TAILIEUCHUNG - Ebook Introduction to fluid mechanics: Part 2
(BQ) Part 2 book "Introduction to fluid mechanics" has contents: Drag and lift, dimensional analysis and law of similarity, measurement of flow velocity and flow rate, measurement of flow velocity and flow rate, flow of a compressible fluid, unsteady flow, computational fluid dynamics, computational fluid dynamics. | Drag and lift In Chapters 7 and 8 our study concerned ‘internal flow’ enclosed by solid walls. Now, how shall we consider such cases as the Aight of a baseball or golf ball, the movement of an automobile or when an aircraft flies in the air, or where a submarine moves under the water? Here, flows outside such solid walls, . ‘external flows’, are discussed. Generally speaking, flow around a body placed in a uniform flow develops a thin layer along the body surface with largely changing velocity, Le. the boundary layer, due to the viscosity of the fluid. Furthermore, the flow separates behind the body, discharging a wake with eddies. Figure shows the flows around a cylinder and a flat plate. The flow from an upstream point a is stopped at point b on the body surface with its velocity decreasing to zero; b is called a stagnation point. The flow divides into the upper and lower flows at point b. For a cylinder, the flow separates at point c producing a wake with eddies. Let the pressure upstream at a, which is not affected by the body, be pbo, the flow velocity be U and the pressure at the stagnation point be p o . Then P u2 Po=Poa+T Fig. Flow around a body () The drag of a body 149 Whenever a body is placed in a flow, the body is subject to a force from the surrounding fluid. When a flat plate is placed in the flow direction, it is only subject to a force in the downstream direction. A wing, however, is subject to the force R inclined to the flow as shown in Fig. . In general, the force R acting on a body is resolved into a component D in the flow direction U and the component L in a direction normal to U. The former is called drag and the latter lift. Drag and lift develop in the following manner. In Fig. , let the pressure of fluid acting on a given minute area dA on the body surface be p, and the friction force per unit area be z. The force pdA due to the pressure p acts normal to dA, while the force due to the friction stress z acts .
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