ABSTRACT
Another aspect of mixing is its role for the understanding and interpretation of observed non-Fickian, or anomalous transport behaviors, which are discussed in Chapter 27. Classical transport approaches based on Fick’s law assume that a smallest length scale exists (the representative elementary volume [REV] of the continuum transport description), on which concentration may be characterized by a single value, and that local-scale concentration changes are attenuated on time scales much smaller than the characteristic observation
time. is means the system is assumed to be in local physical equilibrium. e relaxation times scales are determined by the local-scale mixing mechanisms. On a macroscopic support scale in heterogeneous media, the relaxation time scales may be of the order of or larger than the observation time. us, the requirement of physical equilibrium is in general not met for macroscopic transport descriptions in terms of an average concentration, which invalidates the Fickian paradigm and leads to non-Fickian transport phenomena.
