Friction and Pressure Drag: Bluff Bodies
Both friction and pressure forces contribute to the drag of bluff bodies (see Shapiro, 1960, for a good
discussion of the mechanisms of drag). As an example, consider the drag coef?cient for a smooth sphere
shown in Figure 3.6.5. Transition from laminar to turbulent ?ow in the boundary layers on the forward
portion of the sphere causes a dramatic dip in drag coef?cient at the critical Reynolds number (ReD »
2 ´ 105). The turbulent boundary layer is better able to resist the adverse pressure gradient on the rear
of the sphere, so separation is delayed and the wake is smaller, causing less pressure drag.
Airfoils
Airfoils are shaped to produce lift ef?ciently by accelerating ?ow over the upper surface to produce a
low-pressure region. Because the ?ow must again decelerate, inevitably there must be a region of adverse
pressure gradient near the rear of the upper surface (pressure distributions are shown clearly in Hazen,
1965).
Lift and drag coef?cients for airfoil sections depend on Reynolds number and angle of attack between
the chord line and the undisturbed ?ow direction. The chord line is the straight line joining the leading
and trailing edges of the airfoil (Abbott and von Doenhoff, 1959).
As the angle of attack is increased, the minimum pressure point moves forward on the upper surface
and the minimum pressure becomes lower. This increases the adverse pressure gradient. At some angle
of attack, the adverse pressure gradient is strong enough to cause the boundary layer to separate
completely from the upper surface, causing the airfoil to stall. The separated ?ow alters the pressure
distribution, reducing lift sharply.
Increasing the angle of attack also causes the the drag coef?cient to increase. At some angle of attack
below stall the ratio of lift to drag, the lift?drag ratio, reaches a maximum value.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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