Noise Radiation Sources Research Articles

Measurement of the Convection Velocities in a Hypersonic Turbulent Boundary Layer Using Two-Point Cylindrical-Focused Laser Differential Interferometer

A two-point cylindrical-focused laser differential interferometer (2P-CFLDI) system and a conventional Z-type Schlieren were used to measure the hypersonic turbulent boundary layer on a flat plate at Mach number Ma = 6 and Reynolds number Re = 1.08 × 106 m−1. The boundary layer thickness at the measurement location and the noise radiation angle were obtained by post-processing the Schlieren image. The 2P-CFLDI data underwent cross-correlation analysis to calculate the mean convective velocities at different heights and compared with previous experimental and numerical results. The experimentally measured mean convective velocities agree with the trend of available DNS and experimental results. The mean convective velocity near the wall is significantly larger than the local mean velocity and is the main noise source region. Further filtering treatment shows that the convective velocity of the disturbed structure decreases gradually with the increase in the disturbance scale. The differences between convective velocities at different scales are significantly larger outside the boundary layer than inside the boundary layer, which is in agreement with the findings of the previous hot wire experiments. Near the wall, large-scale disturbances mainly determine the localized mean convective velocity, which are the main source of noise radiation for the hypersonic turbulent boundary layer.

Reduction of vibration and noise in rail transit steel bridges using elastomer mats: Numerical analysis and experimental validation

The noise radiating from a steel bridge can be much greater than the pass-by noise levels of trains at-grade. This is attributed to a special phenomenon known as the steel bridge-borne noise, which occurs when shocks and vibrations due to moving train loads are transferred onto the bridge. To investigate the effectiveness of elastomer mats for reducing vibration and noise in rail transit steel bridges, a frequency domain prediction model is established and calibrated through a field experiment. In this model, the train and track interactions were investigated analytically, and the receptance technique was introduced to determine the force transmitted from a multi-layer track structure to the bridge deck. The local vibrations of the bridge components were solved numerically using a hybrid shell/beam finite element model based on a harmonic response analysis. The bridge vibrations were regarded as the source of noise radiation by considering each bridge component as a rectangular flat plate noise source. There was good agreement between the calculated and measured results in terms of both the magnitude and frequency dependence. The mechanisms of vibration transmission and noise radiation were investigated using numerical simulations. In addition, the vibration and noise reduction effects are discussed by numerically comparing a track system with and without an elastomer mat.

Leading-Edge Profiles for the Reduction of Airfoil Interaction Noise

Airfoils operating in a turbulent flow are an efficient source of noise radiation by scattering vorticity into sound at the leading edge. Much work has been undertaken demonstrating the effectiveness by which serrations, or undulations, introduced onto the leading edge can substantially reduce broadband leading-edge interaction noise. However, most of this work is focused on sinusoidal leading-edge serration profiles. In this paper, a family of serration profiles is proposed that is capable of providing significantly greater noise reductions than single-wavelength serrations at optimal conditions. This new family of profiles will be shown to reduce interaction noise through a fundamentally different noise reduction mechanism than conventional single-wavelength profiles. Unlike single-wavelength profiles, which produce a single compact dominant source region per serration wavelength, these new profiles are designed to produce two or more dominant compact sources per serration wavelength of roughly the same source strength that are separated in the streamwise direction. Because these sources are arranged to be closer together than the turbulence length scale, they are highly coherent and, at certain frequencies, radiate exactly 180 deg out of phase, leading to very high levels of noise reduction in the far field. The experimental noise reduction spectra are first compared against an analytic model obtained as a solution of the acoustic wave equation using the Wiener–Hopf technique. This approach focuses solely on solving for an acoustic field, and not for any secondary vorticity induced by the edge. As such, this comparison, which shows disagreement, proves that secondary vorticity is a dominant mechanism for these new profiles. A simple supplementary analytical model has therefore been developed to explain this noise reduction mechanism for these new profiles, based on interference between sources distributed along the leading edge. Good qualitative agreement is obtained with the experimental results.

Slitted leading edge profiles for the reduction of turbulence-aerofoil interaction noise.

Aerofoils operating in a turbulent flow are an efficient source of noise radiation by scattering vorticity into sound at the leading edge (LE). Much work has now been undertaken demonstrating the effectiveness of serrations, or undulations, introduced onto the LE can substantially reduce broadband LE interaction noise. However, all of this work is focused on sinusoidal LE serration profiles. In this paper, an alternative profile is proposed that is capable of providing significantly greater noise reductions than the maximum noise reductions that can be achieved by single-wavelength serrations of the same peak-to-root amplitude. In its most general realization, the LE serration profiles simply comprise of a sawtooth or single wavelength serration for which every root has a single narrow slit. This simple geometry, upon interaction with a turbulent flow, produces compact source regions at either end of the slit, which then destructively interfere, leading to considerably less efficient noise radiation than conventional single-wavelength geometries. The paper will demonstrate experimentally that even slits by themselves can provide greater levels of noise reduction than conventional profiles of the same peak-to-root amplitude.

Methods and approaches of high resolution spectroscopy for analytical applications

Application of high resolution spectroscopy methods for analyzing vapors and decomposition products of high energy substances is considered. The approaches currently used for detecting such substances are analyzed. Terahertz sources and receivers promising for constructing high resolution spectrometers are identified. A new approach to developing a terahertz spectroscopy technique using phase-diffusion noise radiation sources is proposed. The results of using the high resolution spectroscopy method for analyzing high energy substances vapors are presented.

Controlling smartphone vibration and noise

Vibration of the internal electronics components and its transmission to the casing has been known as a source of noise radiation that degrades the call quality of a smartphone. Here, we expand upon our previous work [J. Acoust. Soc. Am. 137, 2293 (2015)] that employed the nearfield acoustic holography (NAH) for characterization of the smartphone radiated noise. The NAH, when used in conjunction with a vibration model of the smartphone, is shown to be a powerful tool for controlling the smartphone vibration and noise at the initial design phase. The vibration model is largely based on the classical plate theory and yields concrete guidelines on how to reduce vibration and the subsequent noise. The effectiveness of the technique is demonstrated using some of the smartphones currently available on the market.

Modeling of Noisy EM Field Propagation Using Correlation Information

In this paper, an efficient method for the numerical simulation of near- and far-field propagation of stochastic electromagnetic (EM) fields is presented. The method is based on the transformation of field correlation dyadics using Green's functions or the field transfer functions computed for deterministic fields. The method accounts for arbitrary correlations between the noise radiation sources and allows to compute the spatial distribution of the spectral energy density of noisy electromagnetic sources. The introduced methodology can be combined with available electromagnetic modeling tools. It is shown that the method of moments can be applied to solve noisy electromagnetic field problems by network methods applying correlation matrix techniques. Examples demonstrating the strong influence of the correlation between the sources on the spatial distribution of the radiated noise field are presented.

Statistical analysis of “superresolving” methods for direction-of-arrival estimation of noise radiation sources under finite size of training sample

This paper addresses a detailed comparative study of operational performances of a family of the most prominent in the literature so-called “superresolution” methods for the estimation of spatial spectrum of Gaussian noise radiation sources impinging on an antenna array. Real-world operational scenarios are considered, in which small size samples are used for maximum likelihood estimation of the data correlation matrices. We perform an analysis of exact analytically derived and empirically evaluated distribution laws of random operational parameters that determine the resolution performances of the compared direction-of-arrival (DoA) superresolution estimation methods evaluated by statistical and non-statistical criteria. Our study reveals that in realistic scenarios with small size sample data these distribution laws may manifest a behavior significantly different from that theoretically predicted for asymptotic assumptions of “infinitely large” sample data recordings. The addressed detailed analysis of changes in behavior of such distribution laws provides the remedies for improvement of the convergence rates and robustness performances of the proposed modified superresolution procedures for feature-enhanced DoAs estimation of multiple noise radiation sources. The reported simulations corroborate the effectiveness of the proposed modified superresolution DoA estimation techniques.

Operation of a relativistic backward wave tube affected by the noise of electron flows formed by explosive emission cathodes

One of the mechanisms of appearance of the high-frequency noise of intense electron flows formed by explosive emission cathodes is theoretically analyzed. It is shown that the main source of noise radiation in devices using explosive emission electron injectors can be the Shot effect related with the partial character of particle emission. The main energy characteristics of electron flow noise radiation are estimated and the influence of noise on the operation of a relativistic backward wave tube (BWT) is studied. The rate of formation of steady-state oscillations in BWT self-oscillators, the minimal level of the input signal necessary for stable BWT amplifier operation, and the spectral density of the input electromagnetic signal excitation-equivalent to the electron flow noise radiation are estimated.

Paper 1: Relation between Blade Row Spacing and Potential Flow Interaction Effects in Turbomachines

This paper develops a simple relationship between distance upstream and downstream of a row of blades and the velocity disturbances created by the passage of those blades. The relationship is compared with values obtained by numerical computation and measurement in an experimental axial flow compressor rig. The magnitude of vibration exciting forces and/or noise radiation sources due to potential flow interaction on a stationary row of blades is directly related to the magnitude of these velocity fluctuations. The analysis is therefore a basis ( a) for estimating the inter-row spacings for a turbomachine such that potential flow interactions are reduced below the levels of other forms of interaction, and ( b) of translating practical experience of noise and/or vibration excitation in existing machines to give reliable predictions for future designs. It is found that while potential flow interaction effects drop rapidly with increasing blade row spacing for low-speed machines, this is not the case for high speeds and very large spacings may, in fact, be required where the rotor blade speed is equivalent to a high value of Mach number. It is also found that the rate of decrease in effect is related to the circumferential wavelength of the disturbance, and comparisons between machines based on blade chords (as is the present normal practice) are meaningless unless the ratio pitch/chord is constant.