velocity in mass transport

The flux density $N B$ is related to the velocity by the equation

$N B = c B v B$

The vector $v B$ is the macroscopic average velocity at which the species B moves (which is to be distinguished from the random molecular velocity); $c B$ is the concentration of species B ( $mol m −3$ ). Note that the velocity is defined with respect to a frame of reference.

Source:

PAC, 1981, 53, 1827 (Nomenclature for transport phenomena in electrolytic systems) on page 1830