A large number of physical phenomena are associated with the concept of diffusion. They relate to the spread in a “solvent” or porous medium of different populations with different nature or different physical properties. They relate to the spread out of a substance in a solute moving by mean of a gradient of concentration (from higher to lower values of concentration). More in general, they relate to the microscopic movements which are caused by means of the gradient of any physical property. Hence, the concept of diffusion describes how a substance spreads out being moved by a gradient of a characteristic physical magnitude, which generates the specific diffusion phenomenon (i.e. concentration, pressure, electric or magnetic potential).&nbsp; Particles in a solvent diffuse from higher concentrations to lower ones. Heat diffuses inside a material from higher to lower temperatures. Electrons and ions diffuse being attracted by opposite polarity by mean of an electric field gradient . High pressure fluids move toward lower pressure and higher speed regions. The momentum of a viscous fluid diffuses (in a linear way, if we are dealing with a Newtonian fluid).Diffusive phenomena do not necessarily require any visible bulk motion (unlike convection and advection phenomena) [[x]], or may influence convection and advection phenomena without apparent appreciable effects in terms of macroscopic bulk motion.&nbsp;The above general considerations allow thinking to new instruments which can lead to a better comprehension of fluid dynamic phenomena inside a general theoretical framework, which is constituted by irreversible thermodynamics and constructal law. A similar model has been conceived by Biot who has presented a comprehensive view of the thermodynamics of irreversible&nbsp;processes and defined a principle of virtual dissipation. It is a generalization of d'Alembert's law [[xx], [xxi]] into the domain of irreversible thermodynamics and leads&nbsp; to equations&nbsp; of evolution of thermodynamic systems [[xxii]], which can be obtained from either field&nbsp; equations or Lagrangian equations.Biot developed interdisciplinary concepts and methods, which couples pure mechanics, chemistry and the foundations of classical thermodynamics and thermodynamic of open systems. Such as constructal law, the Biot’s approach defines a holistic model which is based on experimental procedures, even in contrast with traditional standard formalism, and defines both energy and entropy of open systems that bypass the classical Gibbs approach and avoids the recourse to quantum statistics and does not require undetermined constants.Biot has introduced the concept of virtual dissipation which can be expressed according to a Lagrangian formulation. Virtual dissipation is a general concept that does not postulate any particular kinetics of the phenomena. Hence, it provides unprecedented flexibility and applies to nonlinear chemical reactions, viscous fluid flow both Newtonian and not Newtonian, and quantum kinetics.

Fluence as an Instrument Towards a Unified Representation of Fluiddynamic Related Phenomena

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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