中文版本 (Chinese Version)

SPH 2D Fluid Simulation

A real-time 2D fluid simulation based on Smoothed Particle Hydrodynamics (SPH), running on Unity URP with core physics computation fully driven by GPU Compute Shaders.

Features

• GPU-driven SPH fluid simulation, supporting up to 131,072 particles
• Spatial hash grid + Bitonic sort for O(n·k) neighbor search
• Real-time density field / pressure field / particle visualization
• Interactive mouse-based particle spawning
• Fully integrated as a Unity URP ScriptableRendererFeature

Showcase

Density Field Visualization

Pressure Field Visualization

Technical Architecture

Assets/Scripts/
├── Core/                           # Foundation: constants, data structures, state
│   ├── FluidConstants.cs
│   ├── FluidParticleData.cs
│   ├── FluidState.cs
│   └── MeshUtility.cs
├── Simulation/                     # Simulation: controller, particle spawning
│   ├── FluidSimulatorController.cs
│   └── ParticleSpawner.cs
├── Rendering/                      # Rendering: URP integration, render passes
│   └── FluidRenderFeature.cs
└── Sorting/                        # Sorting: GPU Bitonic sort
    └── BitonicSort.cs

Assets/Shaders/
├── ComputeShader/
│   ├── FluidParticle.hlsl          # SPH kernel functions + shared data structures
│   ├── FluidParticlesCS.compute    # Physics computation (density/pressure/integration)
│   └── BitonicSortCS.compute       # Bitonic Merge Sort
├── DrawParticles.shader            # GPU Instancing particle rendering
├── DrawGridDensity.shader          # Full-screen density field
├── DrawGridPressure.shader         # Full-screen pressure field
└── VizDensity.shader               # Density deviation coloring

Per-Frame Pipeline

Spatial Hash Build → Bitonic Sort → Density Calculation → Pressure Calculation
→ Velocity/Position Integration → Particle Drawing

SPH Algorithm

Uses a quadratic polynomial kernel function, with spatial hash grid optimizing neighbor search from O(n²) to O(n·k):

1. Compute grid ID for each particle
2. GPU Bitonic sort particles by grid ID
3. Build grid start indices for O(1) grid lookup
4. Traverse 3×3 neighboring grids to compute density and pressure
5. Explicit Euler integration to update velocity and position

Requirements

• Unity 2022.3+ (URP 14.x)
• GPU with Compute Shader support (Shader Model 5.0)

Usage

1. Open the project with Unity Hub
2. Open Assets/Scenes/SampleScene.unity
3. Adjust FluidSimulatorController parameters in the Inspector
4. Click Play, left-click on screen to spawn particles

Inspector Parameters

Parameter	Description
Particle Radius	Particle rendering radius
Smoothing Radius	SPH smoothing kernel radius, affects neighbor search range
Target Density	Target density — particles tend toward this distribution
Pressure Multiplier	Pressure coefficient — higher values increase incompressibility
Energy Damping	Energy decay, controls velocity loss after boundary collision
Gravity	Gravitational acceleration
Enable Update	Enable/pause physics simulation
Draw Particles	Show particles
Draw Grid Density Field	Show density field heatmap
Draw Viz Density Map	Show density deviation coloring

License

MIT