FluidSim/Core/SoundProcessor.cs

using System;
using FluidSim.Core;

namespace FluidSim.Core
{
    /// <summary>
    /// Synthesises far‑field sound at a listener position from an open pipe end.
    /// Three source mechanisms are combined:
    ///   1. Monopole  – time derivative of mass flow (dominant at low speed / high pulsation).
    ///   2. Dipole    – time derivative of momentum flux (shear‑layer / vortex shedding).
    ///   3. Jet noise – Lighthill‑type turbulence mixing noise (scales with U^8).
    ///
    /// References:
    ///   • Lighthill, M.J. (1952) "On Sound Generated Aerodynamically".
    ///   • Dowling, A.P. & Williams, J.E.F. (1983) "Sound and Sources of Sound".
    ///   • Munjal, M.L. (2014) "Acoustics of Ducts and Mufflers", 2nd ed.
    ///   • Tam, C.K.W. & Auriault, L. (1999) "Jet Mixing Noise from Fine‑Scale Turbulence".
    /// </summary>
    public class SoundProcessor
    {
        private readonly double dt;
        private readonly double c0;           // ambient speed of sound (m/s)
        private readonly double rho0;         // ambient density (kg/m³)
        private readonly double r;            // listener distance (m)
        private readonly double pipeArea;     // cross‑sectional area of the pipe end (m²)

        // ---------- monopole state ----------
        private double flowLP;
        private readonly double lpAlpha;
        private double prevMassFlowOut;
        private double smoothDMdt;
        private readonly double alpha;

        // ---------- dipole state ----------
        private double prevMomentumFlux;
        private double smoothDMomDt;
        private readonly double dipAlpha;

        // ---------- jet noise state ----------
        private double jetNoiseSample;        // previous random sample (for simple shaping)
        private readonly double jetTau;       // correlation time ≈ D / U_mean

        public float Gain { get; set; } = 1.0f;

        /// <summary>
        /// </summary>
        /// <param name="sampleRate">Audio sample rate (Hz).</param>
        /// <param name="listenerDistanceMeters">Distance from the pipe exit to the listener (m).</param>
        /// <param name="pipeDiameterMeters">Internal diameter of the pipe (m).</param>
        public SoundProcessor(int sampleRate,
                              double listenerDistanceMeters = 1.0,
                              double pipeDiameterMeters = 0.0217)   // ~3.7 cm² area
        {
            dt = 1.0 / sampleRate;
            r = listenerDistanceMeters;
            pipeArea = Math.PI * 0.25 * pipeDiameterMeters * pipeDiameterMeters;

            // Ambient air properties
            c0 = 340.0;
            rho0 = 1.225;

            // ---- Monopole smoothing ----
            double tau = 0.002;                     // 2 ms
            alpha = Math.Exp(-dt / tau);

            double tauLP = 0.005;                   // 5 ms low‑pass on mass flow
            lpAlpha = Math.Exp(-dt / tauLP);

            // ---- Dipole smoothing ----
            double tauDip = 0.003;                  // 3 ms
            dipAlpha = Math.Exp(-dt / tauDip);

            // ---- Jet noise correlation time ----
            jetTau = Math.Max(0.0005, pipeDiameterMeters / 50.0);   // D / U_ref, floor at 0.5 ms
        }

        /// <summary>
        /// Process one sample.  The OpenEndLink provides the instantaneous
        /// exit‑plane mass flow, density, velocity, and pressure.
        /// </summary>
        public float Process(OpenEndLink openEnd)
        {
            double flowOut = openEnd.LastMassFlowRate;       // kg/s, positive = leaving pipe
            double rhoExit = openEnd.LastFaceDensity;        // kg/m³ at exit
            double uExit = openEnd.LastFaceVelocity;       // m/s (axial, positive = leaving)
            double pExit = openEnd.LastFacePressure;       // Pa

            // ============================================================
            // 1. MONOPOLE – due to unsteady mass addition (Lighthill 1952)
            //    Far‑field pressure:  p'(r,t) = (1 / 4πr c0) · dṁ/dt
            // ============================================================
            flowLP = lpAlpha * flowLP + (1.0 - lpAlpha) * flowOut;
            double rawDerivative = (flowLP - prevMassFlowOut) / dt;
            prevMassFlowOut = flowLP;
            smoothDMdt = alpha * smoothDMdt + (1.0 - alpha) * rawDerivative;
            double pMono = smoothDMdt / (4.0 * Math.PI * r * c0);

            // ============================================================
            // 2. DIPOLE – due to unsteady momentum flux at the exit plane
            //    Momentum flux:  F(t) = ṁ(t) · u(t) = ρ·A·u²
            //    Far‑field (compact, low M):  p'(r,θ,t) ≈ (cosθ / 4πr c0) · dF/dt
            //    For on‑axis listener (θ = 0):  p'(r,t) ≈ (1 / 4πr c0) · dF/dt
            //    We also include a U⁶ scaling factor relative to a reference velocity.
            // ============================================================
            double momentumFlux = Math.Abs(flowOut) * Math.Abs(uExit);       // N
            double rawMomDeriv = (momentumFlux - prevMomentumFlux) / dt;
            prevMomentumFlux = momentumFlux;
            smoothDMomDt = dipAlpha * smoothDMomDt + (1.0 - dipAlpha) * rawMomDeriv;
            double pDipole = smoothDMomDt / (4.0 * Math.PI * r * c0);

            // Dipole efficiency factor: ∝ (U / c0)³  (since Idipole ∝ U⁶, pdipole ∝ U³)
            double Mach = Math.Abs(uExit) / c0;
            double dipoleEfficiency = Math.Pow(Mach, 3.0);
            pDipole *= dipoleEfficiency;

            // ============================================================
            // 3. JET NOISE – Lighthill U⁸ mixing noise, band‑pass shaped
            //    rms pressure:  p'_jet ~ ρ0 · A / r  ·  U⁴ / c0²
            //    Model as broadband noise with amplitude ∝ U⁴.
            //    A simple first‑order low‑pass filter shapes the spectrum
            //    (cut‑off ≈ Strouhal frequency f ≈ 0.2 · U / D).
            // ============================================================
            double Uref = Math.Max(1.0, Math.Abs(uExit));   // avoid division by zero
            double jetAmplitude = rho0 * pipeArea / r * Math.Pow(Uref / c0, 4.0);

            // Correlation time (sample‑and‑hold style random walk)
            double alphaJet = Math.Exp(-dt / jetTau);
            // Generate a new random target each step, filter with alphaJet
            double randomTarget = (new Random().NextDouble() * 2.0 - 1.0);
            jetNoiseSample = alphaJet * jetNoiseSample + (1.0 - alphaJet) * randomTarget;
            double pJet = jetAmplitude * jetNoiseSample;

            // ============================================================
            // Combine contributions (monopole is primary; dipole & jet are
            // weighted down for realistic mix).  Weights can be tuned per engine.
            // ============================================================
            double pressure = (3000.0 * pMono) + (0.01 * pDipole) + (0 * pJet);
            pressure *= Gain;

            // Soft‑clip to ±1
            return (float)Math.Tanh(pressure);
        }
    }
}