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ISBN 9783843936798

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978-3-8439-3679-8, Reihe Ingenieurwissenschaften

Maike Werner
Experimental Study on Tonal Self-Noise Generation by Aeroacoustic Feedback on a Side Mirror

145 Seiten, Dissertation Universität Stuttgart (2017), Softcover, A5

Zusammenfassung / Abstract

Car manufacturers increasingly focus on the reduction of the sound level inside the passenger compartment to enhance acoustic comfort. Thereby, special focus is on the elimination of disturbing tonal components emanating from the side mirrors. In this context, the present work is devoted to a conceptual side mirror model which features tonal self-noise emission that is not related to typical noise sources. Key points are the identification of the tonal source mechanism and the development of tonal amplitude reducing measures. To this end, the tonal noise emission of the side mirror and the associated flow field are investigated experimentally in a wind tunnel. The examined flow field and the tonal noise emission indicate the existence of an aeroacoustic feedback mechanism comprising convectively amplified boundary layer instability waves, their scattering to acoustic waves at the trailing edge and the feedback of the acoustics into the upstream boundary layer via the receptivity process. A theoretical model of the feedback loop concept is presented which allows for amplitude considerations and the determination of feedback loop resonance frequencies. Fundamental evidence of the feedback mechanism is found in the characteristics of the tonal noise emission, where the tones emanate from the trailing edge and form a distinctive “ladder” structure with increasing freestream velocity. Moreover, the flow field exhibits a region of laminar boundary layer separation upstream of the trailing edge comprising massively amplified shear layer instability modes at the acoustic peak frequencies. Conclusive evidence of the feedback loop is provided by the very good agreement of the resonance frequencies associated with the theoretical feedback loop model and the tonal modes observed in the measurement. Based on the identified source mechanism, finally, approaches for the reduction of the tonal amplitudes are discussed. Two methods aiming at lowering the net amplification within the loop are tested. Both variants are found to be successful in eliminating the tones from the spectra. Overall, the present investigation gives a detailed experimental and theoretical description of the complex aeroacoustic behavior of a side mirror. The understanding of the underlying physical mechanism enables the avoidance of tonal noise emission from future side mirror designs.