Relevance of quadrupolar sound diffraction on flow-induced noise from porous-coated cylinders

R. Zamponi*, F. Avallone, D. Ragni, C. Schram, S. van der Zwaag

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review


This paper elucidates the link between the near-wake development of a circular cylinder coated with a porous material in a low Mach-number flow and the related aerodynamic sound attenuation. It accomplishes this by formulating the cylinder flow-induced noise as a diffraction problem. The necessity for such an approach is driven by experimental evidence obtained through acoustic beamforming and particle-image-velocimetry measurements, which reveal that the dominant noise sources for a coated cylinder are not localised on the body surface but rather in the wake, specifically at the outbreak position of the shedding instability. The acoustic field at the vortex-shedding frequency can be hence modelled by considering a compact lateral quadrupole at this location and employing an exact Green's function tailored to a cylindrical geometry. Because of the diffraction of the sound waves radiated by the quadrupolar source on the cylinder surface, the resulting far-field directivity pattern resembles that of a dipole. The study demonstrates that the porous coating has the two-fold effect of decreasing the strength of the point quadrupole in the wake and moving its origin further downstream, reducing, in turn, the efficiency of the sound scattering. Consequently, the diffracted part of the acoustic field, which dominates the far-field noise for a bare cylinder in accordance with classical theory, provides a contribution that is comparable to the direct part. The results eventually indicate that quadrupolar sources must be considered to accurately predict the noise radiated from a porous-coated cylinder, even at low Mach numbers.

Original languageEnglish
Article number118430
Number of pages28
JournalJournal of Sound and Vibration
Publication statusPublished - 2024

Bibliographical note

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project
Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.


  • Aeroacoustics
  • Coated cylinder
  • Noise control
  • Porous materials
  • Sound diffraction
  • Vortex shedding


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