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One of the main reasons for continuing interest in shock focusing is its ability to concentrate energy in a small volume and produce extreme temperatures and pressures in fluids in a controlled laboratory environment. The phenomenon of shock wave focusing leading to extreme conditions in fluids during micro- and nanosecond time intervals is a spectacular example of mechanics at small length and time scales revealing the major properties of shock dynamics including high-temperature gas phenomena. Production of high-energy concentrations in gases and fluids with star-like temperatures and extreme pressures by means of a stable imploding shock is of great interest not only in its own right but also because of the connection to a multitude of phenomena in nature, technology and medicine.



Chapter 1. Introduction

The reader will certainly recognize the everyday situations described in the beginning of this chapter. But did you know that these events are manifestations of exciting and many times unexpected properties of shock waves in nature and technology? One of the main properties of shock waves is an abrupt and large change in density, pressure, and temperature of the carrier medium in which the shock is propagating. This may often result in substantial damage to humans and material structures. At the same time, this harsh property of a shock wave and even its increase in strength in confined spaces opens fascinating possibilities for use in technology and medicine. Inertial confinement fusion and lithotripsy belong to the most known examples. Here we will describe many more and tell the story of their discovery.
Nicholas Apazidis, Veronica Eliasson

Chapter 2. Shock Waves and Blast Waves

This chapter will provide you with a brief introduction to the mathematical description of shock and blast waves and the most common experimental techniques to study shock wave focusing phenomena. In order to fully understand shock wave focusing, it is necessary to first have a good understanding how a shock wave reacts when it encounters another shock or, for example, a concave solid boundary. Therefore, we will introduce the concept of shock wave reflections, how it was discovered and its two main types, regular reflection and irregular reflection, and provide references for the interested reader to learn more. Self-similar solutions are very common tools to predict the motion of converging shocks, and a brief introduction to this topic is provided in this chapter. Lastly, several of the most common experimental techniques to study shock wave focusing, such as shock tubes, exploding wires, and microexplosives, are presented followed by an introduction to different types of visualization techniques (schlieren and interferometry) that are used to photograph the often remarkable beautiful shock wave dynamics phenomena.
Nicholas Apazidis, Veronica Eliasson

Chapter 3. Converging Shocks

In the beginning of this chapter, we give on overview of early experimental work on generation of converging shocks by various methods ranging from annular shock tubes to cylindrical as well as spherical explosion chambers. These early experimental results along with Guderley’s solution raise important questions of self-similarity and stability of converging shocks. Experimental results showing the dependence of the power-law exponent on the adiabatic exponent for various gases are presented and discussed. We then give an overview of theoretical and numerical results on the stability of converging shocks based on the theory of geometrical shock dynamics. A number of experimental results on shock convergence show that converging shock experiences tendency toward planarity, e.g., generation of plane sides and sharp corners in initially cylindrical shock front. In this respect several sections of this chapter are devoted to experimental as well as numerical work on convergence of polygonal shocks and their ability to preserve symmetry and thus enhance the final energy density. Production of cylindrical and spherical converging shocks by a gradual change in the shock tube cross-section has been proposed by several researchers. We discuss the basic theoretical and numerical results as well as their experimental realization leading to extreme conditions at the focal area with gas temperatures in excess of 30,000 K. The end of this chapter is devoted to shock generation and focusing in water by means of exploding wire techniques. Experimental findings showing extreme states of matter at the focal area of a converging shock in water generated by a moderate input of initial energy are discussed.
Nicholas Apazidis, Veronica Eliasson

Chapter 4. Shock Focusing in Nature and Medicine

We will here get acquainted with some spectacular examples of shock focusing occurring in nature, from a tiny bubble that emits light during its periodic compression and expansion to a supernova that rebounds after a gravitational collapse producing the most powerful energy burst known to us with light intensity comparable to that of the whole galaxy. Interestingly, some of the small sea creatures such as the so-called snapping shrimp, just some 20 mm in length, use cavitation to create a powerful outgoing blasts to hunt their pray. Despite the very small scale compared to astronomical events, the tiny bubble functions as a focusing lens and is able to generate extreme accelerations, forces, and temperatures during nano- and picosecond time intervals. Shock wave lithotripsy is one of the most known medical applications of shock wave focusing. The method, developed three decades ago, uses repeated focused pressure pulses and is now the primary method for treatment of kidney stones.
Nicholas Apazidis, Veronica Eliasson
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