Relaxation Effects in Gas Dynamics

Abstract

Classical gas dynamics is by definition the dynamics of gases that are in local thermodynamic equilibrium everywhere in the flow field so that their thermodynamic behaviour is completely specified by two independent variables of state, for example by pressure p and density p. The fundamental assumption of local thermodynamic equilibrium is not valid in many flow situations of practical importance, and this fact has given rise to a revision and generalisation of classical gas dynamics. This generalisation is usually called “relaxation gas dynamics”. Ten years ago J. Ackeret mentioned this subject briefly at the end of the Daniel and Florence Guggenheim Memorial Lecture on The Role of Entropy in the Aerospace Sciences. He gave that lecture at the 2nd International Congress of the Aeronautical Sciences in Zürich (12th September 1960) and he said on that occcasion: “Non-equilibrium states are of growing importance, for strictly speaking, exact (thermodynamic) equilibrium occurs only relatively seldom in gas dynamics. However, a rational theory of nonequilibrium states must necessarily be extremely complicated….