Algorithm or equation? Either way, I'd suggest the Navier-Stokes system of equations, and associated numeric solutions.
Have a look at Chung's 'Computational Fluid Dynamics' (2010) via Google books:
Have a look at Chung's 'Computational Fluid Dynamics' (2010) via Google books:
I would argue that CFD models are surprisingly simple relative to the task of understanding multi-phase, particle-laden, turbulent fluids.