High Quality Content by WIKIPEDIA articles! The convection–diffusion equation is a combination of the diffusion and convection (advection) equations, and describes physical phenomena where particles, energy, or other physical quantities are transferred inside a physical system due to two processes: diffusion and convection. Depending on context, the same equation can be called the advection–diffusion equation, drift–diffusion equation, Smoluchowski equation (after Marian Smoluchowski), or (generic) scalar transport equation.