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# CLASSIFICATION AND DESCRIPTION OF FLUID FLOW

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أستاذ المادة عدي عدنان جهاد الخيكاني       05/10/2012 11:39:34
CLASSIFICATION AND DESCRIPTION OF FLUID FLOW
It is helpful to the understanding of fluid mechanics to broadly classify
certain different types of fluid flow, and here we introduce some of the
terminology used.
Internal and external flows
The distinction between internal and external flows often needs to be made. When the motion of a fluid is between bounding surfaces the flow is described as internal flow. Airflow management systems are widely used to control the quality of air within buildings and vehicles; the movement of air within the ducting which forms part of such a system is an example of an internal flow. Conversely, when a body is surrounded by a fluid in motion, the flow around the immersed body is described as external flow. Examples of external flows are the flows surrounding an aircraft wing, around an entire aircraft, around a road vehicle such as a car or lorry or around a building.

Laminar and turbulent flows
From about 1840, it had been realized that the flow of a fluid could be
of two different kinds. The distinction between them is most easily understood by reference to the work undertaken in the early 1880s by Osborne Reynolds (1842–1912), Professor of Engineering at Manchester University. The apparatus used by Reynolds was as shown in Fig. 1.11. A straight length of circular glass tube with a smoothly rounded, flared inlet was placed in a large glass-walled tank full of water. The other end of the tube passed through the end of the tank. Water from the tank could thus flow out along the tube at a rate controlled by a valve at the outlet end. A fine nozzle connected to a small reservoir of a liquid dye discharged a coloured filament into the inlet of the glass tube. By observing the behaviour of the stream of dye, Reynolds was able to study the way in which the water was flowing along the glass tube If the velocity of the water remained low and especially if the water in the tank had previously been allowed to settle for some time so as to eliminate all disturbances as far as possible, the filament of dye would pass down the tube without mixing with the water, and often so steadily as almost to seem stationary (Fig. 1.12a). As the valve was opened further and the velocity of the water thereby increased, this type of flow would persist until the velocity reached a value at which the stream of dye began to waver (Fig. 1.12b).
Further increase in the velocity of the water made the fluctuations in the
stream of dye more intense, particularly towards the outlet end of tube, until a state was reached, quite suddenly, in which the dye mixed more or less completely with the water in the tube. Thus, except for a region near

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