The solutions so far have all be “in vacuum,” away from sources. In this chapter, we study gravity “in material.” For comparison, we review the continuum form of Newton’s second law and think about Newtonian gravitational predictions for, for example, hydrostatic equilibrium. Then we develop the relativistic version of those equations directly from Einstein’s equation with various source assumptions (spherical symmetry, perfect fluid) and obtain the interior Schwarzschild solution. Cosmology is another example of working “in material,” and we briefly review the Robertson–Walker starting point and solutions both with and without a cosmological constant. At the end of the chapter, spacetimes requiring exotic sources, including the Ellis wormhole and Alcubierre warp drive, are described.

Classical continuum mechanics focuses on the deformation field of moving continua. This deformation field is composed of the trajectories of all material elements, labeled by their initial positions. This initial-condition-based, material description is what we mean here by the Lagrangian description of a fluid motion. In contrast to typical solid-body deformations, however, fluid deformation may be orders of magnitude larger than the net displacement of the total fluid mass. The difficulty of tracking individual fluid elements has traditionally shifted the focus in fluid mechanics from individual trajectories to the instantaneous velocity field and quantities derived from it. These quantities constitutethe Eulerian description of fluids. This chapter surveys the fundamentals of both the Lagrangian and the Eulerian approaches. We also cover notions and results from differential equations and dynamical systems theory that are typically omitted from fluid mechanics textbooks, yet are heavily used in later chapters ofthis book.

Transport barriers offer a simplified global template for the redistribution ofsubstances without the need to simulate or observe numerous different initial distributions in detail. Because of their simplifying role, transport barriers are broadly invoked as explanations for observations in several physical disciplines, including geophysical flows,fluid dynamics,plasma fusion, reactive flowsand molecular dynamics. Despite their frequent conceptual use, however, transport barriers are rarely defined precisely or extracted systematically from data. The purpose of this book is to survey effective and mathematically grounded methods for defining, locating and leveraging transport barriers in numerical simulations, laboratory experiments, technological processes and nature. In the rest of this Introduction, we briefly survey the main topics that we will be covering in later chapters.

This chapter discusses two different approaches to describing fluid flow: a Lagrangian approach (following a fluid element as it moves) and a Eulerian approach (watching fluid pass through a fixed volume in space). Understanding each of these descriptions of flow is needed to fully understand the dynamics of fluids. This chapter is devoted to diving into the differences between the two descriptions of fluid motion. Understanding this chapter will help tremendously in the understanding of the upcoming chapters when the Navier–Stokes equations and energy equation are discussed. This chapter will introduce the material derivative. It is extremely important to understand this derivative before the Navier–Stokes equations themselves are tackled.

In the previous chapter, the forces acting on a moving fluid element were exhaustively studied. Using Newtons second law of motion, the Navier–Stokes equations for both compressible and incompressible flows were obtained. This chapter uses an alternative approach to developing the Navier–Stokes equations. Namely, by starting from a Eulerian description (as opposed to a Lagrangian description), the integral form and conservation form of the Navier–Stokes equations are developed. The continuity and Navier–Stokes equations in its various forms are tabulated and reviewed in this chapter. This chapter ends by solving some very simple, yet common, problems involving the incompressible Navier–Stokes equations.

This chapter develops the Navier–Stokes equations using a Lagrangian description. In doing so, the concept of a stress tensor and its role in the overall force balance on a fluid element is discussed. In addition, the various terms in the stress tensor as well as the individual force terms in the Navier–Stokes equations are investigated. The chapter ends with a discussion on the incompressible Navier–Stokes equations.

This chapter serves as an introduction to the concept of conservation and how conservation principles are used in fluid mechanics. The conservation principle is then applied to mass and an equation known as the continuity equation is developed. Various mathematical operations such as the dot product, the divergence, and the divergence theorem are introduced along the way. The continuity equation is discussed and the idea of an incompressible flow is introduced. Some examples using mass conservation are also given.

In this chapter, a concept known as scaling is introduced. Scaling (also known as nondimensionalization) is essentially a form of dimensional analysis. Dimensional analysis is a general term used to describe a means of analyzing a system based off the units of the problem (e.g. kilogram for mass, kelvin for temperature, meter for length, coulomb for electric change, etc.). The concepts of this chapter, while not entirely about the fluid equations per se, is arguably the most useful in understanding the various concepts of fluid mechanics. In addition, the concepts discussed within this chapter can be extended to other areas of physics, particularly areas that are heavily reliant on differential equations (which is most of physics and engineering).

In addition to the continuity equation, there is another very important equation that is often employed alongside the Navier–Stokes equations: the energy equation. The energy equation is required to fully describe compressible flows. This chapter guides the student through the development of the energy equation, which can be an intimidating equation. A discussion on diffusion and its interplay with advection is also included, leading to the idea of a boundary layer. The chapter ends with the addition of the energy equation in shear-driven and pressure-driven flows.

This chapter starts with a historical review of ideas about blood flow around the body, culminating in an understanding of circulation, the mechanics of which are described. The propagation of the pressure pulse in arteries is discussed, as is the disturbance to smooth flow caused by the complex geometry of arteries. The deformation of blood cells during their passage along the smallest capillaries is considered, as are the interesting effects of gravity on the venous return to the heart in upright animals, notably those with long necks and legs, such as giraffes and dinosaurs.

In this chapter, we show how the (infinite) set of equations known as the Jeans equations is derived by considering velocity moments of the collisionless Boltzmann equation (CBE) discussed in Chapter 9. The Jeans equations are very important for physically intuitive modeling of stellar systems, and they are some of the most useful tools in stellar dynamics. In fact, while the natural domain of existence of the solution of the CBE is the six-dimensional phase space, the Jeans equations are defined over three-dimensional configuration space, allowing us to achieve more intuitive modeling of directly observable quantities. The physical meaning of the quantities entering the Jeans equations is also illustrated by comparison with the formally analogous equations of fluid dynamics. Finally, by taking the spatial moments of the Jeans equations over the configuration space, the virial theorem in tensorial form is derived, complementing the more elementary discussion in Chapter 6.