7708d_c09_532-619

7708d_c09_532-619 - Impulsive start of flow past an array...

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Impulsive start of flow past an array of cylinders: The complex structure of laminar flow past a relatively simple geometric structure illustrates why it is often difficult to obtain exact analytical results for external flows. 1 Dye in water. 21 Photograph courtesy of ONERA, France. 2

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In this chapter we consider various aspects of the flow over bodies that are immersed in a fluid. Examples include the flow of air around airplanes, automobiles, and falling snow flakes, or the flow of water around submarines and fish. In these situations the object is completely surrounded by the fluid and the flows are termed external flows. External flows involving air are often termed aerodynamics in response to the impor- tant external flows produced when an object such as an airplane flies through the atmosphere. Although this field of external flows is extremely important, there are many other examples that are of equal importance. The fluid force 1 lift and drag 2 on surface vehicles 1 cars, trucks, bicycles 2 has become a very important topic. By correctly designing cars and trucks, it has become possible to greatly decrease the fuel consumption and improve the handling charac- teristics of the vehicle. Similar efforts have resulted in improved ships, whether they are sur- face vessels 1 surrounded by two fluids, air and water 2 or submersible vessels 1 surrounded completely by water 2 . Other applications of external flows involve objects that are not completely surrounded by fluid, although they are placed in some external-type flow. For example, the proper de- sign of a building 1 whether it is your house or a tall skyscraper 2 must include consideration of the various wind effects involved. As with other areas of fluid mechanics, two approaches 1 theoretical and experimental 2 are used to obtain information on the fluid forces developed by external flows. Theoretical 1 i.e., analytical and numerical 2 techniques can provide much of the needed information about such flows. However, because of the complexities of the governing equations and the com- plexities of the geometry of the objects involved, the amount of information obtained from purely theoretical methods is limited. With current and anticipated advancements in the area of computational fluid mechanics, it is likely that computer prediction of forces and com- plicated flow patterns will become more readily available. Much of the information about external flows comes from experiments carried out, for the most part, on scale models of the actual objects. Such testing includes the obvious wind tunnel testing of model airplanes, buildings, and even entire cities. In some instances the 533 9 F low Over Immersed Bodies Many practical sit- uations involve flow past objects.
actual device, not a model, is tested in wind tunnels. Figure 9.1 shows tests of vehicles in wind tunnels. Better performance of cars, bikes, skiers, and numerous other objects has re- sulted from testing in wind tunnels. The use of water tunnels and towing tanks also provides useful information about the flow around ships and other objects.

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This note was uploaded on 09/06/2009 for the course CEE cv2601 taught by Professor Kellas,j during the Spring '09 term at Nanyang Technological University.

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7708d_c09_532-619 - Impulsive start of flow past an array...

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