CH.3 The Boundary Layer and Its Control

2. CH.3 The Boundary Layer and Its Control In this chapter we shall show how a knowledge of the behavior of the boundary layer can enable us to improve the lift, drag and general handling characteristics of aircraft.

3. 1./ A Major Breakthrough Prandtl was born in Freising, Germany, in 1875. He studied mechanical engineering in Munich. For his doctoral thesis he worked on a problem on elasticity under August Foppl, who himself did pioneering work in bringing together applied and theoretical mechanics. Later Prandtl became Foppl's son-in-law, following the good German academic tradition in those days. In 1901 he became professor of mechanics at the University of Hanover, where he continued his earlier efforts to provide a sound theoretical basis for fluid mechanics. The famous mathematician Felix Klein, who stressed the use of mathematics in

4. engineering education, became interested in Prandtl and enticed(부추기다) him to come to the University of Gottingen. He served as a professor of applied mechanics at Gottingen from 1904 to1953; the quiet university town of Gottingen became an international center of aerodynamic research.. In 1904 Prandtl conceived the idea of boundary layer, which adjoins the surface of a body moving through a fluid. It is perhaps the greatest single discovery in the history of fluid mechanics. He showed that frictional effects in a slightly viscous fluid are confined to a thin layer near the surface of the body; the rest of the flow can be considered inviscid. The idea led to a rational way of simplifying the equations of motions in the different regions of the flow field. Since then the boundary layer technique has been generalized and has become a most useful tool in many branches of science. He made notable innovations in the design of wind tunnels and other aerodynamics equipment. His advocacy of monoplanes greatly advanced the heavier-than-air aviation. In experimental fluid mechanics he designed the Pitot-static tube for measuring velocity. In turbulence theory he contributed the mixing length theory. Toward to end of his career Prandtl became interested in dynamic meteorology and published a paper generalizing the Ekman spiral for turbulent flows.

5. Prandtl was endowed with rare vision for understanding physical phenomena. His mastery(숙달, 전문적지식) of mathematical tricks was limited; indeed many of his collaborators were better mathematician. However, Prandtl had an unusual ability in putting ideas in simple mathematical forms. In 1948 Prandtl published a simple and popular textbook on fluid mechanics, which has been referred to in several place here. His varied interest and simplicity of analysis is evident throughout this book. Prandtl died in Gottingen in 1953.

6. 2./ Boundary Layer

10. 3./ How the Boundary Layer Form 3./ How the Boundary Layer Form

15. Drag on flat plate is solely due to friction created by laminar, transitional, and turbulent boundary layers.

16. 4./ Surface Friction Drag 4./ Surface Friction Drag

17. 5./ Flow Separation and Stalling

22. 6./ Favorable and Unfavorable Conditions * favorable pressure gradient : * unfavorable pressure gradient : where ; main flow direction 7./ Leading-Edge Separation * For air to travel around a curve, the pressure on the outside of the curve must be greater than on the inside, in order to provide the necessary ‘cornering’(centripetal) force. * Aerofoil with a large radius leading edge are less prone to producing leading-edge separation, and therefore tend to have a more progressive and safer stall characteristics.

23. 8./ Reattachment

24. 9./ Generation of Lift and the Formation of the Starting Vortex It is viscosity, working through the mechanism of boundary layer separation and starting vortex formation, that is ultimately responsible for the generation of lift.

26. 10./ Controlling the Type of Boundary Layer C.F. : laminar boundary layer airfoil

28. 11./ Boundary Layer Control - Preventing Unwanted Flow Separation small tooth-like vortex generators

29. A Gloster Javelin showing the three sets of vortex generators located along the outer portion of the wing Three sets of vortex generators are used along the Javelin's outer wing with one set located near the leading edge, another just before the ailerons, and a third set in between. The generators on both planes serve to break up the shocks formed at transonic speeds thereby delaying the effects of separation. The generators located just ahead of the ailerons on the Javelin wing also help improve the effectiveness of these control surfaces at low speed or high angle of attack, as discussed in the next example.

30. 12./ Boundary Layer and Stalling Problems on Swept Wings 와류생성판

37. 13. Trailing Edge Flap

39. treble ; 3단의

43. 수그러짐

45. slab=평판

46. Lower-surface view of triple-slotted flap on Boeing 737 airplane

47. Upper-surface view showing triple-slotted flap and spoilers on Boeing 737 airplane

48. 14./ Leading Edge Devices

50. Lower-surface view of leading-edge slat on Boeing 737 airplane