Dimensionless Groups by Entropic Similarity: I - Diffusion, Chemical Reaction and Dispersion Processes

Since the time of Buckingham in 1914, dimensional analysis and similarity arguments based on dimensionless groups have served as powerful tools for the analysis of systems in all branches of science and engineering. Dimensionless groups are generally classified into those arising from geometric similarity, based on ratios of length scales; kinematic similarity, based on ratios of velocities or accelerations; and dynamic similarity, based on ratios of forces. We propose an additional category of dimensionless groups based on entropic similarity, defined by ratios of (i) entropy production terms; (ii) entropy flow rates or fluxes; or (iii) information flow rates or fluxes. Since all processes involving work against friction, dissipation, diffusion, dispersion, mixing, separation, chemical reaction, gain of information or other irreversible changes are driven by (or must overcome) the second law of thermodynamics, it is appropriate to analyze them directly in terms of competing entropy-producing and transporting phenomena and the dominant entropic regime, rather than indirectly in terms of forces. In this study, entropic groups are derived for a wide variety of diffusion, chemical reaction and dispersion processes relevant to fluid mechanics, chemical engineering and environmental engineering. It is shown that many dimensionless groups traditionally derived by kinematic or dynamic similarity (including the Reynolds number) can also be recovered by entropic similarity-with a different entropic interpretation-while many new dimensionless groups can also be identified. The analyses significantly expand the scope of dimensional analysis and similarity arguments for the resolution of new and existing problems in science and engineering.

Automated summary

Since Buckingham proposed dimensional analysis and similarity arguments based on dimensionless groups in 1914, they have been widely used in various branches of science and engineering. In this study, an additional category of dimensionless groups based on entropic similarity is proposed, which is defined by comparing ratios of entropy production terms, entropy flow rates or fluxes, and information flow rates or fluxes. The analysis expands the scope of dimensional analysis and similarity arguments for resolving new and existing problems in science and engineering by directly analyzing processes involving friction, dissipation, diffusion, dispersion, mixing, separation, chemical reaction, gain of information, and other irreversible changes in terms of competing entropy-producing and transporting phenomena and the dominant entropic regime, rather than indirectly in terms of forces.