Definition of Convection, Diffusion, and Advection in Fluids
Definition
This section concerns the phenomenon in which matter or heat is transported by a fluid.
- The macroscopic collective movement of a fluid is called advection.
- The homogenization due to microscopic molecular motion is called diffusion.
- The transport of matter or heat resulting from the simultaneous action of advection and diffusion is called convection.
Explanation
Mathematically, advection is described by the fluid’s fluid velocity $\mathbf{u}$, and diffusion is described by diffusion coefficients such as mass diffusivity $D$ or thermal diffusivity $\alpha$.
As an empirical example, imagine a device heating a cold room: a fan heater actively induces advection, whereas a stove can be seen as relying on diffusion. Unlike advection, which imparts momentum to the hot air itself to move it, diffusion is the process by which hot gas molecules undergo Brownian motion and, in the long run, spread throughout the entire room.