Chapter 9 involves shock–boundary-layer interactions that are intrinsic to supersonic engine intakes, transonic gas turbine blade tip gaps and blade passages, scramjet isolator ducts, transonic and supersonic flight vehicle surfaces, and surfaces of rockets, missiles, and reentry vehicles. It is of particular interest because it can result in large temporal and spatial pressure variations, and greatly affect boundary development including causing flow separation that feeds into the flow unsteadiness, and subsequently has a large impact on aerodynamic performance. The outcome of shock–boundary-layer interactions strongly depends on steady and unsteady initial conditions that can be factored into flow control approaches. Methods for these are presented.

Chapter 5 focuses on free shear layers and jets that involve the merging of two flow streams. Away from a boundary, the mean flow that results is inviscidly unstable and rapidly leads to the formation of coherent vortical structures that drive strong fluid mixing. In jets, it can result in large acoustic levels. Free shear layers are also highly sensitive to sound excitation that can lead to resonant growth of the instability, or a means of control. With this understanding, both passive and active methods of free shear layer control are presented.

Chapter 6 deals with 2-D laminar boundary-layer instabilities and their control. It covers the full range of Mach numbers from incompressible to hypersonic. Boundary-layer instabilities leading to turbulence onset is of great practical importance. This chapter reviews methods of analysis of boundary-layer stability and illustrates several linear and nonlinear mechanisms that can play a role in the breakdown to turbulence. Such understanding is intrinsic to the methods of boundary-layer instability control that are presented in the chapter. Both passive and active flow control approaches are presented.

Chapter 3 focuses on the control of bluff-body wakes, where a bluff body is generally categorized as one whose length in the flow direction is approximately the same as its height. Such shapes exhibit a wide wake on the scale of the body height, with aerodynamic drag that is dominated by a low-pressure region that forms in the near wake of the body. Bluff body wakes are complex and highly unsteady, involving boundary layer flow separation and multiple shear layer interactions. The control of bluff body aerodynamics has practical implications to airfoils at high angles of attack, aircraft landing gear, ground vehicles, and buildings and structures. Methods of control that key on the wake instabilities are presented.

Chapter 4 focuses on separated flows that occur in a variety of applications involving external flows, particularly related to aircraft, and internal flows, such as within turbomachines. Flow separation results when the flow does not have sufficient momentum to overcome an adverse pressure gradient, or when viscous dissipation occurs along the flow path. It is almost always associated with some form of aerodynamic penalty, including a loss of lift, an increase in drag, a loss of pressure recovery, and an increase in entropy. This chapter presents both passive and active methods to control these adverse effects.

Chapter 7 deals with 3-D laminar boundary-layer instabilities and their control. It covers the full range of Mach numbers from incompressible to hypersonic. A practical example of a 3-D boundary layer is the flow over a swept wing, which is susceptible to four types of instabilities that can lead to turbulence onset. Of these, cross-flow instability is the most dominant and therefore the most studied 3-D boundary-layer instability mechanism. A fundamental understanding of the instability has led to methods of control that have been successfully demonstrated at incompressible to hypersonic Mach numbers. These and other methods of control are presented.

Chapter 8 focuses on turbulent boundary layers. This considers a proposed autonomous cycle for turbulence production that results from an instability of a distorted mean flow near the wall surface that is produced by a spanwise array of coherent longitudinal vortices whose spacing scales with the viscous shear stress. The instability results in a lift-up and break-up of the longitudinal vortices that are linked to increased turbulence production and increased viscous drag. This and other mechanisms of turbulence production and viscous drag generation are presented. Methods of flow control that key on these specific mechanisms are presented along with significant results.