There are two top-level goals that we aim to address in this book: (1) to provide a description of a gas by considering its most basic constituents, i.e., atoms and molecules, and (2) to introduce readers to computer simulation techniques that are available to analyze a gas at this fundamental level.

The first question we must ask ourselves is, Why should we consider a gas at the molecular level? After all, there are well-established equations and ideas that provide accurate descriptions of gas flow at the macroscopic level that employ variables such as density, flow velocity, temperature, and pressure, and these represent properties that take into account the molecules in an aggregate sense. Certainly, the molecular approach will provide us with a deeper understanding of all gas flows. However, more than that, under certain conditions, the aggregate, or sometimes called fluid, approach is not able to provide a physically accurate picture of the gas.We will find that these conditions arise when there is either not enough time or physical space for a sufficient number of intermolecular collisions to occur to maintain the gas in the well-understood equilibrium state. We refer to such conditions as nonequilibrium. To describe nonequilibium flows accurately, we need to study the molecular nature of the gas.

There are a number of important application areas in aerospace engineering where nonequilibrium gas flows arise. In general, we will find that nonequilibrium occurs when the gas flow is at low density and/or involves very small length scales. One important application area for nonequilibrium is the flight of high-speed vehicles at very high altitude in the Earth's atmosphere. Examples include spacecraft returning from orbit, such as the space shuttle, or hypersonic cruise vehicles. These vehicles have a length scale of several meters, and move at very high speed so that the flow field surrounding them involves very high temperatures. However, it is their operation in the low-density environment of near-space that leads to nonequilibrium gas flow phenomena. We focus on low-density, high-temperature air in many of the examples and analyses presented throughout the book.

A second important technology area involving nonequilibrium gas flow is micro- and nanoelectromechanical systems (MEMS/NEMS) that involve fabrication and operation of very small machines based on microfabrication technology.