Fluid in motion

N. P. Cheremisinoff and R. Gupta, Handbook of Fluids in Motion, Ann Arbor Science PubUshers, Ann Arbor, Mich., 1983, pp. 76—77. 

Inertial forces are developed when the velocity of a fluid changes direction or magnitude. In turbulent flow, inertia forces are larger than viscous forces. Fluid in motion tends to continue in motion until it meets a sohd surface or other fluid moving in a different direction. Forces are developed during the momentum transfer that takes place. The forces ac ting on the impeller blades fluctuate in a random manner related to the scale and intensity of turbulence at the impeller. 

Wen, C. Y. and L. H. Chen, "Flow Modeling Coneepts of Fluidized Beds," n Handbook of Fluids in Motion, N. P. Cheremisinoff (Editor), Butterworth Publishers, Ann Arbor, MI, 1983. 

The governing flow equation describing flow through as porous medium is known as Darcy s law, which is a relationship between the volumetric flow rate of a fluid flowing linearly through a porous medium and the energy loss of the fluid in motion. 

Viewing things from the perspective of his physical theory of contact electricity, Volta was intrigued by the apparently endless power of the battery to keep the electric fluid in motion without the mechanical actions needed to operate the classical, friction, electrostatic machine, and the electrophorus. He called his batteiy alternately the artificial electric organ, in homage to the torpedo fish that had supplied the idea, and the electromotive apparatus, alluding to the perpetual motion (his words) of the electric fluid achieved by the machine. To explain that motion Volta relied, rather than on the concepts of energy available around 1800, on his own notion of electric tension. He occasionally defined tension as the effort each point of an electrified body makes to get rid of its electricity but above all he confidently and consistently measured it with the electrometer. 

By measuring the kinetic effects of the fluid in motion, since at a given pressure drop, low-density steam will move at a much greater velocity than will high-density condensate, and the conversion of pressure energy into kinetic energy can be used to position a valve. 

An understanding of the behavior of fluids in motion, or solids for that matter, requires an understanding of the term inertia. Inertia is the term used by scientists to describe the property possessed by all forms of matter that make it resist being moved when it is at rest and to resist any change in its rate or motion when it is moving. 

Gravity, applied forces, and atmospheric pressure are examples of static factors that apply equally to fluids at rest or in motion. Inertia and friction are dynamic forces that apply only to fluids in motion. The mathematical sum of gravity, applied forces, and atmospheric pressure is the static pressure obtained at any one point in a fluid system at a given point in time. Static pressure exists in addition to any dynamic factors that may also be present at the same time. 

The total energy of a fluid in motion is made up of a number of components. For unit mass of fluid and neglecting changes in magnetic and electrical energy, the magnitutes of the various forms of energy are as follows. 

It has been recognized for some time that fluids in motion, such as the atmosphere or the ocean, disperse added materials. This properly has been exploited by engineers in a variety of ways, such as the use of smoke stacks for boiler furnaces and ocean ontfalls for the release of treated wastewaters. It is now known that dilution is seldom the solution to an enviromnental problem the dispersed pollutants may accumulate to undesirable levels in certain niches in an ecosystem, be transformed by biological and photochemical processes to other pollntants, or have nnanticipated health or ecological effects even at highly dilute concentrations. It is therefore necessary to rmderstand the transport and transformation of chemicals in the natural environment and through the trophic chain ctrlminating in man. 

We have stated that the only stress that can exist in a fluid at rest is pressure, because the shear stresses (which resist motion) are zero when the fluid is at rest. This also applies to fluids in motion provided there is no relative motion within the fluid (because the shear stresses are determined by the velocity gradients, e.g., the shear rate). However, if the motion involves an acceleration, this can contribute an additional component to the pressure, as illustrated by the examples in this section. 

The total energy of a fluid in motion consists of the following components internal, potential, pressure and kinetic energies. Each of these energies may be considered with reference to an arbitrary base level. It is also convenient to make calculations on unit mass of fluid. 

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