Noise Radiation Mechanisms Research Articles

Investigation of the mechanism of noise radiation reduction from an ABH-cavity under interior acoustic excitation considering multistage coupling

The acoustic black hole (ABH) structure exhibits superior vibration and sound radiation control capability compared to its flat counterpart. However, for a coupled plate-cavity system under interior acoustic excitation, the underlying suppression mechanism of noise radiation has not been explored, especially below the critical frequency of the structure. In this paper, the noise suppression mechanism of an ABH-cavity system under interior acoustic excitation is investigated both theoretically and numerically. Based on the theoretical analysis, the noise radiation depends on both the structural modes and the cavity modes, which can be suppressed in two ways. One is to lower the magnitude of structural transfer functions. The other is to change the mapping matrix which maps the structural modal amplitude to the amplitude of radiation modes. For the former, numerical analysis based on FEM demonstrates that a lower structural transfer function is obtained in the ABH-cavity system compared to its flat counterpart because of the damping enhancement characteristic of the ABH structure. Therefore, attenuated flexural vibrations and reduced sound radiation are achieved at corresponding modal frequencies. For the latter, ABH-cavity systems with varying numbers of ABHs are constructed to demonstrate a simple solution of manipulating the mapping matrix to achieve the reduced radiation efficiency of structural panels and the enhanced suppressing effect of noise radiation. The study in this paper indicates a feasible direction for the further optimization analysis of noise radiation suppression of the ABH-cavity system.

Physics-based filtering of irrotational wavepackets from high-speed Schlieren

The eduction of acoustically efficient wavepacket structures from experimental measurements or high-fidelity numerical data aids in understanding different noise generation mechanisms in jets. This is often achieved by employing either signal processing techniques or modal decompositions. The former often require some user-defined thresholds that must be adjusted for each operating condition whereas the latter are more robust and can be applied to any jet flow-field irrespective of the operating conditions. The application of physics-based decompositions requires time-accurate spatio-temporally resolved data of the type obtained from well-validated numerical databases, which require substantial computational resources. There is an inherent advantage in extending decomposition-based approaches to experimental measurements, which can provide a substantially longer time series of data compared to numerical simulations. This investigation explores the application of Doak's Momentum Potential Theory to high-speed schlieren imaging, which is a widely used flow visualization tool in jet noise research. The procedure is applied to different test-cases, selected based on the availability of high-quality experimental measurements and numerical data. The coherent wavepackets obtained exhibit superior spatio-temporal coherence and radiative efficiency that capture the noise radiation mechanisms in the flow.

Analysis of noise radiation mechanisms in hot subsonic jet from a validated large eddy simulation solution

In the framework of jet noise computation, a numerical simulation of a subsonic turbulent hot jet is performed using large-eddy simulation. A geometrical tripping is used in order to trigger the turbulence at the nozzle exit. In a first part, the validity of the simulation is assessed by comparison with experimental measurements. The mean and rms velocity fields show good agreement, so do the azimuthal composition of the near pressure field and the far field spectra. Discrepancies remain close to the nozzle exit which lead to a limited overestimation of the pressure levels in both near and far fields, especially near the 90∘ angular sector. Two point correlation analyses are then applied to the data obtained from the simulation. These enable to link the downstream acoustic radiation, which is the main direction of radiation, to pressure waves developing in the shear layer and propagating toward the potential core end. The intermittency of the downstream acoustic radiation is evidenced and related to the coherent structures developing in the shear layer.

Noise generation mechanisms in claw pole alternators

Noise of claw pole alternators, generated electromagnetically and structurally radiated, has been the subject of an extensive research program. The goal has been to identify and reduce noise radiation mechanisms in claw pole (Lundell) alternators used in automotive applications. Two approaches have been followed. In the first, electromagnetic sources of noise have been investigated by lumped parameter and magnetically equivalent circuit modeling and simulation, and by related experimentation. This is the subject of separate papers. The second, concurrent study reported here has investigated machine and mount responses to an electromagnetically generated torque ripple. Modeling and experimentation has led to the conclusion that there exists a high correlation between electromagnetic sources, torque ripple, and radiated noise. Experimentation also has led to the conclusion that noise characteristics of a given machine are substantially altered by modification of the mounting configuration. The work reported here involves modeling, simulation, and experiment to isolate machine dynamic characteristics and mounting geometries which contribute to strong coupling between torque ripple and machine/mount dynamic response. A low-order model of the alternator which includes shaft flexibility, gyroscopic effects, shaft bearing asymmetry, mounting lug geometry, and mounting structure dynamics has been created. The model provides a rapid simulation of dynamic response in the form of a transfer function between torque ripple and mounting forces. Generic studies of a simplified mounting structure coupled to the machine model are presented here. Acoustic testing of several machine configurations on a production mount has been carried out to investigate 36th order noise in three phase machines and 72nd order noise in six-phase machines. Electromagnetic modeling and dynamic response simulations suggest that the six-phase machine is inherently quieter. This is supported by experimental results. A test fixture for the measurement of torque ripple has been developed. Experimentation shows that the relative amplitudes of torque ripple in three- and six-phase machines correlates with relative noise levels. In addition, in torque ripple measurements, machine mounting characteristics are substantially altered and a predictably large reduction in radiated noise is realized.

Diagnosis of Noise Radiation Mechanisms from a Wind Turbine Generator

Noise from a source such as an engine or gearbox can travel through a surrounding structure by many routes. It can be very difficult to reduce noise transmission to the environment or to operators or passengers, because many paths contribute to the transmission and it may not be clear which need to be treated. In order to apply effective control measures, it is very helpful to start with knowledge of the relative importance of the various possible paths. We report on a method that allows the strengths of the different paths to be quantified and ranked in order of importance, without having to dismantle or disconnect the system. Having measured these, and then having measured the levels of noise being injected into each path by the engine or gearbox, the contribution of each path to transmitted noise can then be estimated. Finally the noise reduction obtained by treating any desired combination of paths can be predicted. We refer to the method as “Transmission Path Audit” (TPA). The method was applied to an operating wind turbine that was giving noise problems. A TPA was carried out: it indicated that the problem was due to the turbine blades and tower, which between them radiated the bulk of the noise at three different tonal frequencies. Based on these results, damping treatments were applied to the blades and tower, and noise radiation was reduced to acceptable levels. The TPA method has very general applicability, and can be used to characterise noise transmission through a wide range of structures. It is likely to be particularly useful in tracing transmission paths through vehicles, from the power plant to the passenger space. Its novelty lies in its being entirely non-intrusive.

Progress towards quieter civil helicopters

AbstractA review of recent developments in helicopter noise reduction is presented. The paper includes a discussion of the present state of understanding of helicopter noise radiation mechanisms and a review of actual noise reductions achieved by the manufacturers over the past decade. An analysis of progress achieved in meeting certification limits is given, together with an evaluation of the impact on design. Future trends in helicopter noise reduction based on current noise research programmes are discussed and suggestions advanced for possible additional areas of study.

Simplified Outdoor Vehicle Noise Testing Methods

A way to vary the source to microphone distance and obtain noise measurements over a reflecting plane adhering to the spherical spreading law is achieved by adjusting the microphone height to maintain the ground interference pattern. Vehicle noise radiation mechanisms are re-examined. The study shows how to devise a stationary test procedure which could be expected to correlate with a given passby vehicle noise test procedure. An example shows how a motorcycle passby test is transformed into a correlatable stationary test with five times as short a measurement distance. The noise testing costs and complexity can be significantly reduced by methods discussed here.

A generalized theory on the noise generation from supersonic shear layers

In 1960, O. M. Phillips introduced a theory on noise generation from supersonic shear layers. The theory is based on a convective wave equation which can describe the aerodynamic parameters and noise generation mechanism more accurately than the original Lighthill equation. The solution to the convective wave equation, as given by Phillips, is restricted to Mach wave radiation at asymptotically large (hypersonic) Mach numbers. The present paper presents a generalization to Phillips' theory. Both Mach wave radiation and non-Mach wave noise radiation mechanisms are considered. The range of validity of Phillips' theory has been expanded to include the low supersonic and transonic ranges. These generalizations are important not only in their analytical rigor, but also in their prospective applications to practical problems in jet noise prediction and control. It has been found that the low frequency noise is refracted by the mean flow while the high frequency noise is refracted and attenuated by the mean flow. For each given noise frequency, the source is limited to a finite layer of the jet. Incidentally, the major portion of the noise spectrum can be analytically classified as high frequencies. The noise generation mechanisms in a supersonic jet are found to differ from those in a subsonic jet in general. This theory is considered to have the prospect of answering important questions in supersonic jet noise such as noise source distribution, mean flow refraction effects, directivity, spectrum, and efficiency of noise radiation.

Cooling tower noise generation and radiation

An investigation has been carried out into the generation of noise by large natural-draught cooling towers, and the mechanism of noise radiation from them, based on measurements made at four large power stations. An expression relating the A-weighted acoustic power of a tower to its physical characteristics has been derived. This, together with the discovery that the towers behave as area sources at low frequencies and as arc sources at high frequencies, has enabled a complete prediction method to be proposed. This method was used to estimate the noise from three further cooling towers and gave excellent agreement with the levels which were subsequently measured.

65. Precision films for electronic components: Anon., New Scientist, 13 (279), 22 March 1962, 674

Noise of claw pole alternators, generated electromagnetically and structurally radiated, has been the subject of an extensive research program. The goal has been to identify and reduce noise radiation mechanisms in claw pole (Lundell) alternators used in automotive applications. Two approaches have been followed. In the first, electromagnetic sources of noise have been investigated by lumped parameter and magnetically equivalent circuit modeling and simulation, and by related experimentation. This is the subject of separate papers. The second, concurrent study reported here has investigated machine and mount responses to an electromagnetically generated torque ripple. Modeling and experimentation has led to the conclusion that there exists a high correlation between electromagnetic sources, torque ripple, and radiated noise. Experimentation also has led to the conclusion that noise characteristics of a given machine are substantially altered by modification of the mounting configuration. The work reported here involves modeling, simulation, and experiment to isolate machine dynamic characteristics and mounting geometries which contribute to strong coupling between torque ripple and machine/mount dynamic response. A low-order model of the alternator which includes shaft flexibility, gyroscopic effects, shaft bearing asymmetry, mounting lug geometry, and mounting structure dynamics has been created. The model provides a rapid simulation of dynamic response in the form of a transfer function between torque ripple and mounting forces. Generic studies of a simplified mounting structure coupled to the machine model are presented here. Acoustic testing of several machine configurations on a production mount has been carried out to investigate 36th order noise in three phase machines and 72nd order noise in six-phase machines. Electromagnetic modeling and dynamic response simulations suggest that the six-phase machine is inherently quieter. This is supported by experimental results. A test fixture for the measurement of torque ripple has been developed. Experimentation shows that the relative amplitudes of torque ripple in three- and six-phase machines correlates with relative noise levels. In addition, in torque ripple measurements, machine mounting characteristics are substantially altered and a predictably large reduction in radiated noise is realized.

61. Stress and magnetic anisotropy in thin permalloy films: M. Prutton, Nature, Lond., 193 (4815), 10 Feb. 1962, 565–566

Noise of claw pole alternators, generated electromagnetically and structurally radiated, has been the subject of an extensive research program. The goal has been to identify and reduce noise radiation mechanisms in claw pole (Lundell) alternators used in automotive applications. Two approaches have been followed. In the first, electromagnetic sources of noise have been investigated by lumped parameter and magnetically equivalent circuit modeling and simulation, and by related experimentation. This is the subject of separate papers. The second, concurrent study reported here has investigated machine and mount responses to an electromagnetically generated torque ripple. Modeling and experimentation has led to the conclusion that there exists a high correlation between electromagnetic sources, torque ripple, and radiated noise. Experimentation also has led to the conclusion that noise characteristics of a given machine are substantially altered by modification of the mounting configuration. The work reported here involves modeling, simulation, and experiment to isolate machine dynamic characteristics and mounting geometries which contribute to strong coupling between torque ripple and machine/mount dynamic response. A low-order model of the alternator which includes shaft flexibility, gyroscopic effects, shaft bearing asymmetry, mounting lug geometry, and mounting structure dynamics has been created. The model provides a rapid simulation of dynamic response in the form of a transfer function between torque ripple and mounting forces. Generic studies of a simplified mounting structure coupled to the machine model are presented here. Acoustic testing of several machine configurations on a production mount has been carried out to investigate 36th order noise in three phase machines and 72nd order noise in six-phase machines. Electromagnetic modeling and dynamic response simulations suggest that the six-phase machine is inherently quieter. This is supported by experimental results. A test fixture for the measurement of torque ripple has been developed. Experimentation shows that the relative amplitudes of torque ripple in three- and six-phase machines correlates with relative noise levels. In addition, in torque ripple measurements, machine mounting characteristics are substantially altered and a predictably large reduction in radiated noise is realized.