho[i] -= _dt * dM;
+                double local = u + c;
-                _rhou[i] -= _dt * dP;
+                if (local > maxWaveSpeed) maxWaveSpeed = local;
-                _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 (maxWaveSpeed < 1e-8) maxWaveSpeed = 1e-8;
-            // if (FrictionFactor > 0) { … }
 
-            // --- Port flows ---
+            int nSub = Math.Max(1, (int)Math.Ceiling(_dt * maxWaveSpeed / (CflTarget * _dx)));
-            PortA.MassFlowRate = _leftBCSet ? Fm[0] * _area : 0.0;
+            double dtSub = _dt / nSub;
-            PortB.MassFlowRate = _rightBCSet ? -Fm[n] * _area : 0.0;
 
-            PortA.SpecificEnthalpy = GetCellTotalSpecificEnthalpy(0);
+            // Accumulators for net mass flows
-            PortB.SpecificEnthalpy = GetCellTotalSpecificEnthalpy(_n - 1);
+            double sumMdotA = 0.0, sumMdotB = 0.0;
+
+            // Accumulators for fluid that ENTERS the volumes (pipe → volume)
+            double massInA = 0.0, energyInA = 0.0;
+            double massInB = 0.0, energyInB = 0.0;
+
+            for (int step = 0; step < nSub; step++)
+            {
+                double[] Fm = new double[n + 1];
+                double[] Fp = new double[n + 1];
+                double[] Fe = new double[n + 1];
+
+                // Left boundary (face 0)
+                if (_leftBCSet)
+                {
+                    HLLCFlux(_rhoLeft, 0.0, _pLeft,
+                             _rho[0], _rhou[0] / Math.Max(_rho[0], 1e-12), Pressure(0),
+                             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);
+                    HLLCFlux(rhoL, uL, pL,
+                             _rhoRight, 0.0, _pRight,
+                             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] -= dtSub * dM;
+                    _rhou[i] -= dtSub * dP;
+                    _E[i] -= dtSub * 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;
+                }
+
+                // Sub‑step mass flow rates (kg/s)
+                double mdotA_sub = _leftBCSet ? Fm[0] * _area : 0.0;    // >0 = into pipe
+                double mdotB_sub = _rightBCSet ? -Fm[n] * _area : 0.0;   // >0 = into pipe from right
+
+                sumMdotA += mdotA_sub;
+                sumMdotB += mdotB_sub;
+
+                // Flow FROM pipe INTO volume A: mdotA_sub < 0
+                if (mdotA_sub < 0 && _leftBCSet)
+                {
+                    double massRate = -mdotA_sub;   // kg/s entering volume A
+                    double h = GetCellTotalSpecificEnthalpy(0);
+                    massInA += massRate * dtSub;
+                    energyInA += massRate * dtSub * h;
+                }
+
+                // Flow FROM pipe INTO volume B: mdotB_sub < 0  (because
+                // mdotB_sub = -Fm[n], and Fm[n] > 0 is flow to the right)
+                if (mdotB_sub < 0 && _rightBCSet)
+                {
+                    double massRate = -mdotB_sub;   // kg/s entering volume B
+                    double h = GetCellTotalSpecificEnthalpy(_n - 1);
+                    massInB += massRate * dtSub;
+                    energyInB += massRate * dtSub * h;
+                }
+            }
+
+            // Averaged net mass flows (sign: positive = into pipe)
+            PortA.MassFlowRate = sumMdotA / nSub;
+            PortB.MassFlowRate = sumMdotB / nSub;
+
+            // Assign enthalpy ONLY for the fluid that physically entered the volume
+            if (massInA > 1e-12)
+                PortA.SpecificEnthalpy = energyInA / massInA;
+
+            if (massInB > 1e-12)
+                PortB.SpecificEnthalpy = energyInB / massInB;
+
+            // If no inflow occurred, leave the port’s enthalpy unchanged.
+            // (It will be set to the volume’s static enthalpy by PushStateToPort
+            //  or overwritten by TransferPipeToVolume if flow reverses later.)
 
             _leftBCSet = _rightBCSet = false;
         }
 
-        double Pressure(int i) =>
+        // Pressure and HLLC flux unchanged
+        private 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,
+        private void HLLCFlux(double rL, double uL, double pL,
-                      out double fm, out double fp, out double fe)
+                              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));
@@ -176,6 +255,7 @@ namespace FluidSim.Components
             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));
 

Core/Simulation.cs

@@ -19,15 +19,14 @@ namespace FluidSim.Core
             dt = 1.0 / sampleRate;
 
             double V = 5.0 * Units.L;
-            volA = new Volume0D(V, 1.1 * Units.atm, Units.Celsius(20), sampleRate);
+            volA = new Volume0D(V, 2.0 * Units.atm, Units.Celsius(20), sampleRate);
             volB = new Volume0D(V, 1.0 * Units.atm, Units.Celsius(20), sampleRate);
 
             double length = 150 * Units.mm;
             double diameter = 25 * Units.mm;
             double area = Units.AreaFromDiameter(25, Units.mm);
-            int nCells = 10;
 
-            pipe = new Pipe1D(length, area, nCells, sampleRate);
+            pipe = new Pipe1D(length, area, sampleRate);
             pipe.SetUniformState(volA.Density, 0.0, volA.Pressure);
             pipe.FrictionFactor = 0.02;
 
@@ -54,13 +53,30 @@ namespace FluidSim.Core
 
         public static void Log()
         {
-            if (stepCount <= 50 || stepCount % 200 == 0)
+            bool logPipe = true;
+
+            if ((stepCount <= 10 || (stepCount <= 1000 && stepCount % 100 == 0)) || stepCount % 1000 == 0 && stepCount < 10000)
             {
+                // Summary line
                 Console.WriteLine(
                     $"t = {time * 1e3:F3} ms  Step {stepCount:D4}: " +
                     $"PA = {volA.Pressure / 1e5:F6} bar, " +
                     $"PB = {volB.Pressure / 1e5:F6} bar, " +
                     $"FlowA = {pipe.PortA.MassFlowRate * 1e3:F2} g/s");
+
+                // Per‑cell state
+                if (logPipe && stepCount <= 1000)
+                {
+                    int n = pipe.GetCellCount();
+                    for (int i = 0; i < n; i++)
+                    {
+                        double rho = pipe.GetCellDensity(i);
+                        double p = pipe.GetCellPressure(i);
+                        double u = pipe.GetCellVelocity(i);
+                        Console.WriteLine(
+                            $"  Cell {i,2}: ρ={rho,8:F4} kg/m³, p={p,10:F2} Pa, u={u,8:F3} m/s");
+                    }
+                }
             }
         }
     }