Introduction

Abstract

There are two main descriptions of fluid flows, namely, (1) from the Boltzmann equation and (2) from the classical continuum theory. The difference comes from the scale at which fluid flows are observed. Therefore they involve the observer or the experimental and numerical devices that may be used. Naturally, at different scales ranging from microscopic (molecular dynamics) to macroscopic (which is the real scale for applications), the fluid flow equations are different. There is also a deeper description, and one of the open problems is the derivation of the fluid flow equations from microscopic Hamiltonian dynamics.