Kenichiro Nagai

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.

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

Airframe noise measurements on JAXA Jet Flying Test Bed “Hisho” using a phased microphone array:

In 2015, the Japan Aerospace Exploration Agency launched the Flight demonstration of QUiet technology to Reduce nOise from High-lift configurations project to verify by flight demonstration the feasibility of practical noise-reducing aircraft modification concepts. In order to serve as a baseline for comparison before modification, airframe noise sources of the JAXA Jet Flying Test Bed “Hisho” were measured with a 30 m diameter array of 195 microphones mounted on a wooden platform built temporary beside the runway of Noto Satoyama Airport in Japan. A classical Delay and Sum in the time domain beamforming algorithm was adapted for the present study, with weight factors introduced to improve the low-frequency resolution and autocorrelations eliminated to suppress wind noise at high frequencies. In the landing configuration at idle thrust, the main landing gear, nose landing gear, and side edges of the six extended flap panels were found to be the dominant “Hisho” airframe noise sources. Deconvolution by the DAMAS and CLEAN-SC algorithms provided clearer positions of these sound sources at low frequencies. Integration of acoustical maps agreed well with the sound pressure level measured by a microphone placed at the center of the microphone array and gave detailed information about the contribution of each noise source.