Fluid mechanics is a branch of physics that studies how fluids react to forces applied to them. It's a field of classical physics with important applications in chemical engineering, hydraulics and aeronautics, meteorology, and zoology. Fluid dynamics is investigated both theoretically and practically, using mathematical and physical interpretations of the results. Petroleum engineers must have a sound understanding of fluid mechanics. Drilling, well completion, production technologies, transportation, and refining are just a few of the areas where it's used.
A multiphase flow is one in which there are multiple phases (e.g., gas, solid, and liquid). In industry, such flows are ubiquitous. These phases can be made up of only one chemical component (for example, water flow and water vapor), or they can be made up of numerous separate chemical components (e.g. flow of oil and water).