Fluids are ubiquitous: the air we breathe, the rivers and lakes we live near, the water that comes from our faucets, and the blood in our veins are all examples of fluids that are familiar to us. Describing how fluids behave is what Fluid Mechanics is all about. In essence, it is the science of liquids and gasses in motion.
Just as a knowledge of mechanics of materials (solid mechanics) allows engineers to design bridges and buildings, a knowledge of how fluids behave enables engineers to design water supply systems, pumps, turbines, aircraft, and even biomedical devices such as artificial hearts.
Before one can study fluids in motion, however, one must have an understanding of fluid properties and fluid statics. The main properties of fluids of interest to civil engineers include density, viscosity, and surface tension. Fluid statics involves the study of forces exerted by a fluid at rest. These principles allow engineers to design dams, ships, and pressure vessels.
The study of fluid motion can be approached in numerous ways. The three main principles involve conservation of mass, conservation of momentum, and conservation of energy. In its most simple form, mass conservation (also called continuity) means that "what goes in must come out." The principle of momentum conservation (based on Newton's second law of motion) allows engineers to design jets, nozzles, turbines, and even rocket motors. The total energy at one location in a fluid system is the sum of the potential and kinetic energies at that point. Conservation of energy means that as fluid moves from one place to another, the total amount of energy does not diminish: potential energy is converted to kinetic energy, and some may be "lost" due to friction. This principle can be used to predict the flow out of a water tank or the power that can be produced by a hydroelectric turbine.
Not everything involved in fluid mechanics can be derived from first principles. In fact, much of what we know about the science of fluids is the result of decades, even centuries of experimental work. Often, engineers wish to conduct tests on a relatively small fluid system (say, in a university laboratory), and then wish to apply the results to the real world (a dam on a river, for example). Relating one to the other is not exactly straightforward; the principles of Dimensional Analysis and Dynamic Similitude provide tools for comparing fluid flows at different scales.
With these principles in hand, engineers can then study and analyze all sorts of fluid-related problems, ranging from design of water supply systems (flow in pipes and pipe networks) to turbomachinery (pumps and turbines) to flow in open channels (canals and rivers).
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