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تاريخ التسجيل : 21/09/2008
 |  موضوع: تقرير عن flow visualization  2008-09-23, 1:14 pm | |
| Appendix FResearch Authorization 201 [529] This is the story of an NACA research authorization. It tells how and why the authorization was opened, executed, and closed. While research authorization 201 had some idiosyncrasies-it lasted longer than most and produced fewer practical results-still it is sufficiently representative to give some idea of how the NACA went about aeronautical research. It is particularly enlightening on the respective roles of headquarters and the laboratory in selecting and conducting research projects, on changes in those roles over the years, on publication policies of the Committee, and on the relations of NACA staff members with clients and colleagues.Boundary-layer research had been going on in Europe for 20 years before the NACA took any official interest. Only when the Europeans began to achieve some success in boundary-layer control did the NACA launch a program of its own. The NACA was always more interested in application than in theory; it never wanted to understand the wind so much as to control it.The boundary layer is a thin film that forms on the surface of a solid body moving through a viscous fluid, like the wing of an airplane moving through the air. Within the film, velocity increases parabolic ally, from zero at the solid surface up to the free-stream velocity at the outer edge of the boundary layer. The depth of the layer varies with the smoothness of the surface, the viscosity of the fluid, and the speed of the flow, but it is never very large. At 5 cm from the leading edge of a fiat plate moving through standard sea-level air at zero angle of incidence and 120 m/sec, the boundary layer will be only .04 cm deep.1  Profile of a boundary layer. (NASA EP-89, 1971, p. 68) The boundary layer was first identified and labeled by Ludwig Prandtl in 1904 in a classic paper that revolutionized this branch of fluid mechanics. The Göttingen University [530] professor actually used the term "boundary layer" only once, while he used "transition layer" seven times. But "boundary layer" became the accepted term, and boundary-layer theory became the descriptor of choice for the entire field. Prandtl had based his paper on empirical investigations, but his concept remained only a theory until it was verified in the 1930s and 1940s by more sophisticated research instruments and techniques. Even today, some of the more complex behavior of the boundary layer is explained only by unconfirmed theory.2Applications of boundary-layer research are as diverse as the circumstances of fluid flow itself. Prandtl was studying the use of a jet of air to blow away sweepings in a factory. Others looked into the flow of fluids in pipes. Many turned their attention to the infant technology of flight, seeking to improve the flow of air over wings.The flying qualities of wings can be enhanced in two ways, and boundary-layer control can help in both. The first is to decrease drag; the second is to increase lift. The most desirable way to decrease drag is to maintain laminar flow within the boundary layer and prevent a transition to turbulent flow. Laminar flow occurs when successive layers of air within the boundary layer slide smoothly over one another, from the stationary film at the surface up to the free-stream velocity of the outside air. Turbulent flow within the boundary layer occurs when these "streamlines break up and a fluid element moves in a random, irregular, and tortuous fashion," as when the smoke rising from a cigarette in a still room ceases to travel smoothly up but tumbles instead in eddies and curls. Over a normal wing, the boundary layer remains laminar over only a small portion of the wing chord before breaking up into turbulent flow. The area of turbulent flow experiences significantly greater skin-friction drag than the laminar flow.3  Transition from laminar to turbulent flow can be seen occurring down the length of this missile-body model captured by shadowgraph in high-speed flow. (ARC) The second way to improve the flying qualities of a wing through boundary-layer control is to increase the lift, especially the maximum lift, of the wing. Maximum lift can be increased by delaying the onset of separation of the boundary layer. As a wing's angle of incidence increases-as its leading edge is tipped up above the plane of flow of the.... [531] This smoke-flow visualization of the same wing at differing angles (6°, 12°, 14° top to bottom) of incidence reveals how tipping a wing above the plane of flow can bring on separation of the boundary layer, and stalling. (LaRC) [532] ....free-stream air-its lift also increases, up to a point. Finally, however, the boundary layer on the upper surface breaks free of the wing altogether, reducing lift drastically. This is known as stalling. If the boundary layer can be kept from separating, the maximum lift of the aircraft can be increased, an important consideration in increasing takeoff-weight capacity and reducing landing speed. Furthermore, the same energizing of the boundary layer that delays separation can also help to maintain the boundary layer in fast laminar flow, increasing total lift even at low angles of incidence.  Separation of the boundary layer. (NASA TN-1384, 1947) | |
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