Research Papers

A High-Resolution Continuous-Scan Acoustic Measurement Method for Turbofan Engine Applications

[+] Author and Article Information
Parthiv N. Shah

ATA Engineering, Inc.,
San Diego, CA 92128

Håvard Vold

ATA Engineering, Inc.,
Charleston, SC 29412

Dan Hensley

ATA Engineering, Inc.,
Lakewood, CO 80401

Edmane Envia, David Stephens

NASA Glenn Research Center,
Cleveland, OH 44135

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received April 6, 2015; final manuscript received August 10, 2015; published online September 23, 2015. Assoc. Editor: Ron Bunker.This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Turbomach 137(12), 121002 (Sep 23, 2015) (11 pages) Paper No: TURBO-15-1064; doi: 10.1115/1.4031341 History: Received April 06, 2015; Revised August 10, 2015

Detailed mapping of the sound field produced by a modern turbofan engine, with its multitude of overlapping noise sources, often requires a large number of microphones to properly resolve the directivity patterns of the constituent tonal and broadband components. This is especially true at high frequencies where the acoustic wavelength is short, or when shielding, scattering, and reflection of the sound field may be present due to installation effects. This paper presents a novel method for measuring the harmonic and broadband content of complex noncompact noise sources using continuously moving (referred to here as continuous-scan (CS)) microphones in conjunction with a state-of-the-art phase-referencing technique. Because the microphones are moving through the sound field produced by the noise sources, they effectively provide infinite spatial resolution of the sound directivity over the scan path. In this method, harmonic (i.e., shaft-coherent) content at the integer multiples of the instantaneous shaft rotational frequency is first extracted from the time signal using a tachometer signal and the Vold-Kalman (VK) filter. The residual broadband signal is then filtered in the time domain in fractional octave bands. The broadband spectra of the signals from the moving microphones are then computed at arbitrary positions along their scan paths using weighted averages (based on Chebyshev polynomial zero-crossings) and the assumption of a complex envelope that varies slowly over a spatial scale whose lower bound is set by the acoustic wavenumber. A benefit of this method is that the decomposition of the total measured sound field into a stochastic superposition of components preserves a meaningful phase definition for each “partial field” associated with a given shaft order (SO). This preservation of phase data enables the forward or backward projection of each of these partial fields using acoustical holography (AH). The benefits of the CS method are demonstrated using acoustic data acquired for a 22-in. scale-model fan stage run at the NASA Glenn Research Center's 9-foot by 15-foot wind tunnel. Two key outcomes of the work include (1) significant improvement in the spatial resolution of the measured sound field and (2) reduction in the overall data acquisition time. Additionally, the methods described here lead to new opportunities for noise source diagnostics and visualization.

Copyright © 2015 by ASME
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Fig. 1

Top-view schematic of wind tunnel facility. Unless otherwise labeled, dimensions are in centimeters (inches in parentheses).

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Fig. 2

Photograph of 22-inch SDT fan stage installed in the NASA Glenn 9 × 15 low-speed acoustic wind tunnel

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Fig. 3

FI Narrowband SPL spectrum from microphone 1 acquired at z = −1.02 m (θ = 114.5 deg (exhaust arc))

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Fig. 4

FI Narrowband SPL spectrum from microphone 1 acquired at z = 1.38 m (θ = 58.5 deg (inlet arc))

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Fig. 5

Shaft mechanical RPM time history during Fast CS (4in./s) acquisition

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Fig. 6

Fast CS (4 in./s) Wold decomposition for microphone 1 signal

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Fig. 7

1BPF (SO 22) partial field (complex envelope magnitude) as a function of geometric (sideline) angle

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Fig. 8

Same as Fig. 7, zoomed in over a 25-deg sideline angle

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Fig. 9

4BPF (SO 88) partial field (complex envelope magnitude) as a function of geometric (sideline) angle

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Fig. 10

Same as Fig. 9, zoomed in over a 25-deg sideline angle

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Fig. 11

SO 86 partial field (complex envelope magnitude) as a function of geometric (sideline) angle. Two scan speeds and fixed index data are compared.

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Fig. 12

Same as Fig. 11, zoomed in over a 25-deg sideline angle

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Fig. 13

First 136 SOs at z = −1.02 m (i.e., at θ = 114.5 deg sideline angle)

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Fig. 14

First 136 SOs at z = 1.38 m (i.e., at θ = 58.5 deg sideline angle)

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Fig. 15

First six BPF harmonics compared at z = −1.02 m (θ = 114.5 deg) and z =+1.38 m (θ = 58.5 deg)

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Fig. 16

One-third-octave directivities at 1000 Hz

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Fig. 17

One-third-octave directivities at 3981 Hz. See legend in Fig. 16.

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Fig. 18

One-third-octave directivities at 15,849 Hz. See legend in Fig. 16.

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Fig. 19

One-third-octave spectra at z = −1.02 m (i.e., 114.5 deg sideline angle). Refer to Fig. 20 for legend.

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Fig. 20

One-third-octave spectra at z = +1.38 m (58.5 deg sideline angle)

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Fig. 21

Wavenumber spectra of 1BPF and 4BPF partial fields, assuming n = 22 and 88, respectively

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Fig. 22

AH sound-field reconstruction of 4BPF tone in near field. Blue asterisk shows assumed source origin at z = 0 m. Scan line shown in dashed black.

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Fig. 23

Same as Fig. 22 with assumed source origin at z = 1.5 m

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Fig. 24

Reconstruction of BPF 4 complex envelope along CS line using two different assumed source origins




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