Hideshi Oinuma

High-resolution Fly-over Beamforming Using a Small Practical Array

The paper describes a commercially available fly-over beamforming system based on methodologies already published, but using an array that was designed for quick and precise deployment on a concrete runway rather than for minimum sidelobe level. Time domain tracking Delay And Sum (DAS) beamforming is the first processing step, followed by Deconvolution in the frequency domain to reduce sidelobes, enhance resolution, and get absolute scaling of the source maps. The system has been used for a series of fly-over measurements on a Business Jet type MU300 from Mitsubishi Heavy Industries. Results from a couple of these measurements are presented: Contribution spectra from selected areas on the aircraft to the sound pressure level at the array are compared against the total sound pressure spectrum measured by the array. One major aim of the paper is to verify that the system performs well although the array was designed with quick deployment as a main criterion. The results are very encouraging. A second aim is to elaborate on the handling of the array shading function in connection with the calculation of the Point Spread Function (PSF) used in deconvolution. Recent publications have used a simple formula to compensate for Doppler effects for the case of flat broadband spectra. A more correct formula is derived in the present paper, covering also a Doppler correction to be made in the shading function, when that function is used in the PSF calculation.

Acoustic Measurement and Prediction of Solid Rockets in Static Firing Tests

Acoustic measurements are executed in two series of static-firing tests of a solid rocket motor. The obtained data are quantitatively compared with calculation results of an empirical prediction method, NASA SP-8072 and CFD. According to the results, the NASA SP-8072 overestimates the sound pressure levels at the 20° and 35° points from the jet axis in the far field, although the SPLs at other measured points are reasonably predicted. On the other hand, the CFD calculation can clearly explain the generation and propagation mechanism of the acoustic wave and reasonably predict the SPLs at all the measured points. From the results, it is confirmed that the prediction accuracy of the CFD calculation is within 5 [dB] in overall sound pressure level, which is within the experimental uncertainty involved in the measured data, and the CFD is effective for the prediction of both the near and the far field acoustics generated from the rocket motors.

Experimental Study of a Claw Mixer

This experimental study proposes a claw mixer as a jet noise reduction device. The claw, which is composed of sharp-edged parts, i.e., nails, extends from the nozzle end into the jet. It was hypothesized that when the sharp-edged faces of the nails are immersed in the jet plume at an inclined angle relative to the jet axis, the mixing between the jet and surrounding air is improved, leading to a reduction in jet noise. To test this hypothesis, the authors carried out a series of noise tests as a first step. The tests involved a cold-jet test in an anechoic room, a hot-jet test using a model jet engine to simulate heated air, and a largescale test with a turbojet engine in an outdoor environment. The scale-model tests with the hot and cold jets showed the expected noise reduction performance. With increasing penetration of the nail into the plume, the noise benefit in the side direction either decreased or disappeared. The engine noise test, which adopted a relatively deep penetration, resulted in a conventional configuration of noise reduction, with the reduction directivity in the rear direction. Sound source localization supported the far-field noise results, distinguishing the source intensities both with and without the claw mixer. When the thrust was measured simultaneously with the noise, there still remained relatively large thrust penalty compared to the baseline nozzle.

Experimental Study on a Notched Nozzle for Jet Noise Reduction

This paper describes an experimental study on a notched nozzle for jet noise reduction. The notch, a tiny tetrahedral dent formed at the edge of a nozzle, is expected to enhance mixing within a limited region downstream of the nozzle. The enhanced mixing leads to the suppression of broadband peak components of jet noise with little effect on the engine performance. To investigate the noise reduction performances of a six-notch nozzle, a series of experiments have been performed at an outdoor test site. Tests on the engine include acoustic measurement in the far field to evaluate the noise reduction level with and without the notched nozzle, and pressure measurement near the jet plume to obtain information on noise sources. The far-field measurement indicated the noise reduction by as much as 3 dB in terms of overall sound pressure level in the rear direction of the engine. The use of the six-notch nozzle though decreased the noise-benefit in the side direction. Experimental data indicate that the high-frequency components deteriorate the noise reduction performance at wider angles of radiation. Although the increase in noise is partly because of the increase in velocity, the penetration of the notches into the jet plume is attributed to the increase in sound pressure level in higher frequencies. The results of near-field measurement suggest that an additional sound source appears up to x/D = 4 due to the notches. In addition, the total pressure maps downstream of the nozzle edge, obtained using a pressure rake, show that the notched nozzle deforms the shape of the mixing layer, causing it to become wavy within a limited distance from the nozzle. This deformation of the mixing layer implies strong vortex shedding and thus additional noise sources. To improve the noise characteristics, we proposed a revised version of the nozzle on the basis of a computational prediction, which contained 18 notches that were smaller than those in the 6-notched nozzle. Ongoing tests indicate greater noise reduction in agreement with the computational prediction.© 2011 ASME

Hot-Jet Noise Test of a Revised Notched Nozzle

Jet noise remains a significant noise component in modern commercial aero-engines. A high-speed flow mixing with the surrounding air constitutes noise sources behind the nozzle. One noise-reduction technology is a mixing device attached to the nozzle. Several fixed-geometry mixers such as chevrons have been studied by both computational and experimental approaches. The authors have previously proposed a notched nozzle with dents allocated along the nozzle lip and discussed its ability to reduce the noise level. The revised notch was expected to suppress the broadband jet-mixing noise as well as additional noise at higher frequencies. However, further assessments are required before proceeding to a large-scale engine test in an outdoor environment. First, the influence of the gas temperature on acoustic results must be tested because the temperature affects the mean jet velocity and sound propagation. As the preliminary noise test in the previous paper was limited to the cold-jet condition, far-field noise data under the hot-jet condition should be investigated. Second, the aerodynamic performance must be evaluated. Data on the flow rate and thrust would help in considering the aerodynamic performances between the baseline, notched, and chevron nozzles. This study focuses on noise tests for the finer-notched nozzle under the hot-gas condition. A small jet engine for model jet planes was employed to generate a high-temperature jet. An engine test stand was designed to monitor the engine performance data, consisting of the pressure and temperature at several positions, the fuel flow rate, and the thrust. The hot-jet test with and without the mixing device served as a compact and flexible test for aerodynamic evaluation of the nozzle. The noise test results under the hot-jet condition with this rig showed that the noise reduction characteristics of the finer-notched nozzle are different from those of conventional mixers.Copyright