Helmholtz test, sod shock tube

Scenarios/HelmholtzResonatorScenario.cs

@@ -0,0 +1,133 @@
+using System;
+using FluidSim.Components;
+using FluidSim.Interfaces;
+using FluidSim.Utils;
+using SFML.Graphics;
+using SFML.System;
+
+namespace FluidSim.Core
+{
+    public class HelmholtzResonatorScenario : Scenario
+    {
+        private Solver solver;
+        private Volume0D cavity;
+        private Pipe1D neck;
+        private Connection coupling;
+        private int stepCount;
+        private double time;
+        private double dt;
+        private double ambientPressure = 1.0 * Units.atm;
+
+        public override void Initialize(int sampleRate)
+        {
+            dt = 1.0 / sampleRate;
+
+            // 1‑litre cavity, 10% over‑pressure
+            double cavityVolume = 1e-3;
+            double initialCavityPressure = 1.1 * ambientPressure;
+            cavity = new Volume0D(cavityVolume, initialCavityPressure, 300.0, sampleRate)
+            {
+                Gamma = 1.4,
+                GasConstant = 287.0
+            };
+
+            // Neck: length 10 cm, radius 1 cm
+            double neckLength = 0.1;
+            double neckRadius = 0.01;
+            double neckArea = Math.PI * neckRadius * neckRadius;
+            neck = new Pipe1D(neckLength, neckArea, sampleRate, forcedCellCount: 40);
+            neck.SetUniformState(1.225, 0.0, ambientPressure);
+
+            coupling = new Connection(neck.PortA, cavity.Port)
+            {
+                Area = neckArea,
+                DischargeCoefficient = 0.62,
+                Gamma = 1.4
+            };
+
+            solver = new Solver();
+            solver.SetTimeStep(dt);
+            solver.AddVolume(cavity);
+            solver.AddPipe(neck);
+            solver.AddConnection(coupling);
+
+            // Port A (left) = volume coupling, Port B (right) = open end
+            solver.SetPipeBoundary(neck, isA: true, BoundaryType.VolumeCoupling);
+            solver.SetPipeBoundary(neck, isA: false, BoundaryType.OpenEnd, ambientPressure);
+        }
+
+        public override float Process()
+        {
+            float sample = solver.Step();
+            time += dt;
+            stepCount++;
+
+            double pOpen = neck.GetCellPressure(neck.GetCellCount() - 1);
+            float audio = (float)((pOpen - ambientPressure) / ambientPressure);
+
+            if (stepCount % 20 == 0)
+            {
+                double pCav = cavity.Pressure;
+                double mdotA = neck.PortA.MassFlowRate;   // positive = into pipe (leaving cavity)
+                Console.WriteLine(
+                    $"t={time * 1e3:F2} ms step={stepCount} " +
+                    $"P_cav={pCav:F1} Pa, P_open={pOpen:F1} Pa, " +
+                    $"mdot_A={mdotA * 1e3:F4} g/s, audio={audio:F4}");
+            }
+
+            return audio;
+        }
+
+        public override void Draw(RenderWindow target)
+        {
+            float winW = target.GetView().Size.X;
+            float winH = target.GetView().Size.Y;
+            float centerY = winH / 2f;
+
+            // Cavity rectangle
+            float cavityWidth = 120f;
+            float cavityHeight = 180f;
+            var cavityRect = new RectangleShape(new Vector2f(cavityWidth, cavityHeight));
+            cavityRect.Position = new Vector2f(40f, centerY - cavityHeight / 2f);
+            cavityRect.FillColor = PressureColor(cavity.Pressure);
+            target.Draw(cavityRect);
+
+            // Neck drawn as tapered pipe
+            int n = neck.GetCellCount();
+            float neckStartX = 40f + cavityWidth + 10f;
+            float neckEndX = winW - 60f;
+            float neckLenPx = neckEndX - neckStartX;
+            float dx = neckLenPx / (n - 1);
+            float baseRadius = 20f;
+
+            Vertex[] vertices = new Vertex[n * 2];
+            for (int i = 0; i < n; i++)
+            {
+                float x = neckStartX + i * dx;
+                double p = neck.GetCellPressure(i);
+                float r = baseRadius * (float)(0.5 + 0.5 * Math.Tanh((p - ambientPressure) / (ambientPressure * 0.2)));
+                if (r < 4f) r = 4f;
+                Color col = PressureColor(p);
+                vertices[i * 2] = new Vertex(new Vector2f(x, centerY - r), col);
+                vertices[i * 2 + 1] = new Vertex(new Vector2f(x, centerY + r), col);
+            }
+            target.Draw(vertices, PrimitiveType.TriangleStrip);
+
+            // Open end indicator
+            var arrow = new CircleShape(8f);
+            arrow.Position = new Vector2f(neckEndX - 4f, centerY - 4f);
+            arrow.FillColor = Color.White;
+            target.Draw(arrow);
+        }
+
+        private Color PressureColor(double pressure)
+        {
+            double range = ambientPressure * 0.1;
+            double t = Math.Clamp((pressure - ambientPressure) / range, -1.0, 1.0);
+            byte r = (byte)(t > 0 ? 255 * t : 0);
+            byte b = (byte)(t < 0 ? -255 * t : 0);
+            byte g = (byte)(255 * (1 - Math.Abs(t)));
+            return new Color(r, g, b);
+        }
+    }
+}