FluidSim/Scenarios/SodShockTubeScenario.cs

using System;
using FluidSim.Components;
using FluidSim.Utils;
using SFML.Graphics;
using SFML.System;

namespace FluidSim.Core
{
    public class SodShockTubeScenario : Scenario
    {
        private Solver solver;
        private Pipe1D pipe;
        private int stepCount;
        private double time;
        private double dt;
        private double ambientPressure = 1.0 * Units.atm;
        private const double GasConstant = 287.0;

        public override void Initialize(int sampleRate)
        {
            dt = 1.0 / sampleRate;
            double length = 1.0;
            double area = 1.0;
            int nCells = 200;

            pipe = new Pipe1D(length, area, sampleRate, forcedCellCount: nCells);
            pipe.SetUniformState(0.125, 0.0, 0.1 * ambientPressure);  // right state

            // Left half high pressure
            for (int i = 0; i < nCells / 2; i++)
                pipe.SetCellState(i, 1.0, 0.0, ambientPressure);

            solver = new Solver();
            solver.SetTimeStep(dt);
            solver.AddPipe(pipe);
            solver.SetPipeBoundary(pipe, isA: true, BoundaryType.ClosedEnd);
            solver.SetPipeBoundary(pipe, isA: false, BoundaryType.ClosedEnd);
        }

        public override float Process()
        {
            float sample = solver.Step();
            time += dt;
            stepCount++;

            double pMid = pipe.GetPressureAtFraction(0.5);
            float audio = (float)((pMid - ambientPressure) / ambientPressure);

            bool log = true;

            if (log)
            {
                int n = pipe.GetCellCount();
                Console.WriteLine($"step {stepCount}:");
                Console.WriteLine("i    rho (kg/m³)   p (Pa)       T (K)     u (m/s)");
                for (int i = 0; i < n; i++)
                {
                    if (i % 10 == 0)
                    {
                        double rho = pipe.GetCellDensity(i);
                        double p = pipe.GetCellPressure(i);
                        double u = pipe.GetCellVelocity(i);
                        double T = p / (rho * GasConstant);   // GasConstant = 287.0
                        Console.WriteLine($"{i,-4} {rho,10:F4}   {p,10:F1}   {T,8:F2}   {u,10:F4}");
                    }
                }
                Console.WriteLine();
            }

            return audio;
        }

        public override void Draw(RenderWindow target)
        {
            float winW = target.GetView().Size.X;
            float winH = target.GetView().Size.Y;
            float centerY = winH / 2f;
            float margin = 40f;
            float pipeStartX = margin;
            float pipeEndX = winW - margin;
            float pipeLenPx = pipeEndX - pipeStartX;
            int n = pipe.GetCellCount();
            float dx = pipeLenPx / (n - 1);
            float baseRadius = 60f;

            Vertex[] vertices = new Vertex[n * 2];
            for (int i = 0; i < n; i++)
            {
                float x = pipeStartX + i * dx;

                double p = pipe.GetCellPressure(i);
                double rho = pipe.GetCellDensity(i);
                double T = p / (rho * GasConstant);           // temperature in Kelvin

                // Radius from pressure (exaggerated deviation)
                float r = baseRadius * (float)(p / ambientPressure * 2);
                if (r < 4f) r = 4f;

                // Colour from temperature
                Color col = TemperatureColor(T);

                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);

            // Diaphragm marker (faint white line at initial interface)
            float diaphragmX = pipeStartX + (n / 2) * dx;
            var line = new RectangleShape(new Vector2f(2f, winH * 0.5f));
            line.Position = new Vector2f(diaphragmX - 1f, centerY - winH * 0.25f);
            line.FillColor = new Color(255, 255, 255, 80);
            target.Draw(line);
        }

        /// <summary>
        /// Custom temperature‑to‑hue mapping that matches the given Sod‑tube hue values:
        ///   250 K → 176, 300 K → 122, 350 K → 120?, 450 K → 71.
        /// Interpolates piecewise linearly, clamping outside [250,450].
        /// </summary>
        private Color TemperatureColor(double T)
        {
            // 1. Map temperature to hue (0–360)
            double[] Tknots = { 250, 282, 353, 450 };
            double[] Hknots = { 176, 179, 122, 71 };
            double hue;
            if (T <= Tknots[0]) hue = Hknots[0];
            else if (T >= Tknots[^1]) hue = Hknots[^1];
            else
            {
                int i = 0;
                while (i < Tknots.Length - 1 && T > Tknots[i + 1]) i++;
                double frac = (T - Tknots[i]) / (Tknots[i + 1] - Tknots[i]);
                hue = Hknots[i] + frac * (Hknots[i + 1] - Hknots[i]);
            }

            // 2. Convert hue to RGB (S = 1, V = 1)
            double h = hue / 60.0;
            int sector = (int)Math.Floor(h);
            double f = h - sector;
            byte p = 0;
            byte q = (byte)(255 * (1 - f));
            byte tByte = (byte)(255 * f);
            byte v = 255;

            byte r, g, b;
            switch (sector % 6)
            {
                case 0: r = v; g = tByte; b = p; break;
                case 1: r = q; g = v; b = p; break;
                case 2: r = p; g = v; b = tByte; break;
                case 3: r = p; g = q; b = v; break;
                case 4: r = tByte; g = p; b = v; break;
                default: r = v; g = p; b = q; break;
            }
            return new Color(r, g, b);
        }
    }
}