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Circular cylinders with soft porous cover for flow noise reduction

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Abstract

The use of porous materials is one of several approaches to passively control or minimize the generation of flow noise. In order to investigate the possible reduction of noise from struts and other protruding parts (for example components of the landing gear or pantographs), acoustic measurements were taken in a small aeroacoustic wind tunnel on a set of circular cylinders with a soft porous cover. The aim of this study was to identify those materials that result in the best noise reduction, which refers to both tonal noise and broadband noise. The porous covers were characterized by their air flow resistivity, a parameter describing the permeability of an open-porous material. The results show that materials with low air flow resistivities lead to a noticeable flow noise reduction. Thereby, the main effect of the porous cylinder covers is that the spectral peak of the aeolian tone due to vortex shedding appears much narrower, but is not suppressed completely. Based on the measurement results, a basic model for the estimation of the total peak level of the aeolian tone was derived. In addition to the minimization of the vortex shedding noise, a reduction of broadband noise can be observed, especially at higher Reynolds numbers. The noise reduction increases with decreasing air flow resistivity of the porous covers, which means that materials that are highly permeable to air result in the best noise reduction.

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Notes

  1. In order to simplify the present case, it was assumed that the porous cylinders lead to the same blockage as a non-porous cylinder.

  2. Sometimes the tortuosity \(\tau\) is defined as the squared ratio of the effective length of the flow path through the pores to the minimum length between flow inlet and outlet.

  3. A similar parameter is known for the non-dimensional description of the lining of absorptive mufflers, where instead of a diameter the thickness of the lining is used (Mechel 2008).

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Acknowledgments

The authors thank Martin Noack for his help with the preparation of the cylinders and the measurements.

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Authors and Affiliations

  1. Brandenburg University of Technology Cottbus - Senftenberg, Cottbus, Germany

    Thomas F. Geyer & Ennes Sarradj

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  1. Thomas F. Geyer
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  2. Ennes Sarradj
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Correspondence to Thomas F. Geyer.

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Geyer, T.F., Sarradj, E. Circular cylinders with soft porous cover for flow noise reduction. Exp Fluids 57, 30 (2016). https://doi.org/10.1007/s00348-016-2119-7

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  • DOI: https://doi.org/10.1007/s00348-016-2119-7

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