FluidSim/Core/Solver.cs

using System;
using System.Collections.Generic;
using FluidSim.Components;

namespace FluidSim.Core
{
    public class Solver
    {
        private readonly List<Volume0D> _volumes = new();
        private readonly List<Pipe1D> _pipes = new();
        private readonly List<PipeVolumeConnection> _connections = new();

        private double _dt;
        private double _ambientPressure = 101325.0;

        public void SetAmbientPressure(double p) => _ambientPressure = p;
        public void AddVolume(Volume0D v) => _volumes.Add(v);
        public void AddPipe(Pipe1D p) => _pipes.Add(p);
        public void AddConnection(PipeVolumeConnection c) => _connections.Add(c);
        public void SetTimeStep(double dt) => _dt = dt;

        public void SetPipeBoundary(Pipe1D pipe, bool isA, BoundaryType type, double ambientPressure = 101325.0)
        {
            if (isA)
            {
                pipe.SetABoundaryType(type);
                if (type == BoundaryType.OpenEnd) pipe.SetAAmbientPressure(ambientPressure);
            }
            else
            {
                pipe.SetBBoundaryType(type);
                if (type == BoundaryType.OpenEnd) pipe.SetBAmbientPressure(ambientPressure);
            }
        }

        public float Step()
        {
            // 1. For each connection, handle flow or closed wall
            foreach (var conn in _connections)
            {
                double area = conn.OrificeArea;
                if (area < 1e-12)   // valve closed → treat as solid wall
                {
                    conn.Volume.MassFlowRateIn = 0.0;
                    conn.Volume.SpecificEnthalpyIn = conn.Volume.SpecificEnthalpy; // not used

                    // Set ghost to a reflective wall (u = -u_pipe, same p, ρ)
                    int cellIdx = conn.IsPipeLeftEnd ? 0 : conn.Pipe.GetCellCount() - 1;
                    double rho = Math.Max(conn.Pipe.GetCellDensity(cellIdx), 1e-6);
                    double p   = Math.Max(conn.Pipe.GetCellPressure(cellIdx), 100.0);
                    double u   = conn.Pipe.GetCellVelocity(cellIdx);
                    if (conn.IsPipeLeftEnd)
                        conn.Pipe.SetGhostLeft(rho, -u, p);
                    else
                        conn.Pipe.SetGhostRight(rho, -u, p);
                    continue;
                }

                // Valve open → use the nozzle model
                double downstreamPressure = conn.IsPipeLeftEnd
                    ? conn.Pipe.GetCellPressure(0)
                    : conn.Pipe.GetCellPressure(conn.Pipe.GetCellCount() - 1);

                NozzleFlow.Compute(conn.Volume, area, downstreamPressure,
                                out double mdot, out double rhoFace, out double uFace, out double pFace,
                                gamma: conn.Volume.Gamma);

                // Clamp mdot to available mass
                double maxMdot = conn.Volume.Mass / _dt;
                conn.LastMassFlowIntoVolume = mdot;
                if (mdot > maxMdot) mdot = maxMdot;
                if (mdot < -maxMdot) mdot = -maxMdot;

                conn.Volume.MassFlowRateIn = mdot;
                // enthalpy: if inflow, use pipe enthalpy; if outflow, use cylinder enthalpy
                if (mdot >= 0)
                {
                    int cellIdx = conn.IsPipeLeftEnd ? 0 : conn.Pipe.GetCellCount() - 1;
                    double pPipe = Math.Max(conn.Pipe.GetCellPressure(cellIdx), 100.0);
                    double rhoPipe = Math.Max(conn.Pipe.GetCellDensity(cellIdx), 1e-6);
                    conn.Volume.SpecificEnthalpyIn = (conn.Volume.Gamma / (conn.Volume.Gamma - 1.0)) * pPipe / rhoPipe;
                }
                else
                {
                    conn.Volume.SpecificEnthalpyIn = conn.Volume.SpecificEnthalpy;
                }

                // Integrate the volume
                conn.Volume.Integrate(_dt);

                // Set ghost from nozzle face state (but don't allow zero density)
                if (rhoFace < 1e-6) rhoFace = Constants.Rho_amb;
                if (pFace < 100.0)  pFace = Constants.P_amb;
                if (conn.IsPipeLeftEnd)
                    conn.Pipe.SetGhostLeft(rhoFace, uFace, pFace);
                else
                    conn.Pipe.SetGhostRight(rhoFace, uFace, pFace);
            }

            // 2. Sub‑step pipes
            int nSub = 1;
            foreach (var p in _pipes)
                nSub = Math.Max(nSub, p.GetRequiredSubSteps(_dt));
            double dtSub = _dt / nSub;

            for (int sub = 0; sub < nSub; sub++)
                foreach (var p in _pipes)
                    p.SimulateSingleStep(dtSub);

            // 3. Clear ghost flags
            foreach (var p in _pipes)
                p.ClearGhostFlag();

            // 4. Return exhaust tailpipe mass flow
            if (_pipes.Count > 0)
                return (float)_pipes[0].GetOpenEndMassFlow();
            return 0f;
        }
    }
}