Fluid Mechanics

Fluid mechanics is the branch of science that deals with the study of fluids (liquids and gases) in a state of rest or motion. Fluid mechanics is a science concerned with the response of fluids to forces exerted upon them. A fluid is a substance which deforms continuously under the application of a shear stress. The applications of fluid mechanics is enormous: breathing, blood flow, swimming, pumps, fans, turbines, airplanes, ships, rivers, windmills, pipes, missiles, icebergs, engines, filters, jets and sprinklers etc. It is an important subject of civil, mechanical and chemical engineering. 

From the point of view of fluid mechanics, all matter consists of only two states, fluid and solid. The difference between the two is perfectly obvious. The technical distinction lies with the reaction of the two to an applied shear or tangential stress. A solid can resist a shear stress by a static deflection; a fluid cannot. Any shear stress applied to a fluid, no matter how small, will result in motion of that fluid. The fluid moves and deforms continuously as long as the shear stress is applied. As a corollary, we can say that a fluid at rest must be in a state of zero shear stress.

Fluid

A substance that flows is called as fluid. All liquid and gaseous substances are considered to be fluids. Water, oil and others are very important in our day-to-day life as they are used for various applications. For instance water is used for generation of electricity in hydroelectric power plants and thermal power plants, water is also used as the coolant in nuclear power plants, oil is used for the lubrication of automobiles etc.

Whether the fluid is at rest or motion, it is subjected to different forces and different climatic conditions and it behaves in these conditions as per its physical properties. Its various branches are fluid statics, fluid kinematics and fluid dynamics. 

Fluid Statics 

The fluid which is in state of rest is called as static fluid and its study is called as fluid statics. 

Fluid Kinematics 

The fluid which is in state of motion is called as moving fluid. The study of moving fluid without considering the effect of external pressures is called as fluid kinematics. 

Fluid Dynamics 

The branch of science which studies the effect of all pressures including the external pressures on the moving fluid is called as fluid dynamics.

Liquid and Gases

A fixed amount of a liquid has a definite volume which varies only slightly with temperature and pressure. If the capacity of the containing vessel is greater than this definite volume, the liquid occupies only part of the container and it forms an interface separating it from its own vapour, the atmosphere or any other gas present.

A fixed amount of a gas, by itself in a closed container, will always expand until its volume equals that of the container. Only then can it be in equilibrium. In the analysis of the behaviour of fluids an important difference between liquids and gases is that, whereas under ordinary conditions liquids are so difficult to compress that they may for most purposes be regarded as incompressible, gases may be compressed much more readily. Where conditions are such that an amount of gas undergoes a negligible change of volume, its behaviour is similar to that of a liquid and it may then be regarded as incompressible. If, however, the change in volume is not negligible, the compressibility of the gas must be taken into account in examining its behaviour.

The continuum

An absolutely complete analysis of the behaviour of a fluid would have to account for the action of each individual molecule. In most engineering applications, however, interest centres on the average conditions of velocity, pressure, temperature, density and so on. Therefore, instead of the actual conglomeration of separate molecules, we regard the fluid as a continuum that is a continuous distribution of matter with no empty space. This assumption is normally justifiable because the number of molecules involved in the situation is so vast and the distances between them are so small. The assumption fails, when these conditions are not satisfied as, for example, in a gas at extremely low pressure.