In this chapter, we explore the basics of fluid mechanics. We will think about how to describe fluids and look at the kinds of things they can do.

Unusually, and a little defensively, the title of this chapter highlights what we won’t talk about, rather than what we will. Fluids have a property known as viscosity. This is an internal friction force acting within the fluid as diferent layers rub together. It is crucially important in many applications. In spite of its importance, we will start our journey into the world of fluids by ignoring viscosity altogether. Such flows are called inviscid. This will allow us to build intuition for the equations of fluid mechanics without the complications that viscosity brings. Moreover, the flows that we find in this section will not be wasted work. As we will see later, they give a good approximation to viscous flows in certain regimes where the more general equations reduce to those studied here.

When it comes to natural disasters, earthquakes and tsunamis all too often top the list of worst calamities. Using several examples we will try to improve our understanding of how they occur. In later chapters, we discuss whether science indeed has techniques that can lead to statistical modeling. The examples discussed include the 2004 Boxing Day tsunami, killing more than 220 000 people, the 2011 Tōhoku earthquake and tsunami, which included the major nuclear disaster in Fukushima, and the volcanic explosion at the kingdom of Tonga on January 15, 2022. From each of these events we discuss specifics concerning risk, both in understanding as well as communication. We start the chapter with a brief, non-technical discussion of (Daniel) Bernoulli’s principle in incompressible fluids. This allows us to learn how tsunamis are formed and propagate across oceans causing catastrophic inundations to lower-lying coastal areas, often very far away. Especially for the Tōhoku and Fukushima case, we discuss the crucial difference between an "if" approach to risk management versus a "what if" one. The Tonga explosion highlights the importance of modeling such extremal events, taking the global geometric shape of our planet into account.