FluidSim/Core/OrificeLink.cs

using System;
using FluidSim.Components;
using FluidSim.Interfaces;

namespace FluidSim.Core
{
    /// <summary>
    /// Connects a port (volume or atmosphere) to one end of a pipe via an orifice.
    /// Uses the isentropic nozzle model for the steady‑state relationship,
    /// and includes acoustic inertance for dynamic (Helmholtz) behaviour.
    /// </summary>
    public class OrificeLink
    {
        public Port VolumePort { get; }
        public Pipe1D Pipe { get; }
        public bool IsPipeLeftEnd { get; }
        public Func<double> AreaProvider { get; set; }
        public double DischargeCoefficient { get; set; } = 0.62;

        // Acoustic length (wall thickness + end correction) – controls the resonance frequency
        public double EffectiveLength { get; set; } = 0.001;   // 1 mm

        // Whether to include inertance; if false, uses the steady‑state nozzle model directly
        public bool UseInertance { get; set; } = true;

        // Current mass flow (kg/s, positive = volume → pipe)
        private double _mdot;

        public double LastMassFlowRate { get; private set; }
        public double LastFaceDensity { get; private set; }
        public double LastFaceVelocity { get; private set; }
        public double LastFacePressure { get; private set; }

        public OrificeLink(Port? volumePort, Pipe1D pipe, bool isPipeLeftEnd, Func<double> areaProvider)
        {
            VolumePort = volumePort;   // null is allowed
            Pipe = pipe ?? throw new ArgumentNullException(nameof(pipe));
            IsPipeLeftEnd = isPipeLeftEnd;
            AreaProvider = areaProvider ?? throw new ArgumentNullException(nameof(areaProvider));
            _mdot = 0.0;
        }

        public void Resolve(double dtSub)
        {
            double area = AreaProvider();
            // Closed wall or missing volume port => reflective boundary
            if (area < 1e-12 || VolumePort == null)
            {
                SetClosedWall();
                return;
            }

            // Gather volume state
            double volP   = VolumePort.Pressure;
            double volRho = VolumePort.Density;
            double volT   = VolumePort.Temperature;
            double volH   = VolumePort.SpecificEnthalpy;

            // Gather pipe interior state at the connected end
            (double pipeRho, double pipeU, double pipeP) = IsPipeLeftEnd
                ? Pipe.GetInteriorStateLeft()
                : Pipe.GetInteriorStateRight();

            double pipeT = pipeP / Math.Max(pipeRho * 287.0, 1e-12);
            double gamma = 1.4;
            double R     = 287.0;

            // ---- Steady‑state mass flow from isentropic nozzle ----
            double mdotSS;   // positive = volume → pipe
            double rhoFace, uFace, pFace;

            if (volP >= pipeP)
            {
                IsentropicOrifice.Compute(volP, volRho, volT, pipeP, gamma, R, area, DischargeCoefficient,
                    out double mdotUpToDown, out rhoFace, out uFace, out pFace);
                mdotSS = mdotUpToDown;   // volume → pipe
            }
            else
            {
                IsentropicOrifice.Compute(pipeP, pipeRho, pipeT, volP, gamma, R, area, DischargeCoefficient,
                    out double mdotUpToDown, out rhoFace, out uFace, out pFace);
                mdotSS = -mdotUpToDown;  // pipe → volume → negative for volume→pipe convention
            }

            // ---- Inertance ODE (optional) ----
            if (UseInertance)
            {
                double rhoUp = _mdot >= 0 ? volRho : pipeRho;
                double inertance = rhoUp * EffectiveLength / area;
                double dp = volP - pipeP;
                double resistance = Math.Abs(dp) / Math.Max(Math.Abs(mdotSS), 1e-12);
                double dmdot_dt = (dp - resistance * _mdot) / inertance;
                _mdot += dmdot_dt * dtSub;
            }
            else
            {
                _mdot = mdotSS;
            }

            // Clamp outflow to available mass (if finite volume)
            if (VolumePort.Owner is Volume0D vol)
            {
                double maxOut = vol.Mass / dtSub;
                if (_mdot > maxOut) _mdot = maxOut;
            }

            // ---- Ghost state ----
            // Density = upstream density (consistent with current flow direction)
            rhoFace = _mdot >= 0 ? volRho : pipeRho;
            // Pressure = downstream pressure (consistent with nozzle exit)
            pFace   = _mdot >= 0 ? pipeP : volP;
            // Velocity magnitude derived from actual mass flow
            double mdotMag = Math.Abs(_mdot);
            uFace = mdotMag / (rhoFace * area);

            if (IsPipeLeftEnd)
                uFace = _mdot >= 0 ? uFace : -uFace;    // left end: positive u = into pipe
            else
                uFace = _mdot >= 0 ? -uFace : uFace;    // right end: positive u = out of pipe

            // Apply ghost to pipe
            if (IsPipeLeftEnd)
                Pipe.SetGhostLeft(rhoFace, uFace, pFace);
            else
                Pipe.SetGhostRight(rhoFace, uFace, pFace);

            // ---- Store results ----
            double mdotIntoVolume = -_mdot;   // positive = into volume
            LastMassFlowRate = mdotIntoVolume;
            LastFaceDensity  = rhoFace;
            LastFaceVelocity = uFace;
            LastFacePressure = pFace;

            VolumePort.MassFlowRate = mdotIntoVolume;

            // Enthalpy for volume integration
            if (mdotIntoVolume >= 0)   // inflow → pipe enthalpy
            {
                double hPipe = gamma / (gamma - 1.0) * pipeP / Math.Max(pipeRho, 1e-12);
                VolumePort.SpecificEnthalpy = hPipe;
            }
            else                       // outflow → volume's own enthalpy
            {
                VolumePort.SpecificEnthalpy = volH;
            }
        }

        private void SetClosedWall()
        {
            var (rInt, uInt, pInt) = IsPipeLeftEnd
                ? Pipe.GetInteriorStateLeft()
                : Pipe.GetInteriorStateRight();

            if (IsPipeLeftEnd)
                Pipe.SetGhostLeft(rInt, -uInt, pInt);
            else
                Pipe.SetGhostRight(rInt, -uInt, pInt);

            LastMassFlowRate = 0.0;
            LastFaceDensity  = rInt;
            LastFaceVelocity = 0.0;
            LastFacePressure = pInt;
            // Don't touch VolumePort if it's null
            if (VolumePort != null)
                VolumePort.MassFlowRate = 0.0;
        }
    }
}