Computational Aeroacoustics
Computational aeroacoustics (CAA) deals with the prediction of an aerodynamic noise source and its propagation numerically with the help of time-dependent equations. Some of the great books in this field are the 1976 book by Goldstein [3] and a recent boo
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Computational Aeroacoustics
Computational aeroacoustics (CAA) deals with the prediction of an aerodynamic noise source and its propagation numerically with the help of time-dependent equations. Some of the great books in this field are the 1976 book by Goldstein [3] and a recent book by Wagner et al. [12]. After identification of the sound sources, the noise generated in the field must be transported outside to an observer. Computational fluid dynamics (CFD) was designed generally to solve a near-field problem because the perturbations from the mean flow vanish quickly. Furthermore, the flow in this region is usually highly nonlinear but basically stationary, or at least the changes are slow. On the other hand, acoustics is clearly a far-field problem, in which the sound is generated locally in the aerodynamic area and passively radiated outside to an observer with a smaller exponent of decrease with radial distance. Otherwise the area where the sound is generated is aerodynamically active, the perturbations are small, and linear descriptions are usually sufficient. However, noise due to turbulence is inherently unsteady quite comparable to turbulent eddies even if spatial wavelengths are large compared with aerodynamic ones by an order of the reciprocal of the Mach number. Aerodynamic noise occurs basically because of three different phenomena. The first one is due to a fluctuating mass flow rate, as the noise that can happen from sirens or in exploding burning droplets in a combustion chamber (combustion noise) as a result of the chemical reactions and the subsequent introduction of energy into the flow (entropy noise). The second one is deterministic impulsive noise, which is a result of moving surfaces in nonuniform flow conditions. The displacement effect of an immersed body in motion and the nonstationary aerodynamic loads on the body’s surface, generate loads on the bodys’ surface creating pressure fluctuations that are radiated as sound. This type of noise is relatively easy to extract from aerodynamic simulations because the required resolution in space and time to predict the acoustics is similar to the demands from aerodynamic computations. Aerodynamic noise arises primarily from rotating systems (e.g., helicopter rotors, wind turbines, cooling fans, and ventilators). If the surface moves at speeds comparable to the speed of T. Bose, Aerodynamic Noise: An Introduction for Physicists and Engineers, Springer Aerospace Technology 7, DOI 10.1007/978-1-4614-5019-1 6, © Springer Science+Business Media New York 2013
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sound or there is an interaction between a rotor and a stator wake, then the tonal engine components can be dominant [12]. Because the frequency of interaction between a rotor and a stator can be determined by the product of the number of blades and the revolutions per minute, analysis of the noise level is an easy way of determining problems in a particular blade row. The third noise mechanism is the result of turbulence and therefore arises in nearly every
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