Fluid mechanics deals with the flow of fluids. Fluid mechanics, a special branch of general mechanics, describes the laws of liquid and gas motion.
Fluid mechanics is the study of fluids either in motion (fluid dynamics) or at rest (fluid statics) and the subsequent effects of the fluid upon the boundaries, which may be either solid surfaces or interfaces with other fluids. Both gases and liquids are classified as fluids, and the number of fluids engineering applications is enormous: breathing, blood flow, swimming, pumps, fans, turbines, airplanes, ships, flows in living organisms, atmospheric circulation, oceanic currents, tidal flows in rivers,, windmills, pipes, missiles, icebergs, engines, filters,valves, hydraulic systems jets, and sprinklers, to name a few. When you think about it, almost everything on this planet either is a fluid or moves within or near a fluid.
Liquids and gases, often termed fluids, in contrast to rigid bodies cause only little resistance when they are slowly deformed, as long as their volume does not change. The resistance is so much less the slower the deformation is. It can therefore be concluded, that the arising tangential stresses are small when the deformations are slow and vanish in the state of rest. Hence, liquids and gases can be defined as bodies, which do not build up tangential stresses in the state of rest. If the deformations are fast, there results a resistance proportional to the friction forces in the fluid. The ratio of the inertia to the friction forces is therefore of great importance for characterizing fluid flows. This ratio is called Reynolds number.
If liquid and gas motion is to be described, in general, not the motion of single atoms or molecules is described, neither is their microscopic behavior taken into account; the flowing medium is considered as a continuum. It is assumed to consist out of very small volume elements, the overall dimensions of which, however, being much larger than the intermolecular distances. In a continuum the mean free path between the collisions of two molecules is small compared to the characteristic length of the changes of the flow quantities. Velocity, pressure and temperature, density, viscosity, thermal conductivity, and specific heats are described as mean values, only depending on position and time. In order to be able to define the mean values, it is necessary, that the volume element is small compared to the total volume of the continuum.
All matter consists of only two states, fluid and solid. The difference between the two is perfectly obvious to the layperson, and it is an interesting exercise to ask a layperson to put this difference into words. 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 deformation; 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, a state often called the hydrostatic stress condition in structural analysis. In this condition, Mohr’s circle for stress reduces to a point, and there is no shear stress on any plane cut through the element under stress.
Given the definition of a fluid above, every layperson also knows that there are two classes of fluids, liquids and gases. Again the distinction is a technical one concerning the effect of cohesive forces. A liquid, being composed of relatively close-packed molecules with strong cohesive forces, tends to retain its volume and will form a free surface in a gravitational field if unconfined from above.
