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NIA Seminar by Alex Povitsky |
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Date: March 29, 2006
Time: 11:00am
Location: NIA, Rm 137
Additional Information: Presentation (PDF)
Sound Generation by Street of Vortices Impinging into Lifting Bodies
Alex Povitsky, University of Akron
Although the general theme of vortex-body interaction has been under scientific scrutiny for many years especially for point-wise vortices and/or compact rigid bodies, much less is known about the noise generated by non-compact vortices, that is, the size of vortex and body are of the same order and neither of them is compact relative to the acoustic wavelength. The role that the vortex strength, vortex core size, vortex street frequency, distance between vortex streets, the Mach number of mean flow, and the vortex profile play on the sound radiation of lifting bodies are examined in order to obtain simple scaling laws where it is possible.
Propagation of acoustic waves originating from periodic vortices deforming in the non-uniform flow about a rigid body is studied numerically using high-order compact finite difference approximation. The goal of this study is to determine the sound directivity and sound strength as a result of vortex street interaction with a solid body in presence of subsonic mean flow.
When low-frequency vortex streets interact with cylinder, they produce sound waves that are quite different from those originated from high-frequency vortex streets. The interaction mechanism and sound generation and propagation in a non-uniform flow are quite different for the Taylor vortex street and the Vatistas vortex street. For the low-frequency Vatistas vortex street, the RMS of acoustic pressure has well-defined sound directivity and sound amplitude where the sound directivity is greatly affected by the Mach number of mean flow. For the high frequency Vatistas vortex street, the RMS of acoustic pressure becomes highly non-monotonic in the angular direction while the Mach number of mean flow has a moderate effect on the RMS angular profile.
The lifting flow about cylinder not only amplifies the sound strength but also shifts the sound directivity. The wave originated by impingement of Vatistas vortex is much more sensitive to the non-zero circulation of the mean flow compare to that of Taylor vortex. For vortex deforming in the flow around two cylinders, tandem and transverse set-ups of cylinders were investigated. It was found that the vortex trapped between transverse cylinders radiated much stronger noise than the vortex trapped between tandem cylinders. The directivity of sound and maximum of acoustic pressure at cylinder surfaces were found for both cases. The striking differences in sound amplitude and directivity for Taylor and Vatistas vortices are discussed in terms of their vorticity distribution.
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