Fluids & Combustion (real CFD)
Two-Stream Fluid Mixing
advection–diffusion of two streams

Two opposing jets of fluid collide in the middle and interleave into turbulent mixing — advection and diffusion resolved by the Navier–Stokes solver.
Computed & rendered in pure C# on the frozen OnlyCSharp 1.8 library — animation baked to GIF with the library's own
Gif encoder (no GPU, no ffmpeg, no external packages).