Lokalisation strömungsinduzierter Schallquellen aus numerisch berechneten Oberflächendruckfluktuationen

A fundamental challenge of aeroacoustics is the reduction of flow-induced noise. For low Mach number flow, it is well known that the dominant sound is caused by fluctuating surface pressure. Thus, the localization of surface sources is an important task, for which beamforming techniques are used. Their application allow a source reconstruction based on the sound pressure recorded in the farfield. Unfortunately, the obtained results are usually constrained by artificial components and a poor resolution, especially for low frequencies, can be observed. For this reason, the present report presents the development of an alternative approach for the localization of flow-induced surface sound sources. CAA simulations can be used to determine not only acoustic farfield quantities but also the sound-generating fluctuating surface pressure. Since this represents the underlying source mechanism in the area of sound generation, it is chosen as the underlying quantity of the approach being developed. Based on the example of an SD7003-airfoil, various source hypotheses are formulated and tested for their suitability. A comparison with known beamforming results serves as a reference. First, it is shown that the surface pressure itself does not allow a sound source localization, since it is characterized by a dominant hydrodynamic component, which is mostly not capable of propagation. A similar picture is shown by selected aeroacoustic source terms derived from flow quantities. This also refers to the extracted acoustic component of the surface pressure, which is obtained from a filter approach developed especially for this purpose. Finally, by utilizing the mirror principle, the acoustically radiating component of the surface pressure is determined, which represents a clear image of sound radiation. Based on this result, a further development of the process is carried out, which makes exactly those hydrodynamic fluctuations of the flow visible, that are responsible for the sound generation. In addition, a spatial visualization of the radiated sound field is performed, which allows to identify propagation effects as well as the directivity of the radiating body. From the entirety of the quantities found, a comprehensive picture of the flow-induced sound generation is obtained, showing not only the radiation but also the originating locations and the underlying source mechanisms. Generally, results similar to beamforming are found, but with improved resolution, especially remarkable for low frequencies. Since the approach is based on a simple evaluation of the Helmholtz-Kirchhoff integral, it has the complexity of a conventional sound propagation computation.