 Péclet number

The Péclet number compares advection over length $L$ at speed $U$ with molecular diffusion of diffusivity $D$.

The Péclet number (Pe) is a dimensionless number used in fluid mechanics and heat transfer to characterize the relative importance of advection and diffusion in a flowing system. It is defined as the ratio of the rate of advective transport of a quantity (such as heat or mass) to the rate of diffusive transport of that same quantity.