318 lines
12 KiB
C#
318 lines
12 KiB
C#
using System;
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using FluidSim.Interfaces;
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namespace FluidSim.Components
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{
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/// <summary>
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/// 1‑D compressible Euler pipe (finite‑volume, HLLC flux).
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/// Boundary conditions are set externally via SetGhostLeft/Right.
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/// Enforces that ghosts are always valid before stepping.
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/// Uses exponential damping and Newtonian energy relaxation.
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/// </summary>
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public class Pipe1D : IComponent
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{
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public Port PortA { get; }
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public Port PortB { get; }
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public double Area { get; }
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public double DampingMultiplier { get; set; } = 1.0;
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public double EnergyRelaxationRate { get; set; } = 0.0; // 1/s
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// Ambient pressure for the energy relaxation term (default 101325 Pa)
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private double _ambientPressure = 101325.0;
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public double AmbientPressure
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{
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get => _ambientPressure;
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set
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{
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_ambientPressure = value;
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_ambientEnergyReference = value / (_gamma - 1.0);
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}
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}
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// Geometry
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private readonly int _n; // number of real cells
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private readonly double _dx; // cell size (m)
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private readonly double _diameter; // m
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private readonly double _gamma = 1.4;
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// Conserved variables [0 .. _n-1]
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private double[] _rho;
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private double[] _rhou;
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private double[] _E;
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// Face fluxes [0 .. _n]
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private double[] _fluxM;
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private double[] _fluxP;
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private double[] _fluxE;
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// Ghost cells (set externally)
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private double _rhoGhostL, _uGhostL, _pGhostL;
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private double _rhoGhostR, _uGhostR, _pGhostR;
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private bool _ghostLValid, _ghostRValid;
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// Pre‑computed damping coefficient
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private double _laminarCoeff;
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private double _ambientEnergyReference; // internal energy density at ambient pressure
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/// <summary>
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/// Initialise a pipe with a given cell count.
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/// </summary>
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/// <param name="length">Pipe length (m).</param>
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/// <param name="area">Cross‑sectional area (m²).</param>
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/// <param name="cellCount">Number of finite‑volume cells (≥ 4).</param>
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public Pipe1D(double length, double area, int cellCount)
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{
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if (cellCount < 4) throw new ArgumentException("cellCount must be at least 4");
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_n = cellCount;
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_dx = length / _n;
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Area = area;
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_diameter = 2.0 * Math.Sqrt(area / Math.PI);
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_rho = new double[_n];
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_rhou = new double[_n];
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_E = new double[_n];
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_fluxM = new double[_n + 1];
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_fluxP = new double[_n + 1];
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_fluxE = new double[_n + 1];
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// Laminar damping coefficient for air at 20°C (multiplied by DampingMultiplier each step)
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double mu_air = 1.8e-5;
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double radius = _diameter * 0.5;
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_laminarCoeff = 8.0 * mu_air / (radius * radius);
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// Ambient energy reference (internal energy per unit volume at 101325 Pa)
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_ambientEnergyReference = 101325.0 / (_gamma - 1.0);
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PortA = new Port { Owner = this };
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PortB = new Port { Owner = this };
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// Initial state = still air at ambient conditions
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SetUniformState(1.225, 0.0, 101325.0);
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}
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IReadOnlyList<Port> IComponent.Ports => new[] { PortA, PortB };
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// No integration needed for pipes – state is advanced via sub‑steps
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public void UpdateState(double dt) { }
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// ---------- Ghost cell interface ----------
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public void SetGhostLeft(double rho, double u, double p)
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{
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_rhoGhostL = rho;
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_uGhostL = u;
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_pGhostL = p;
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_ghostLValid = true;
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}
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public void SetGhostRight(double rho, double u, double p)
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{
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_rhoGhostR = rho;
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_uGhostR = u;
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_pGhostR = p;
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_ghostRValid = true;
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}
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public void ClearGhostFlags()
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{
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_ghostLValid = false;
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_ghostRValid = false;
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}
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public (double rho, double u, double p) GetInteriorStateLeft()
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{
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double rho = Math.Max(_rho[0], 1e-12);
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double u = _rhou[0] / rho;
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double p = PressureScalar(0);
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return (rho, u, p);
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}
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public (double rho, double u, double p) GetInteriorStateRight()
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{
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double rho = Math.Max(_rho[_n - 1], 1e-12);
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double u = _rhou[_n - 1] / rho;
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double p = PressureScalar(_n - 1);
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return (rho, u, p);
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}
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public int CellCount => _n;
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public double GetCellDensity(int i) => _rho[i];
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public double GetCellVelocity(int i)
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{
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double rho = Math.Max(_rho[i], 1e-12);
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return _rhou[i] / rho;
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}
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public double GetCellPressure(int i) => PressureScalar(i);
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// ---------- Sub‑stepping ----------
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public int GetRequiredSubSteps(double dtGlobal, double cflTarget = 0.8)
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{
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double maxW = 0.0;
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for (int i = 0; i < _n; i++)
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{
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double rho = Math.Max(_rho[i], 1e-12);
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double u = Math.Abs(_rhou[i] / rho);
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double p = PressureScalar(i);
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double c = Math.Sqrt(_gamma * p / rho);
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double local = u + c;
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if (local > maxW) maxW = local;
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}
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maxW = Math.Max(maxW, 1e-8);
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return Math.Max(1, (int)Math.Ceiling(dtGlobal * maxW / (cflTarget * _dx)));
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}
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// ---------- Main simulation step (per sub‑step) ----------
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public void SimulateSingleStep(double dtSub)
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{
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// Enforce that both ends have been provided with ghost states
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if (!_ghostLValid || !_ghostRValid)
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throw new InvalidOperationException("Pipe boundary ghosts not set before SimulateSingleStep.");
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double dt = dtSub;
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int n = _n;
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// Left boundary face (index 0)
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HLLCFlux(_rhoGhostL, _uGhostL, _pGhostL, _rho[0], _rhou[0] / _rho[0], PressureScalar(0),
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out _fluxM[0], out _fluxP[0], out _fluxE[0]);
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// Internal faces (1 .. n-1)
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for (int f = 1; f < n; f++)
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{
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double rhoL = Math.Max(_rho[f - 1], 1e-12);
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double uL = _rhou[f - 1] / rhoL;
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double pL = PressureScalar(f - 1);
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double rhoR = Math.Max(_rho[f], 1e-12);
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double uR = _rhou[f] / rhoR;
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double pR = PressureScalar(f);
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HLLCFlux(rhoL, uL, pL, rhoR, uR, pR, out _fluxM[f], out _fluxP[f], out _fluxE[f]);
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}
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// Right boundary face (index n)
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HLLCFlux(_rho[_n - 1], _rhou[_n - 1] / _rho[_n - 1], PressureScalar(_n - 1),
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_rhoGhostR, _uGhostR, _pGhostR,
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out _fluxM[n], out _fluxP[n], out _fluxE[n]);
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// Cell update
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double dt_dx = dt / _dx;
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double coeff = _laminarCoeff * DampingMultiplier;
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double relaxRate = EnergyRelaxationRate;
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for (int i = 0; i < n; i++)
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{
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double r = _rho[i];
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double ru = _rhou[i];
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double E = _E[i];
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double dM = _fluxM[i + 1] - _fluxM[i];
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double dP = _fluxP[i + 1] - _fluxP[i];
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double dE_flux = _fluxE[i + 1] - _fluxE[i];
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double newR = r - dt_dx * dM;
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double newRu = ru - dt_dx * dP;
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double newE = E - dt_dx * dE_flux;
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// Wall friction damping (laminar)
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double dampingFactor = Math.Exp(-coeff / Math.Max(r, 1e-12) * dt);
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newRu *= dampingFactor;
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// Newtonian cooling toward ambient energy
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double relaxFactor = Math.Exp(-relaxRate * dt);
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newE = _ambientEnergyReference + (newE - _ambientEnergyReference) * relaxFactor;
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// Clamps – minimum density 1e-12, minimum pressure 100 Pa
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newR = Math.Max(newR, 1e-12);
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double kin = 0.5 * newRu * newRu / Math.Max(newR, 1e-12);
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double eMin = 100.0 / (_gamma - 1.0) + kin;
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newE = Math.Max(newE, eMin);
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_rho[i] = newR;
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_rhou[i] = newRu;
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_E[i] = newE;
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}
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// Update port states to reflect the current interior state (for audio / monitoring)
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(double rhoA, double uA, double pA) = GetInteriorStateLeft();
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PortA.Pressure = pA;
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PortA.Density = rhoA;
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PortA.Temperature = pA / (rhoA * 287.0);
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PortA.SpecificEnthalpy = _gamma / (_gamma - 1.0) * pA / rhoA;
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(double rhoB, double uB, double pB) = GetInteriorStateRight();
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PortB.Pressure = pB;
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PortB.Density = rhoB;
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PortB.Temperature = pB / (rhoB * 287.0);
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PortB.SpecificEnthalpy = _gamma / (_gamma - 1.0) * pB / rhoB;
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}
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// ---------- Private helpers ----------
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private double PressureScalar(int i)
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{
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double rho = Math.Max(_rho[i], 1e-12);
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return (_gamma - 1.0) * (_E[i] - 0.5 * _rhou[i] * _rhou[i] / rho);
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}
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/// <summary>
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/// HLLC approximate Riemann solver (Toro, 1997).
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/// Computes the numerical flux at a face given left and right states.
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/// </summary>
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private void HLLCFlux(double rL, double uL, double pL, double rR, double uR, double pR,
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out double fm, out double fp, out double fe)
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{
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double cL = Math.Sqrt(_gamma * pL / rL);
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double cR = Math.Sqrt(_gamma * pR / rR);
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double EL = pL / ((_gamma - 1.0) * rL) + 0.5 * uL * uL; // specific total energy
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double ER = pR / ((_gamma - 1.0) * rR) + 0.5 * uR * uR;
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// Wave speed estimates (Davis, 1988)
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double SL = Math.Min(uL - cL, uR - cR);
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double SR = Math.Max(uL + cL, uR + cR);
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double denom = rL * (SL - uL) - rR * (SR - uR);
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double Ss = (pR - pL + rL * uL * (SL - uL) - rR * uR * (SR - uR)) / denom;
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double Fm_L = rL * uL;
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double Fp_L = rL * uL * uL + pL;
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double Fe_L = (rL * EL + pL) * uL;
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double Fm_R = rR * uR;
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double Fp_R = rR * uR * uR + pR;
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double Fe_R = (rR * ER + pR) * uR;
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if (SL >= 0) { fm = Fm_L; fp = Fp_L; fe = Fe_L; }
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else if (SR <= 0) { fm = Fm_R; fp = Fp_R; fe = Fe_R; }
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else if (Ss >= 0)
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{
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double rsL = rL * (SL - uL) / (SL - Ss);
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double ps = pL + rL * (SL - uL) * (Ss - uL);
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double EsL = EL + (Ss - uL) * (Ss + pL / (rL * (SL - uL)));
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fm = rsL * Ss;
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fp = rsL * Ss * Ss + ps;
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fe = (rsL * EsL + ps) * Ss;
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}
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else
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{
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double rsR = rR * (SR - uR) / (SR - Ss);
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double ps = pR + rR * (SR - uR) * (Ss - uR);
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double EsR = ER + (Ss - uR) * (Ss + pR / (rR * (SR - uR)));
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fm = rsR * Ss;
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fp = rsR * Ss * Ss + ps;
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fe = (rsR * EsR + ps) * Ss;
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}
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}
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/// <summary>Initialise all cells to a uniform state (rho, u, p).</summary>
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public void SetUniformState(double rho, double u, double p)
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{
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double e = p / ((_gamma - 1.0) * rho);
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double E = rho * e + 0.5 * rho * u * u;
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for (int i = 0; i < _n; i++)
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{
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_rho[i] = rho;
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_rhou[i] = rho * u;
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_E[i] = E;
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}
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}
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}
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} |