Transfer

We are concerned here with the transfer of properties from one place to another. This transfer may be by bodily contact as when momentum is transferred from one airstream to another by the action of pressure forces. Transfer may be by pure advection with the bulk motion of the fluid, as when water vapour is carried along at constant mixing ratio together with the dry air. Such simple advection may be complicated by the relative motion of a different phase as when water droplets fall relative to the air. Molecular diffusion may be important, as when warm air is brought close to cold air then brought back to where it started, but cooler. To form a cloud droplet, water vapour diffuses towards a cloud particle and the latent heat released is conducted away from a hot particle. Electromagnetic radiation transfers energy from one absorber (emitter) to another, and sound waves carry pressure information.

Here we concentrate on the advection by fluid motion, but find that we must be aware of these other processes at the same time. For example, when the fluid motion is inexactly known, the statistical fluctuations round the known state may resemble diffusion in some respects, but not in others. It is necessary at least to be aware of the possible differences. The transfer of a variety of properties by fluctuating unresolved, but not necessarily random, fluid motion, is at the heart of the turbulence problem.