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Flow Around Immersed Bodies – Drag and Lift

الكلية كلية الهندسة     القسم  الهندسة البيئية     المرحلة 3
أستاذ المادة عدي عدنان جهاد الخيكاني       05/10/2012 11:34:53
Flow Around Immersed Bodies – Drag and Lift
When fluid flows around a body or the body moves in a fluid there is a relative motion between the fluid and the body. The body will experience a force in such a situation. In the case of a flat plate positioned parallel to the direction of the flow, the force is parallel to the surface.
But generally in the case of blunt bodies, the force will neither be parallel nor perpendicular to the surface. The force can be resolved into two components one parallel to the flow and the other perpendicular to the flow. The former may be called shear force and the other, the pressure force. The component parallel to the direction of motion is called drag force FD and the component perpendicular to the direction of motion is called lift force, FL. Determination of these forces is very important in many applications, an obvious example being aircraft wings. Simple analytical methods are found to be insufficient for the determination of such forces. So experimentally measured coefficients are used to compute drag and lift.


Drag Force and Coefficient of Drag
Drag is the component of force acting parallel to the direction of motion. Using the method of dimensional analysis the drag force can be related to flow Reynolds number by

FD/?AV^2= f (Re) ……………….(10-31)
For generality velocity is indicated as V Defining coefficient of drag as the ratio of drag to dynamic pressure, it is seen that

CD = f (Re) …………….(10-32)
CD =FD/A?V ?^2(1/ 2) ?

This applies to viscous drag only. In case wave drag is encountered, then
CD = f (Re, Fr) ………..(10.3.3)
If compressibility effect is to be considered
CD = f (Re, M) …………. (10.3.4)
Friction coefficient over flat plate in laminar flow, at a location was defined by
Cfx = ?w /(1/2) ? A V^2 = 0.664/Rex


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