General Equations and Classification of Variable Density Fluid Motions

Abstract

The first goal of this chapter is to recall the “general, instantaneous, local” equations governing variable density fluid motions, according to the classical approach of continuum thermo-mechanics and using the local equilibrium hypothesis. On this basis, the classical numbers associated with density changes in a fluid motion are introduced and the departure from the solenoidal condition is discussed. The final part of the chapter is devoted to the presentation of a general time-scale analysis which is relevant to distinguishing and classifying several turbulent flows of variable density fluid.