Shocks and Rarefactions

Abstract

The word " shock " is used very widely in common experience. One is shocked by an unexpected event; a wounded victim goes into shock; and one shocks a material by suddenly cooling it. A " shock wave " is a sudden transition in the properties of a fluid medium, involving a difference in flow velocity across a narrow (ideally, abrupt) transition. In high-energy-density physics, nearly any experiment involves at least one shock wave. Such shock waves may be produced by applying pressure to a surface or by creating a collision between two materials. In astrophysics, nearly every sudden event produces a shock wave. Yet in common experience one encounters very few shock waves. We hear thunder after lightning, which is a long-term consequence of the shock wave produced by the lightning channel, but as we shall see below one would hope never to directly experience this shock wave. Most of us hear sonic booms infrequently, but they are the only shock wave of human origin we typically encounter. We have more direct experience with rarefactions or " rarefaction waves, " in which a fluid begins to move, expanding and becoming less dense, with the edge of the moving region propagating into an initial body of fluid. Household drafts may be due to rarefactions, which can occur in a house, for example, when a gust of wind drops the pressure at an open door, by the Bernoulli ef-fect. Rarefactions also have real practical uses, notably in refrigeration where they are used to produce expansion cooling. It is also true that nearly every high-energy-density experiment involves at least one rarefaction wave. Moreover, most high-energy-density experiments involve at least one in-terface, where the density (and perhaps the equation of state) changes. When-ever a shock wave or a rarefaction wave reaches an interface, there are trans-mitted and reflected waves in response. In each of these two directions, these waves might be either shock waves or rarefaction waves, so that there are four possible responses. Which of these four occurs depends on the details. One can find systematic discussions of this in books on shock physics. As we proceed to consider various cases, we will encounter specific examples. It should be clear that shock waves, rarefaction waves, and their interactions merit a serious examination, which we undertake in this chapter.