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Unformatted text preview: Chapter 7 External Forced Convection Chapter 7 EXTERNAL FORCED CONVECTION Drag Force and Heat Transfer in External Flow 71C The velocity of the fluid relative to the immersed solid body sufficiently far away from a body is called the freestream velocity , V . The upstream (or approach ) velocity V is the velocity of the approaching fluid far ahead of the body. These two velocities are equal if the flow is uniform and the body is small relative to the scale of the freestream flow. 72C A body is said to be streamlined if a conscious effort is made to align its shape with the anticipated streamlines in the flow. Otherwise, a body tends to block the flow, and is said to be blunt . A tennis ball is a blunt body (unless the velocity is very low and we have creeping flow). 73C The force a flowing fluid exerts on a body in the flow direction is called drag . Drag is caused by friction between the fluid and the solid surface, and the pressure difference between the front and back of the body. We try to minimize drag in order to reduce fuel consumption in vehicles, improve safety and durability of structures subjected to high winds, and to reduce noise and vibration. 74C The force a flowing fluid exerts on a body in the normal direction to flow that tend to move the body in that direction is called lift . It is caused by the components of the pressure and wall shear forces in the normal direction to flow. The wall shear also contributes to lift (unless the body is very slim), but its contribution is usually small. 75C When the drag force F D , the upstream velocity V , and the fluid density are measured during flow over a body, the drag coefficient can be determined from A F C D D 2 2 1 V = where A is ordinarily the frontal area (the area projected on a plane normal to the direction of flow) of the body. 76C The frontal area of a body is the area seen by a person when looking from upstream. The frontal area is appropriate to use in drag and lift calculations for blunt bodies such as cars, cylinders, and spheres. 77C The part of drag that is due directly to wall shear stress w is called the skin friction drag F D , friction since it is caused by frictional effects, and the part that is due directly to pressure P and depends strongly on the shape of the body is called the pressure drag F D , pressure . For slender bodies such as airfoils, the friction drag is usually more significant. 78C The friction drag coefficient is independent of surface roughness in laminar flow , but is a strong function of surface roughness in turbulent flow due to surface roughness elements protruding further into the highly viscous laminar sublayer. 79C As a result of streamlining, ( a ) friction drag increases, ( b ) pressure drag decreases, and ( c ) total drag decreases at high Reynolds numbers (the general case), but increases at very low Reynolds numbers since the friction drag dominates at low Reynolds numbers....
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 Spring '08
 BENARD
 Heat Transfer

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