text: There are some notable similarities in equations for momentum, energy, and mass transfer which can all be transported by diffusion, as illustrated by the following examples:
  * Mass: the spreading and dissipation of odors in air is an example of mass diffusion.
  * Energy: the conduction of heat in a solid material is an example of heat diffusion.
  * Momentum: the drag experienced by a rain drop as it falls in the atmosphere is an example of momentum diffusion (the rain drop loses momentum to the surrounding air through viscous stresses and decelerates).
  The molecular transfer equations of Newtons law for fluid momentum, Fouriers law for heat, and Fick's law for mass are very similar.  One can convert from one transport coefficient to another in order to compare all three different transport phenomena.
  A great deal of effort has been devoted in the literature to developing analogies among these three transport processes for turbulent transfer so as to allow prediction of one from any of the others. The Reynolds analogy assumes that the turbulent diffusivities are all equal and that the molecular diffusivities of momentum (μ/ρ) and mass (D) are negligible compared to the turbulent diffusivities. When liquids are present and/or drag is present, the analogy is not valid. Other analogies, such as von Karmans and Prandtls, usually result in poor relations.
  The most successful and most widely used analogy is the Chilton and Colburn J-factor analogy. This analogy is based on experimental data for gases and liquids in both the laminar and turbulent regimes. Although it is based on experimental data, it can be shown to satisfy the exact solution derived from laminar flow over a flat plate.  All of this information is used to predict transfer of mass.