Research Papers

Far-Field Radiation of Tip Aerodynamic Sound Sources in Axial Fans Fitted With Passive Noise Control Features

[+] Author and Article Information
Stefano Bianchi, Alessandro Corsini, Franco Rispoli

Dipartimento di Ingegneria Meccanica e Aerospaziale Sapienza, University of Rome, Via Eudossiana 18, I-00184 Rome, Italy

Anthony G. Sheard

 Fläkt Woods Limited, Axial Way, Colchester CO4 5AR, UK

J. Vib. Acoust 133(5), 051001 (Jul 26, 2011) (11 pages) doi:10.1115/1.4003931 History: Received October 06, 2009; Revised January 19, 2011; Published July 26, 2011; Online July 26, 2011

This study investigates the causal relationship between flow aerodynamics and radiated noise in low-speed axial fans. Using blades with three distinctive tip configurations, including two that were developed with a view to reducing noise emissions, the pressure fluctuations of the exhaust flow in the near field are correlated with the noise measured in the far field in an anechoic chamber. By varying the far-field microphone’s azimuthal position, the study investigates the source signatures and directivity of noise sources distributed along the blade span. Several distinctive features in the noise directivity pattern are identified and correlated with the noise sources of aerodynamic origin dissected along the blade span. Utilizing the directional far-field autospectra of the three blade configurations in combination with the near-field/far-field cross-spectra, the emission characteristics of the aerodynamic sources are analyzed and their roles with respect to the overall acoustic signature of the fan are discussed. It is apparent that the ability to decompose the output of the aerodynamic noise sources in the near field is a useful tool in designing fans to achieve desirable low-noise targets. The results confirm that the tip-flow appendages influence the noise radiation pattern for the investigated family of fans. These phenomena are linked with the control of aerodynamic noise sources related to the tip-leakage vortex and the hub corner separation.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Test fan rotor blades and tip end-plates (not to scale) (19)

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Figure 2

Chordwise evolution of tip-leakage vortex skewing angle βLV and rotation number Ro

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Figure 3

Static pressure rise and ηstat characteristic curves (dashed lines: data fan; solid lines: TF fan; line-symbols: TFvte)

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Figure 4

Lw(A) (bottom lines, left axis) and Ks (top lines, right axis) characteristic curves

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Figure 5

Sketch of the test-rig setup (not to scale)

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Figure 6

Directivity of the integrated Lp for the different geometries for data (dashed-line), TF (solid line), and TFvte (line-symbols)

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Figure 7

Directivity map of the Lp spectral for the different geometries for (a) data, (b) TF, and (c) TFvte

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Figure 8

Lp autospectra for the different geometries; data (dashed-line), TF (solid line), TFvte (line-symbols), at 0 deg (right axis) and 90 deg (left axis)

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Figure 9

Directivity of the Lp for the different geometries at (a) BPF, (b) second BPF, (c) third BPF, and (d) fourth BPF for data (dashed-line), TF (solid line), and TFvte (line-symbols)

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Figure 10

Directivity of the Lp at frequency greater then 1 kHz for data (dashed-line), TF (solid), and TFvte (line-symbols)

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Figure 11

Lp spanwise cross-spectrum between near and far fields: data (dashed-line), TF (solid), and TFvte (line-symbols). Lp integrated over the range (a) 50 Hz–10 kHz and (b) 1–10 kHz.

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Figure 12

Lp spanwise cross-spectrum between near and far fields: data (dashed-line), TF (solid), and TFvte (line-symbols). (a) BPF, (b) fifth and sixth BPF, and (c) seventh and eighth BPF harmonics

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Figure 13

κp spanwise distributions: data (dashed-line), TF (solid), and TFvte (line-symbols). (a) 30 deg and (b) 90 deg off the axis




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