Phased microphone arrays have become a well-established tool for performing aeroacoustic measurements in wind tunnels (both open-jet and closed-section), flying aircraft, and engine test beds. This paper provides a review of the most well-known and state-of-the-art acoustic imaging methods and recommendations on when to use them. Several exemplary results showing the performance of most methods in aeroacoustic applications are included. This manuscript provides a general introduction to aeroacoustic measurements for non-experienced microphone-array users as well as a broad overview for general aeroacoustic experts.

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A review of acoustic imaging methods using phased microphone arrays

The present work describes a numerical reproduction of the 22-in source diagnostic test fan rig of the NASA Glenn Research Center. Numerical flow simulations are performed for three different rotor...

https://arc.aiaa.org/doi/pdf/10.2514/1.J055674

Turbofan Broadband Noise Prediction Using the Lattice Boltzmann Method

Computation of rotor wake turbulence noise

This paper is concerned with utilizing the acoustic analogy approach to predict the sound from unheated supersonic jets. Previous attempts have been unsuccessful at making such predictions over the Mach number range of practical interest. The present paper, therefore, focuses on implementing the necessary refinements needed to accomplish this objective. The important effects influencing peak supersonic noise turn out to be source convection, mean flow refraction, mean flow amplification, and source non-compactness. It appears that the last two effects have not been adequately dealt with in the literature. The first of these because the usual parallel flow models produce most of the amplification in the so called critical layer where the solution becomes singular and, therefore, causes the predicted sound field to become infinite as well. We deal with this by introducing a new weakly non parallel flow analysis that eliminates the critical layer singularity. This has a strong effect on the shape of the peak noise spectrum. The last effect places severe demands on the source models at the higher Mach numbers because the retarded time variations significantly increase the sensitivity of the radiated sound to the source structure in this case. A highly refined (non-separable) source model is, therefore, introduced in this paper.

/pdf/the-aeroacoustics-of-slowly-diverging-supersonic-jets-59gg7l3mzh.pdf

The aeroacoustics of slowly diverging supersonic jets

Experimental results from exploring the flowfield and noise generated by turbulent, high subsonic, single-stream jets are presented. We study the nature and directly measure the distribution of subsonic jet noise sources and attempt to establish their connection to some of its turbulence characteristics. Detailed measurements of the jet turbulence characteristics and of the spatial distribution of noise sources (using a linear phased array) are performed for an Mj = 0.6, Re D ≅10 6 isothermal jet. Noise source distributions are investigated for a wide range of jet Mach numbers (0.5 ≤ Mj ≤ 0.9) with and without heating (T j =80, 1000°F). The turbulence and source distribution measurements provide a unique experimental database for high subsonic turbulent jets with which to validate aeroacoustic prediction models

High Subsonic Jet Experiments: Turbulence and Noise Generation Studies

An advanced phased array system, OptiNav Array 48, and a new deconvolution algorithm, TIDY, have been used to make octave band images of supersonic and subsonic jet noise produced by the NASA Glenn Small Hot Jet Acoustic Rig (SHJAR). The results are much more detailed than previous jet noise images. Shock cell structures and the production of screech in an underexpanded supersonic jet are observed directly. Some trends are similar to observations using spherical and elliptic mirrors that partially informed the two-source model of jet noise, but the radial distribution of high frequency noise near the nozzle appears to differ from expectations of this model. The beamforming approach has been validated by agreement between the integrated image results and the conventional microphone data.

/pdf/improved-phased-array-imaging-of-a-model-jet-4j62732g4k.pdf

Improved Phased Array Imaging of a Model Jet

Results are presented of an experiment conducted to investigate potential sources of noise in the flow developed by two 22-in. diameter turbofan models. The R4 and M5 rotors that were tested were designed to operate at nominal take-off speeds of 12,657 and 14,064 RPMC, respectively. Both fans were tested with a common set of swept stators installed downstream of the rotors. Detailed measurements of the flows generated by the two were made using a laser Doppler velocimeter system. The wake flows generated by the two rotors are illustrated through a series of contour plots. These show that the two wake flows are quite different, especially in the tip region. These data are used to explain some of the differences in the rotor/stator interaction noise generated by the two fan stages. In addition to these wake data, measurements were also made in the R4 rotor blade passages. These results illustrate the tip flow development within the blade passages, its migration downstream, and (at high rotor speeds) its merging with the blade wake of the adjacent (following) blade. Data also depict the variation of this tip flow with tip clearance. Data obtained within the rotor blade passages at high rotational speeds illustrate the variation of the mean shock position across the different blade passages.

/pdf/fan-noise-source-diagnostic-test-ldv-measured-flow-field-4a98skkpxx.pdf

Fan Noise Source Diagnostic Test: LDV Measured Flow Field Results

/pdf/improved-phased-array-imaging-of-a-model-jet-33enboi558.pdf

/pdf/an-assessment-of-current-fan-noise-prediction-capability-17raay0pqi.pdf

An Assessment of Current Fan Noise Prediction Capability

This work documents two-point space-time correlation measurements made in a cold, M=OS round jet using a pair of two-component hotwire probes, From the simultaneously measured velocities space-time correlations are computed for axial and radial displacements for several radial locations each at three axial stations (4, 8, and 20 diameters). This rich type of turbulence data is required in many jet noise prediction theories, such as the Mani-Gliebe-Balsa (MGB) theory. In the past, these terms have been estimated from an isotropic, homogeneous turbulence model with a Gaussian form for space and time dependence. This turbulence model resulted in a particular partitioning of turbulent kinetic energy among the many quadrupole components. The work presented here indicates that such models are in error and proposes an axisymmetric model with lower order exponent. This model results in a different ratio of source directivities, which will impact the predicted jet noise source directivity. Using a lower-order exponent in the time-dependent part of the space-time correlation also shows better agreement with data and will change the predicted jet noise spectral characteristics.

Gary Podboy

Papers

Improved Phased Array Imaging of a Model Jet

Fan Noise Source Diagnostic Test: LDV Measured Flow Field Results

Improved Phased Array Imaging of a Model Jet

An Assessment of Current Fan Noise Prediction Capability

Measurements of two-point velocity correlations in a round jet with application to jet noise