Convection - A Widespread Mechanism in Nature

Last week I derived the PDE starting from an SDE describing a family of short rate models (see here). When we take a look at the resulting PDE we see that it is go the kind of a reaction-diffusion-convection equation. In a blog post a few months ago I explained the effect of diffusion for the example of osmotic pressure (see Diffusion and Osmotic Pressure). Starting with this friday I will make some blog posts to show examples of the physical effect of convection in nature.

Before we can give examples we should make something like a qualitative definition what convection is. Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding fluid (liquids or a gases). This fluid transfers the internal energy along with its mass. Although the heat is initially transferred between the object and the fluid by conduction, the bulk transfer of energy comes from the motion of the fluid. Convection can occur spontaneously/naturally through the creation of convection cells. Forced convection occurs either when the fluid is propelled across the object or the object is propelled through the fluid.

From a more microscopic view convection is a collective movement of groups or aggregates of molecules within fluids and rheids, either through diffusion or by advection (the term advection sometimes serves as a synonym for convection, but technically, convection covers the sum of transport both by diffusion and by advection - advection itself is the transport mechanism related to the fluid's bulk motion).

We will close todays blog post with a very common example where we observe convection: Cooking a pot of water. The water at the bottom of the pot is close to the flame (heat source) and heats up (red arrows). It rises because it expands and has lower density. Then the water releases heat as it cools down at the top (blue arrows) and sinks down again.  Basically, the moving (circulating) water is a conveyor belt for heat transport from the hot flame to the cooler surface.


Next friday I will give some more examples of convection in nature and how this influences our live.