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(BQ) Part 2 book "Aerodynamics for engineering students" has contents: Compressible flow, viscous flow and boundary layers, flow control and wing design, propellers and propulsion. | Compressible flow 274 Aerodynamics for Engineering Students 6.1 Introduction In previous chapters the study of aerodynamics has been almost exclusively restricted to incompressible flow. This theoretical model is really only suitable for the aerodynamics of low-speed flight and similar applications. For incompressible flow the air density and temperature are assumed to be invariant throughout the flow field. But as flight speeds rise, greater pressure changes are generated, leading to the compression of fluid elements, causing in turn a rise in internal energy and, in consequence, temperature. The resulting variation of these flow variables throughout the flow field makes the results obtained from incompressible flow theory less and less accurate as flow speeds rise. For example, in Section 2.3.4 we showed how use of the incompressibility assumption led to errors in estimating the stagnation-pressure coefficient of 2% at M = 0.3, rising to 6% at M = 0.5, and 28% at M = 1. But these errors in estimating pressures and other flow variables are not the most important disadvantage of using the incompressible flow model. Far more significant is the marked qualitative changes to the flow field that take place when the local flow speed exceeds the speed of sound. The formation of shock waves is a particularly important phenomenon and is a consequence of the propagation of sound through the air. In incompressible flow the fluid elements are not permitted to change in volume as they pass through the flow field. And, since sound waves propagate by alternately compressing and expanding the medium (see Section 1.2.7), this is tantamount to assuming an infinite speed of sound. This has important consequences when a body like a wing moves through the air otherwise at rest (or, equivalently, a uniform flow of air approaches the body). The presence of the body is signalled by sound waves propagating in all directions. If the speed of sound is infinite the presence of the body is
