This book deals with multiphase flows, i.e. systems in which different fluid phases, or fluid and solid hases, are simultaneously present. The fluids may be different phases of the same substance, such as a liquid and its vapor, or different substances, such as a liquid and a permanent gas, or two liquids. In fluid–solid systems, the fluid may be a gas or a liquid, or gases, liquids, and solids may all coexist in the flow domain.