Update

include/FluidSim/Fluid.hpp

@@ -189,10 +189,14 @@ public:
         float dt = std::numeric_limits<float>::max();
         for (std::size_t i = 0; i < cells.size(); ++i) {
             states_[i] = primitive(cells[i]);
+            waves_[i] = wave(states_[i]);
+        }
+        // Keep the arithmetic loop free of exceptional control flow so SIMD
+        // can evaluate independent cells, then validate before any flux update.
+        for (std::size_t i = 0; i < cells.size(); ++i) {
             const auto &s = states_[i];
             if (!(s.rho > 0) || !(s.pressure > 0) || !std::isfinite(s.velocity) || !std::isfinite(s.pressure))
                 throw std::runtime_error("nonphysical pipe state in preparation");
-            waves_[i] = wave(s);
             dt = std::min(dt, cfl * cell_lengths[i] / waves_[i]);
         }
         prepared_ = true;
@@ -265,7 +269,12 @@ public:
                 en = minimum;
                 ++positivity_repairs;
             }
-            next_cells_[i] = {rho, mom, en, std::clamp(air / rho, 0.0f, 1.0f)};
+            // Purged tracer tails below the smallest normal float carry less
+            // than 1.18e-38 of the gas mass. Avoid repeatedly transporting
+            // subnormal concentrations; total gas mass and energy are unchanged.
+            const float fraction = std::clamp(air / rho, 0.0f, 1.0f);
+            next_cells_[i] = {rho, mom, en,
+                             fraction < std::numeric_limits<float>::min() ? 0.0f : fraction};
         }
         cells.swap(next_cells_);
         prepared_ = false;
@@ -399,6 +408,7 @@ private:
         float *area{};
         float cd{1}, length{}, loss{}, mdot{};
         std::size_t budget{std::numeric_limits<std::size_t>::max()};
+        double previous_pressure{};
     };
     struct Open {
         int pipe;
@@ -491,7 +501,8 @@ private:
         const double zero_pressure = s.pressure * std::pow(zero_sound / c, 2 * g / (g - 1));
         double low = std::min(zero_pressure, double(v.pressure));
         double high = std::max(zero_pressure, double(v.pressure));
-        double duct_pressure = .5 * (low + high);
+        double duct_pressure = o.length == 0 && o.previous_pressure > low && o.previous_pressure < high
+                                   ? o.previous_pressure : .5 * (low + high);
         const double flow_sign = v.pressure >= zero_pressure ? 1 : -1;
         const double flow_scale = area * o.cd;
         const double density_ratio_scale = std::pow(double(s.pressure) / v.pressure, 1 / g);
@@ -537,6 +548,7 @@ private:
                                     ? candidate
                                     : .5 * (low + high);
             }
+        o.previous_pressure = duct_pressure;
         float mdot = static_cast<float>(nozzle_flow(duct_pressure));
         const bool upstream_port = mdot >= 0;
         const float pup = upstream_port ? v.pressure : static_cast<float>(duct_pressure);