FlipSolver2d

A 2d testing ground for writing FLIP/PIC fluid solver

Building

1. After cloning, get submodules to setup imgui:

git submodule init
git submodule update --recursive

2. Create and go to build directory
3. Run:

cmake PATH_TO_CMakeLists_IN_REPO_ROOT -DCMAKE_BUILD_TYPE=Release

4. Wait for cmake to fetch dependencies and finish
5. Run:

cmake --build . --config Release -j NUM_OF_CORES

6. Outputs will be in following dirs:

• Liquid2dRender:
  • scenes directory contains all scenes to load
  • config.json specifies what scene to load
• bench:
  • Test benchmark, chages time to time, used as quick test for various things

Directory structure

• FlipSolver2dLib:
  Main solver library sources
• FlipSolver2dLib/threading:
  Sources for thread pool
• Liquid2dRender:
  Main app sources
• Liquid2dRender/include:
  glad and khr headers
• Liquid2dRender/src:
  glad source
• Liquid2dRender/scenes:
  All scenes that are watched by cmake and that will be copied to output scenes directory
• bench:
  Testing benchmark sources, change randomly, used to quickly test stuff outside of full gui app
• devDocs:
  Various files to not forget some aspects of the simulation.
  Are not meant to replace documentation and may be confusing

Scene file specification

Each scene file is a json with two top-level keys:

• settings:
  Holds global scene settings. Scene 0,0 is in top left, (X,I,U) points down, (Y,J,V) points right
  • simType:
    flip, nbflip, smoke, fire
    Specifies simulation type
  • domainSizeI, domainSizeJ:
    Domain size in units (read meters)
  • resolution:
    Number of grid cells along the shortest dimension, other dimension is calculated automatically
  • fps:
    Target number of steps per second
  • density:
    Fluid desity
  • maxSubsteps:
    Maximum number of substeps per one big step
  • pcgIterLimit:
    Max number of PCG solver iterations
  • seed:
    Seed for RNG
  • particlesPerCell:
    Target number of particles to keep in each cell
  • particleScale:
    Radius of particles for SDF update, relative to grid cell size (0.5 means particle radius is half of grid cell size length). Keep 0.8 for 2d cases
  • picRatio:
    PIC/FLIP velocity update methods ratio. Lower ratio - More FLIP, more splash, but less stability. 0.03 is usually a good spot
  • cflNumber:
    Solver will try to keep particles from traveling more than this many cells per time step, substepping if necessary. 5 is a good start
  • scale:
    Scale to be applied for whole scene. So domain 1x1 meter in size with scale 0.01 becoms 1x1 centimeter
  • viscosityEnabled:
    Toggle viscosity solver for water sims
  • heavyViscosity:
    If true, use more expensive but potentially more accurate viscosity model
  • globalAcceleration:
    2 number array, holds gravitational acceleration. Mind the coordinate system.
  • parameterHandlingMethod:
    grid, particle (default - particle)
    Advect secondary parameters (viscosity, soot, fuel) via grid or particles. May cause interesting results, experimental
  • ambientTemperature:
    Gas solver parameter. Ambient air temperature
  • temperatureDecayRate:
    Gas solver parameter. How quickly does the gas cool down?
  • concentrationDecayRate:
    Gas solver parameter. How quickly does smoke dissolve?
  • buoyancyFactor:
    Gas solver parameter. Temperature weight in buoyancy calculations. Keep at 1 for realistic math
  • sootFactor:
    Gas solver parameter. Soot weight in buoyancy calculations. Higher number - heavier the smoke is.
  • ignitionTemp:
    Fire solver parameter. Fuel needs to be hotter than this to burn.
  • burnRate:
    Fire solver parameter. How quickly the fuel burns?
  • smokeEmission:
    Fire solver parameter. How much smoke does fuel emit when burnt?
  • heatEmission:
    Fire solver parameter. How much heat does fuel produce when burnt?
  • billowing:
    Fire solver parameter. How much does the fuel expand when burnt? Experimental, may not work in many versions.
• solver:

  • objects:
    Holds all objects in the scene. For now, the only key under solver section.
    Each object has:

    • type:
      solid, source, sink, fluid
      What type of object this object is
    • enabled:
      Allows toggling objects for quick tinkering
    • verts:
      Array of 2-number arrays. Specifies verticies of the object. Winding order does not matter, last vertex is connected to first

    Solids have:

    • friction:
      How much does the obstacle slow down the fluid?

    Fluid sources and initial fluids have:

    • viscosity:
      Water solver parameter. Initial fluid viscosity
    • temperature:
      Gas solver parameter. Initial gas temperature
    • concentration:
      Gas solver parameter. Initial smoke concentration
    • fuel:
      Fire solver parameter. Initial amount of fuel in the emitted gas
    • transferVelocity:
      If true, initial fluid velocity is set to velocity value
    • velocity:
      Array of two numbers. If transferVelocity is true, gives fluid this initial velocity.