Aerodynamic noise reducing techniques by using pile-fabrics

Nishimura, Masafumi; Kudo, Toshifumi; Nishioka, Masato

doi:10.2514/6.1999-1847

Cited by 26 publications

(18 citation statements)

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“…Apart from the described increases in flow noise, the fur is responsible for the complete suppression of the first and second harmonics of the aeolian tone. This is in good agreement with the results of Nishimura et al [16], who performed measurements on cylinders covered by pile fabrics and found a noticeable decrease of the vortex shedding peak. The aft body modification also helps with suppressing the harmonics of the vortex shedding noise.…”

Section: Results Of the Acoustic Measurementssupporting

confidence: 93%

“…In general, solid structures covered with hairs or fibres, such as fluffy bodies, are known to affect the generation of flow induced noise [16,17]. The present investigation intends to describe the consequence of the presence of hairy flaps on the noise generated by cylinders in a flow, especially regarding the spectral modification and possible acoustic damping impact of such flaps.…”

Section: Introductionmentioning

confidence: 99%

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Vortex shedding noise of a cylinder with hairy flaps

Kamps

Geyer

Sarradj

et al. 2017

Journal of Sound and Vibration

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This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractThis study describes the modification of acoustic noise emitted from cylinders in a stationary subsonic flow for a cylinder equipped with flexible hairy flaps at the aft part as a passive way to manipulate the flow and acoustics. The study was motivated by the results from previous water tunnel measurements, which demonstrated that hairy flaps can modify the shedding cycle behind the cylinder and can reduce the wake deficit. In the present study, wind tunnel experiments were conducted on such a modified cylinder and the results were compared to the reference case of a plain cylinder. The acoustic spectrum was measured using two microphones while simultaneously recording the flap motion. To further examine the flow structures in the downstream vicinity of the cylinder, constant temperature anemometry measurements as well as flow visualizations were also performed. The results show that, above a certain Reynolds number, the hairy flaps lead to a jump in the vortex shedding frequency. This phenomenon is similarly observed in the water flow experiments as a jump in the non-dimensional Strouhal number that is related to the change of the shedding cycle. This jump appears to be coupled to a resonant excitation of the flaps and a considerable increase of the flapping amplitude. The specific Reynolds number at which the jump Email addresses: Laura.Kamps@imfd.tu-freiberg.de (Laura Kamps), Thomas.Geyer@b-tu.de (Thomas F. Geyer) Preprint submitted to Journal of Sound and Vibration October 11, 2016 occurs is higher in the present case, which is attributed to the lower added mass in air as compared with the one in water. The flow visualizations confirmed that such action of the flaps lead to a more slender elongated shape of the time-averaged separation bubble. In addition, the hairy flaps induce a noticeable reduction of the tonal noise as well as broadband noise as long as the flaps do not touch each other.

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Section: Results Of the Acoustic Measurementssupporting

confidence: 93%

Section: Introductionmentioning

confidence: 99%

Vortex shedding noise of a cylinder with hairy flaps

Kamps

Geyer

Sarradj

et al. 2017

Journal of Sound and Vibration

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“…Although the modification of structural shapes is effective in reducing aerodynamic noise, such measures are limited to a certain extent due to the various functions that must be maintained. The reduction of such noise has been achieved by covering the surface of objects with fur materials [4], [5] -a technique based on the reduction of vorticity. However, it is difficult with the application of fur materials to satisfy the requirements of durability against weathering in railway vehicles.…”

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confidence: 99%

Aerodynamic Noise Reduction using Porous Materials and their Application to High-speed Pantographs

2009

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“…In the system, ambient air is taken in as an entrainment flow, and the surplus flow is released to the ambient air through an outlet. The pile-fabric material is a furlike material with fine fibers of about 10mm long applied on the inner surface of the collector to reduce aerodynamic noise (10)(11). The acoustic opening is an original countermeasure developed in this investigation.…”

Section: Results Of Investigationmentioning

confidence: 99%

“…For example, the diffuser duct downstream of the fan was reshaped with a cross-section area that increased more gradually than that in the original duct. Furthermore, the entrainment system (10) was applied to smooth the flow conducted by the collector, and a pile-fabric material (10)(11) was set on the inner surface of the collector to reduce the pressure fluctuations.…”

Section: Swaying Componentmentioning

confidence: 99%

Techniques for Reducing Low-Frequency Fluctuations in Aeroacoustic Wind Tunnels

Kudo

Komatsu

Maeda

et al. 2009

JEE

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Aeroacoustic wind tunnels, which consist of large measuring sections, such as anechoic rooms, and open jets, are generally applied as test facilities to measure aerodynamic noise generated around vehicles. In aeroacoustic wind tunnels, it is easy to generate flow-induced low-frequency fluctuations resulting in pressure fluctuations or velocity fluctuations associated with vortices or turbulence in the open jet. These low-frequency fluctuations are generated by various flow-induced phenomena, such as swaying that occurs in various elements of the circuit, fluctuations propagated from the shear layers or mixing layers of the open jet, and the acoustic resonances of the whole circuit, which often become serious problems, generally called pulsations, at particular wind speeds. Because these low-frequency fluctuations may affect the measured pressure and velocity of a flow around vehicles in measuring sections, we should aim to reduce or control them when designing aeroacoustic wind tunnels to ensure measurement with high reliability. In this study, we investigate the generating mechanisms and techniques for reducing these low-frequency fluctuations in aeroacoustic wind tunnels by performing experiments on a model wind tunnel. In particular, new techniques for reducing acoustic resonance components such as by installing an acoustic opening in the circuit are proposed and verified by simulations and experiments. Using these techniques for reducing low-frequency fluctuations, the fluctuations of pressure and velocity in the measuring section are markedly decreased over the entire wind speed range.

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Aerodynamic noise reducing techniques by using pile-fabrics

Cited by 26 publications

References 0 publications

Vortex shedding noise of a cylinder with hairy flaps

Vortex shedding noise of a cylinder with hairy flaps

Aerodynamic Noise Reduction using Porous Materials and their Application to High-speed Pantographs

Techniques for Reducing Low-Frequency Fluctuations in Aeroacoustic Wind Tunnels

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