Fluid Simulation Solver

What this is

• Real-time 2D LBM fluid simulation solver from FetchCFD running on GPU for fast, interactive visualization, qualitative analysis, and rapid prototyping
• “Physical” inputs (Mach or m/s, Re, chord, speed of sound) are used to auto-scale the lattice to a stable regime.
• Best used for learning and visualization (flow features, separation, vortices), not certified engineering results.

Quick start

• Obstacle Shape: Airfoil / Circle / Upload Mask (PNG).
• Speed Input Mode: choose Mach or m/s, then adjust the speed slider.
• Re + chord + speed of sound: set your target conditions; the solver updates τ, Ma(LBM), U∞, and effective Re in the stats.
• Reset: rebuilds the flow field using your current settings.
• Pause: stops time stepping (handy for reading probe values and force estimates).
• Snapshot: saves a PNG of the current view including the legend overlay.

Obstacle options

• Airfoil: NACA 4-digit style geometry using AoA, thickness, and camber.
• Circle: cylinder; the “thickness” slider acts as a diameter control.
• Upload Mask (PNG):
  • Opaque pixels (alpha > ~20) become solid obstacle; transparent = fluid.
  • Use the transform tools: rotate, scale, Flip Y (fix upside-down), and optional Use AoA.
  • You can drag the mask on the canvas to reposition it.

Flow models & boundary conditions

• Collision model:
  • BGK (SRT) – simple and fast.
  • TRT – includes a “magic” parameter Λ. By default it uses the recommended safe preset Λ = 3/16. Enable Advanced to adjust Λ.
• Turbulence:
  • Laminar – no subgrid model.
  • LES (Smagorinsky) – adds subgrid viscosity for stability at higher Re, but it changes the solution (not “ground truth”).
• Domain type:
  • External: open top and bottom.
  • Ground vehicle: moving-wall bottom (“belt”) with adjustable Belt speed × U∞. (Legend moves to the top.)
  • Symmetry: bottom free-slip / specular reflection.
• Outlet sponge: optional damping zone near the outlet to reduce reflections; tune start and strength.

Scaling, stability, and what the stats mean

• LBM Mach Target: caps lattice Mach for stability (lower is safer).
• Viscosity is auto-computed from target Re + scaling and then clamped to keep τ > ~0.52. (The viscosity slider is intentionally read-only.)
• Re: the stats show effective Re and your target Re; a warning appears if the target exceeds what’s achievable under stability limits.

Probe (Sensor) & data logging

• Sensor: enables a draggable probe showing local speed (m/s), Mach, speed (LBM), Cp, pressure (LBM), and density.
• Local Cl/Cd: shown near the probe when paused (estimated from nearby surface samples).
• Data Collection: logs probe values to a table (~4 Hz). Includes Stop/Resume, Copy as CSV, and Clear. (Table keeps up to ~250 rows.)

Known limitations (important)

• 2D only (no spanwise/3D physics).
• Low-Mach assumption: high “physical Mach” is mainly visual; this is not a transonic/shock solver.
• Not a validated aero tool: Cl/Cd are approximate and depend strongly on resolution, pixel geometry, and the sampling method.
• Grid/geometry dependence: obstacles are rasterized on the lattice (stair-stepping), so results change with resolution and mask quality.
• Simple inlet/outlet: outlet is simplified; use the sponge to reduce reflections in wake flows.
• Hardware/extension limits: requires WebGL 2 and float render targets (some devices may not support this reliably).

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