Engine working

Components/Volume0D.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using FluidSim.Interfaces;
 
@@ -8,8 +8,9 @@ namespace FluidSim.Components
     {
         public List<Port> Ports { get; } = new List<Port>();
 
-        public double Mass { get; set; }           // made public setter
-        public double InternalEnergy { get; set; }  // made public setter
+        private double _airMass;
+        private double _exhaustMass;
+        public double InternalEnergy { get; set; }
         public double Volume { get; set; }
         public double Dvdt { get; set; }
         public double Gamma { get; set; } = 1.4;
@@ -18,6 +19,8 @@ namespace FluidSim.Components
         public double AmbientPressure { get; set; } = 101325.0;
 
         // Derived quantities
+        public double Mass => _airMass + _exhaustMass;
+        public double AirFraction => _airMass / Math.Max(Mass, 1e-12);
         public double Density => Mass / Math.Max(Volume, 1e-12);
         public double Pressure => (Gamma - 1.0) * InternalEnergy / Math.Max(Volume, 1e-12);
         public double Temperature => Pressure / Math.Max(Density * GasConstant, 1e-12);
@@ -32,7 +35,8 @@ namespace FluidSim.Components
             Dvdt = 0.0;
 
             double rho0 = initialPressure / (GasConstant * initialTemperature);
-            Mass = rho0 * Volume;
+            _airMass = rho0 * Volume;   // starts with all air
+            _exhaustMass = 0.0;
             InternalEnergy = (initialPressure * Volume) / (Gamma - 1.0);
         }
 
@@ -43,44 +47,53 @@ namespace FluidSim.Components
             port.Density = Density;
             port.Temperature = Temperature;
             port.SpecificEnthalpy = SpecificEnthalpy;
+            port.AirFraction = AirFraction;
             Ports.Add(port);
             return port;
         }
 
-        /// <summary>
-        /// Set the pressure to a specific value while keeping the current temperature constant.
-        /// Updates Mass and InternalEnergy accordingly.
-        /// </summary>
         public void SetPressure(double pressure, double? temperature = null)
         {
             double V = Math.Max(Volume, 1e-12);
             double T = temperature ?? Temperature;
             double rho = pressure / (GasConstant * T);
-            Mass = rho * V;
+            double totalMass = rho * V;
+            // Keep current air fraction when setting pressure?
+            double af = AirFraction;
+            _airMass = totalMass * af;
+            _exhaustMass = totalMass * (1.0 - af);
             InternalEnergy = pressure * V / (Gamma - 1.0);
         }
 
         public void UpdateState(double dt)
         {
-            double totalMdot = 0.0;
+            double totalMdotAir = 0.0;
+            double totalMdotExhaust = 0.0;
             double totalEdot = 0.0;
 
             foreach (var port in Ports)
             {
-                totalMdot += port.MassFlowRate;
-                totalEdot += port.MassFlowRate * port.SpecificEnthalpy;
+                double mdot = port.MassFlowRate;   // positive INTO volume
+                double af = mdot >= 0 ? port.AirFraction : AirFraction;   // inflow: use port's fraction; outflow: well-mixed
+                totalMdotAir += mdot * af;
+                totalMdotExhaust += mdot * (1.0 - af);
+                totalEdot += mdot * port.SpecificEnthalpy;
             }
 
-            double dm = totalMdot * dt;
+            double dAir = totalMdotAir * dt;
+            double dExhaust = totalMdotExhaust * dt;
             double dE = totalEdot * dt - Pressure * Dvdt * dt;
 
-            Mass += dm;
+            _airMass += dAir;
+            _exhaustMass += dExhaust;
             InternalEnergy += dE;
 
             double V = Math.Max(Volume, 1e-12);
-            if (Mass < 1e-9)
+            double totalMass = _airMass + _exhaustMass;
+            if (totalMass < 1e-9)
             {
-                Mass = 1e-9;
+                _airMass = 1e-9;
+                _exhaustMass = 0.0;
                 InternalEnergy = AmbientPressure * V / (Gamma - 1.0);
             }
             else if (InternalEnergy < 0.0)
@@ -88,16 +101,17 @@ namespace FluidSim.Components
                 InternalEnergy = AmbientPressure * V / (Gamma - 1.0);
             }
 
-            if (Mass < 0.0) Mass = 1e-9;
-            if (InternalEnergy < 0.0) InternalEnergy = AmbientPressure * V / (Gamma - 1.0);
+            if (_airMass < 0.0) _airMass = 0.0;
+            if (_exhaustMass < 0.0) _exhaustMass = 0.0;
 
-            double p = Pressure, rho = Density, T = Temperature, h = SpecificEnthalpy;
+            double p = Pressure, rho = Density, T = Temperature, h = SpecificEnthalpy, afrac = AirFraction;
             foreach (var port in Ports)
             {
                 port.Pressure = p;
                 port.Density = rho;
                 port.Temperature = T;
                 port.SpecificEnthalpy = h;
+                port.AirFraction = afrac;
             }
         }