Update

tests/fluid_validation.cpp

@@ -48,6 +48,23 @@ int main() {
             std::cout << "FAIL " << n << ": " << e.what() << '\n';
         }
     };
+    test("tracer underflow preserves bulk gas and normal concentrations", [] {
+        for (float fraction : {1e-40f, 1e-30f, .5f}) {
+            Pipe1D p(.001f, 1, 8);
+            for (int i=0; i<8; ++i) set(p, i, 101325, 1.2f, 0, fraction);
+            p.left_ghost = p.state(0);
+            p.right_ghost = p.state(7);
+            const double before_mass = mass(p), before_energy = energy(p);
+            p.advance(1e-6f);
+            near(mass(p), before_mass, 0, "tracer truncation changed total mass");
+            near(energy(p), before_energy, 0, "tracer truncation changed energy");
+            for (auto cell : p.cells) {
+                check(std::fpclassify(cell.air) != FP_SUBNORMAL, "subnormal tracer survived");
+                near(cell.air, fraction < std::numeric_limits<float>::min() ? 0 : fraction,
+                     fraction * 1e-6, "normal tracer concentration changed");
+            }
+        }
+    });
     test("invalid uniform geometry", [] {
         bool caught = false;
         try {

tests/saab_turbo_regression.cpp

@@ -0,0 +1,46 @@
+#include "FluidSim/Scenario.hpp"
+#include <cmath>
+#include <iostream>
+#include <stdexcept>
+using namespace fluidsim::port;
+void check(bool ok, const char* message) { if (!ok) throw std::runtime_error(message); }
+int main() {
+    Ecu ecu;
+    ecu.boost_control_enabled = true;
+    ecu.step(.001f, 1, 3500);
+    for (int i=0;i<1000;++i) ecu.control_boost(.001f,1,3500,110000,115000);
+    check(ecu.wastegate_duty()>0 && ecu.wastegate_duty()<=.85f,"no spool duty");
+    ecu.control_boost(.001f,1,3500,190000,190000);
+    check(ecu.fuel_cut() && ecu.wastegate_duty()==0,"overboost protection failed");
+    ecu.control_boost(.001f,1,3500,140000,140000);
+    check(!ecu.fuel_cut(),"overboost recovery failed");
+    ecu.control_boost(.001f,0,3500,100000,100000);
+    check(ecu.wastegate_duty()==0 && ecu.boost_target()==0,"lift-off did not restore spring control");
+    ecu.control_boost(.001f,1,3500,NAN,100000);
+    check(ecu.fuel_cut() && ecu.wastegate_duty()==0,"boost sensor fail-safe failed");
+    FourStrokeScenario s(44100,FourStrokePreset::SaabB205,true);
+    check(s.pipes.size()==12 && s.turbo && s.ecu.boost_control_enabled,"Saab turbo topology missing");
+    double peak_rpm=0, peak_map=0, peak_audio=0, settled_pressure=0;
+    for(int second=0;second<10;++second) {
+        for(int i=0;i<44100;++i) {
+            s.throttle = second>=1 && second<7 ? 1.f : 0.f;
+            if(second<7) s.crankshaft.omega=3500*2*pi/60;
+            const auto a=s.process();
+            if(second>=4 && second<7) settled_pressure+=s.intake_plenum.pressure()/(3.*44100);
+            check(std::isfinite(a.left) && std::isfinite(a.right),"nonfinite audio");
+            peak_audio=std::max(peak_audio,double(std::abs(a.left)));
+        }
+        auto t=s.turbo->snapshot();
+        peak_rpm=std::max(peak_rpm,t.rpm);
+        peak_map=std::max(peak_map,double(s.intake_plenum.pressure()));
+        check(std::isfinite(t.rpm) && std::abs(t.energy_residual)<.1,"turbo energy balance failed");
+        check(t.limited_transfers==0,"housing donor limit hit");
+        std::cout<<second+1<<" MAP="<<s.intake_plenum.pressure()/1000<<" charge="<<t.charge_pressure/1000
+                 <<" rpm="<<t.rpm<<" duty="<<s.ecu.wastegate_duty()<<" gate_mm="<<t.wastegate_lift*1000<<'\n';
+    }
+    check(peak_rpm>50000 && peak_map>110000,"GT17 failed to spool/boost");
+    check(peak_audio<1,"audio exceeds monitoring range");
+    check(std::abs(settled_pressure-146325)<8000,"boost did not settle near target");
+    check(s.ecu.wastegate_duty()==0 && s.turbo->snapshot().rpm<peak_rpm,"lift-off control/spool-down failed");
+    for(auto& p:s.pipes) check(p.positivity_repairs==0,"pipe state repaired");
+}