Irreversibility in Fluid Dynamics

Abstract

Let us begin with a broad view of fluid behavior and then isolate the part which is dependent on the cause of irreversibility. By “fluid” we mean any macroscopic and inanimate sample of matter, solid, liquid, or gas. It is true that solids are not very “fluid,” but they do flow slowly and they do the other things (melt, conduct, react….) covered by theories about “fluid dynamics,” so we have no reason to exclude them here. Nor do we exclude plasma (ionized gases), often referred to as a fourth state of matter, or surface films or even rarefied gases. But we do restrict ourselves at every opportunity to the simplest fluid predicament that can offer the needed examples. We are not trying to construct a fluid dynamics but merely to identify the cause of the irreversible part.