Aero-acoustic features of internal and external chamfered Hartmann whistles: A comparative study

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The efficient way of chamfering at the mouth of Hartmann whistles in generating higher acoustic emission levels are experimentally demonstrated in this paper. The relevant parameters of the present work comprise internal and external-chamfer angles (15, 30), cavity-length, nozzle-to-cavity-distance and jet pressure ratios. The frequency and amplitude characteristics of internal and external, chamfered-Hartmann whistles are compared in detail to ascertain the role of chamfering in enhancing acoustic radiations. The high frequencies possessed by the internal chamfered whistles as compared to the external ones indicate that it amplifies the resonance. It is observed that the internal chamfered whistles exhibit higher directivity than the external chamfered ones. Further, it is noticed that the acoustic-power and efficiency are also higher for the internal chamfered whistles. The shadowgraph sequences reveal the variance in flow-shock oscillations as well as the spill-over features at the mouth of internal and external, chamfered cavities. The presence of large mass flow as well as its subsequent increase of spill-over as a result of enlarged mouth in internal chamfered whistles, leads to the generation of high intensity acoustic radiation than the external chamfered ones. Thus, the internal chamfer proves to be the best passive control device for augmented sound pressure levels and acoustic efficiencies in resonance cavities. © 2013 Elsevier Ltd.