FluidSimCpp

Native C++20 port of FluidSim. The C# application remains the reference while the component models and behavior are being ported and checked against it.

Build (Windows / Visual Studio)

Open a Visual Studio Developer PowerShell (or initialize the MSVC environment with VsDevCmd.bat), then run from this directory:

cmake -S . -B build -G "NMake Makefiles" -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build --output-on-failure

The first configure fetches SFML 3.0.2 with CMake FetchContent for the desktop window and audio stream. -DFLUIDSIM_BUILD_DESKTOP=OFF builds only the headless runner and physics tests.

Run the interactive simulator with:

.\build\fluidsim_desktop.exe

Controls: hold W for throttle, C for clutch, A / D to shift, Z for neutral, up/down to adjust throttle target, left/right to adjust engine load, M toggles mechanical engine noise, mouse wheel changes simulation playback speed, and Space toggles real time.

The command-line runner is useful for repeatable checks:

.\build\fluidsim_cpp.exe --seconds 1 --throttle 0.35
.\build\fluidsim_cpp.exe --scenario two-stroke --seconds 1 --throttle 0.3
.\build\fluidsim_cpp.exe --scenario helmholtz --seconds 1
.\build\fluidsim_cpp.exe --scenario pulse --seconds 1

To compare deterministic checkpoints from the active C# twin scenario against the C++ port (energy variation is disabled for repeatability), build the C++ trace runner and execute the PowerShell comparison script:

cmake --build build-msvc --target fluidsim_parity_trace
.\verification\Compare-ScenarioTrace.ps1

Current port status

The C++ core now includes the shared EngineCylinder model with four-stroke and two-stroke cycle implementations, crankshaft, crankcase, atmosphere and configurable 0-D volume components (including thermal relaxation and moving volume work), profiled compressible 1-D pipes and boundaries, adaptive solver stepping, the impulse-clutch motorcycle/road-load model, and the active 600 cc twin topology. Audio includes the flow-derivative source, spatialized intake/exhaust radiation, outdoor feedback-delay reverb, optional mechanical noise, a lock-free simulation audio buffer, a SoundEngine lifecycle wrapper, and an SFML desktop frontend. The shared 50-RPM-bin dyno history and graph are also ported. A crankcase-scavenged 125 cc two-stroke scenario is available in the core.

This is still an in-progress port, not a claimed complete replacement. All four current C# scenarios (600 cc twin, 125 cc two-stroke, Helmholtz resonator, and pressure pulse) have headless C++ counterparts and can be selected by the CLI. Remaining parity work includes completing the C# drawing/diagnostic controls in the desktop UI and resolving numerical differences reported by the comparison harness. The latest MSVC build completed, CTest passed, and the one-second headless twin run completed with finite output. The desktop executable opened, but its client area rendered blank in the captured run, so desktop rendering is not yet verified.

The one-second twin trace remains substantially outside its 1% tolerance. The latest comparison's largest relative difference is 40.6745 for cylinder 1 pressure at checkpoint 33075 (69.8 kPa in C# versus 2.91 MPa in C++). A dense intake trace matches closely at first, then the C# plenum pressure begins rising at checkpoint 63 while C++ remains near ambient. This divergence needs to be resolved before claiming numerical parity. CTest covers cylinder geometry and valve timing, dyno-bin interpolation, configurable 0-D volume thermodynamics, crankcase and atmosphere state, profiled-pipe geometry, inertive and closed boundaries, gearing and clutch momentum transfer, finite runs for every scenario, and finite audio/reverb output.