Formulation and validation of a Ritz-based analytical model of high-frequency periodically layered isolators in compression

Periodically layered isolators exhibit transmissibility “stop bands” or frequency ranges in which there is very low transmissibility. A two-dimensional axisymmetric model was developed to accurately predict the location of these stop bands for isolators in compression. A Ritz approximation method was used to model the axisymmetric elastic behavior of layered cylindrical isolators. A modal analysis was performed for a single elastomer and metal layer combination or cell. A modal synthesis approach was then used to obtain a model of an n-celled isolator, from which overall isolator modal properties are determined. This model of the dynamic behavior of layered isolators was validated with experiments. Analytical and experimental transmissibilities are compared for test specimens having identical elastomer components, but different geometries and different numbers of cells. In all cases, experimental and analytical transmissibilities are in close agreement at frequencies ranging from zero to those associated with the initial roll-off of the stop bands. For three and four cell cases, minimum stop band analytical transmissibilities lie below the minimum experimental measurements, although an experimental noise floor imposed a minimum transmissibility measurement of approximately 1.4×10⁻⁴. Experiment suggests a practical isolator design could limit the minimum number of cells to three or four to ensure a pronounced stop band attenuation effect. In addition, analytical and experimental transmissibilities are compared for geometrically similar test specimens with differing elastomeric damping properties. The analytical and experimental results show that stop band effectiveness is not appreciably affected by the addition of modest damping.     

水下对转桨无空化噪声调制理论分析与试验研究

水下对转桨无空化噪声由流场-桨叶相互作用引起,调制特性是水下对转桨无空化噪声的重要特征,本文研究了水下对转桨无空化辐射噪声调制机理。首先利用广义声类比方法得到了无空化条件下水下对转桨的远场声压谱,建立了水下对转桨无空化噪声的调制模型,然后数值仿真了模型对转桨无空化噪声的功率谱和调制谱,最后在空泡水筒中进行了模型对转桨的无空化噪声测量试验,数值仿真结果和试验验证了调制模型的准确性。该模型对于水下对转桨无空化噪声调制特性预报及目标识别具有重要价值.     

Theoretical analysis and experimental research on non-cavitation noise modulation mechanism of underwater counter-rotation propeller

The underwater counter-rotation propeller non-cavitation noise has an obvious modulation characteristic which is due to the interaction of flow and blade. A modulation mechanism is presented in this paper. A sound pressure spectrum model is presented to describe its non-cavitation noise with application of generalized acoustic analogy method, the modulation mechanism is expressed with the improvement of sound pressure model. The power spectrum and modulation spectrum are presented by numerical simulation. Theoretical analysis and numerical simulation results are verified by the cavitation tunnel experiment. The modulation model of counter-rotation propeller is beneficial to the prediction modulation characteristics and identification of underwater high-speed vehicles.     

Reduction of coating thermal noise by using an etalon

Reduction of coating thermal noise is a key issue in precise measurements like a gravitational-wave detector, where each mirror is coated by a few tens of dielectric layers to achieve high reflectivity while the noise level increases with the number of layers. One way to realize the reduction is mechanical separation of the first few layers from the rest. Using an etalon with a few layers on the front and more on the back surface is a way to realize such a system. We perform a noise analysis of an etalon using the Fluctuation-dissipation theorem with probes on both sides.     

Relationship between low-frequency aircraft noise and annoyance due to rattle and vibration

A near-replication of a study of the annoyance of rattle and vibration attributable to aircraft noise [Fidell et al., J. Acoust. Soc. Am. 106, 1408-1415 (1999)] was conducted in the vicinity of Minneapolis-St. Paul International Airport (MSP). The findings of the current study were similar to those reported earlier with respect to the types of objects cited as sources of rattle in homes, frequencies of notice of rattle, and the prevalence of annoyance due to aircraft noise-induced rattle. A reliably lower prevalence rate of annoyance (but not of complaints) with rattle and vibration was noted among respondents living in homes that had been treated to achieve a 5-dB improvement in A-weighted noise reduction than among respondents living in untreated homes. This difference is not due to any substantive increase in low-frequency noise reduction of acoustically treated homes, but may be associated with installation of nonrattling windows. Common interpretations of the prevalence of a consequential degree of annoyance attributable to low-frequency aircraft noise may be developed from the combined results of the present and prior studies.     

水下对转螺旋桨流致辐射噪声机理与预报方法*

水下对转螺旋桨流致辐射噪声的预报对于水下目标的特征提取和分类识别具有重要意义。由桨叶的旋转引起的湍流场是水下对转螺旋桨流致辐射噪声的源场。分述了水下对转螺旋桨湍流边界层脉动、旋转干涉效应和空化效应引发的水动力噪声机制和研究进展,比较了目前工程应用中的3种对转螺旋桨流致辐射噪声预报方法的特点。在分析对转螺旋桨流致辐射噪声数值预报难点的基础上,综述了对转螺旋桨流致辐射噪声计算方法的研究进展,指出间接数值模拟方法是工程中进行对转螺旋桨流致辐射噪声预报的有效方法。     

Effective suppression of pneumatic vibration isolators by using input-output linearization and time delay control

This paper presents a new state space representation of pneumatic vibration isolators (PVIs) and a design of a robust control, Time Delay Control (TDC), based on it. The new state space model, derived by using the input-output linearization method, is of the phase variable form with the air mass-flow as the control input. This model offers a framework that enables simultaneous suppression of both seismic vibration and direct disturbance (or payload disturbance) with an accelerometer only. Based on this model, TDC is designed and verified with experiments on a single chamber PVI with an accelerometer only. In the experiment, the PVI with TDC successfully suppresses seismic vibration and direct disturbance, both individually and simultaneously. Faced with seismic vibration, the transmissibility of the PVI with TDC has virtually no resonance peak at low frequency; under direct disturbance, the former achieves a 68 percent reduction in settling time of the latter. The final analysis of experimental result shows that TDC effectively estimates the modeling error along with other uncertainties and cancels them, while achieving desired closed-loop dynamics.     

Analytical predictions for vibration phase shifts along fluid-conveying pipes due to Coriolis forces and imperfections

Resonant vibrations of a fluid-conveying pipe are investigated, with special consideration to axial shifts in vibration phase accompanying fluid flow and various imperfections. This is relevant for understanding elastic wave propagation in general, and for the design and trouble-shooting of phase-shift measuring devices such as Coriolis mass flowmeters in particular. Small imperfections related to elastic and dissipative support conditions are specifically addressed, but the suggested approach is readily applicable to other kinds of imperfection, e.g. non-uniform stiffness or mass, non-proportional damping, weak nonlinearity, and flow pulsation. A multiple time scaling perturbation analysis is employed for a simple model of an imperfect fluid-conveying pipe. This leads to simple analytical expressions for the approximate prediction of phase shift, providing direct insight into which imperfections affect phase shift, and in which manner. The analytical predictions are tested against results obtained by pure numerical analysis using a Galerkin expansion, showing very good agreement. For small imperfections the analytical predictions are thus comparable in accuracy to numerical simulation, but provide much more insight. This may aid in creating practically useful hypotheses that hold more generally for real systems of complex geometry, e.g. that asymmetry or non-proportionality in axial distribution of damping will induce phase shifts in a manner similar to that of fluid flow, while the symmetric part of damping as well as non-uniformity in mass or stiffness do not affect phase shift. The validity of such hypotheses can be tested using detailed fluid-structure interaction computer models or laboratory experiments.     

水下加筋板振动声辐射的代理模型研究

基于kriging模型建立了水下结构振动声辐射计算的代理模型,可对水下结构的共振频率和声功率级进行预测。还将主分量分解和kriging模型相结合建立了可对整个计算频段内的振动声辐射响应进行预测的代理模型。这些代理模型可以基于样本信息在整个设计空间对水下结构的振动声辐射响应进行实时预测。文中以水下加筋板的振动声辐射为例进行了代理模型的构造和验证。计算结果表明:使用基于kriging方法的代理模型预测水下结构的共振频率及声功率级具有非常高的精度;使用基于主分量分解和kriging模型的代理模型预测整个计算频段内非共振频率处的声功率级响应也具有很好的精度。在水下结构振动声辐射的设计优化中,可以根据需要单独或联合使用这些代理模型。     

声强测量技术在水下结构辐射噪声近场测量中的应用

本文介绍了强测量技术在水下结构辐射近场测量中的实际测量系统，特别是水下声系统的扫描平面的实现方法及定位误差控制方法，最后讨论和分析了实际测量结果，从而说明该系统用水下声强测量是可行的。     

Reduction of motor fan noise using CFD and CAA simulations

Motor fans used for cooling electric motors have long been recognized as one of the major noise sources. Current paper focuses on design of motor fan for electric motors that are used in submarines for pumping sea water. Noise reduction at source is very important and the critical task, for under water applications. An attempt has been made for reduction of motor fan noise by modification of noise sources. For this purpose computational fluid dynamics and computational aeroacoustics code FLUENT package is used to identify the noise sources and to know the overall sound pressure level of motor fan. From these results it is observed that aerodynamic noise is the dominate fan noise source. Aerodynamic noise of motor fan can be reduced by modifying fan geometry. The aerodynamic noise level of motor fan has been reduced by replacing the straight blades with various digits of NACA (National Advisory Committee for Aeronautics) 65 series airfoil sections. From the numerical results it is observed that the minimum sound pressure level for NACA 65-010 is 65.4 dB(A). These numerical results are compared with measurements in a semi-anechoic chamber. It is found that there is good agreement between numerical and experimental results.     

Reduction of line radiation of an absorbing plasma to an optically transparent plasma

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 49, No. 3, pp. 393–400, September, 1988.     

Excitation of shear waves due to the interband absorption of laser radiation in a piezoelectric semiconductor-dielectric layered structure

Optical piezoelectric generation of shear bulk acoustic waves by volume-distributed electric fields in the vicinity of a hard planar interface between a piezoelectric semiconductor and a dielectric is described theoretically. Nonstationary and nonuniform electric fields are formed as a result of the spatial separation of electrons and holes photoexcited in the piezoelectric semiconductor due to the interband absorption of laser radiation. Frequency regions where the efficiency of the optical piezoelectric excitation of shear waves increases when the surface of the piezoelectric semiconductor is loaded by another piezoelectric with a high acoustic impedance are found. Numerical estimates of the applicability of the immobile hole model are presented.     

Estimation and interpolation of underwater low frequency ambient noise spectrum using artificial neural networks

In this work, estimation of ambient noise spectrum influenced by wind speed and wave height carried out for the frequency range of 500 Hz to 5 kHz using Feed forward Neural Network (FNN) is presented. Ocean ambient noise measurements were made in the shallow waters of Bay of Bengal using a portable data acquisition system with a high sensitivity hydrophone at a depth of 5 m from the surface.100 sets of data covering a rage of wind speeds from 2.5 m/s to 8.5 m/s with approximately 15 sets of data falling within 1 m/s over the range of wind speed were used for training the FNN. The parameter wave height which contributes to the noise producing mechanism is also used for training along with wind speed. The results revealed that the proposed method is useful in the estimation and interpolation of underwater noise spectrum level and hence in simulation for the considered frequency range. These were confirmed by calculating the Mean Squared Error (MSE) between the experimental data and the simulation. As the measurements of the underwater ambient noise level are very difficult in remote oceanic regions, where conditions are often inhospitable, these studies seem to be relevant.     

Numerical analysis of isolation of the vibration due to moving loads using pile rows

The isolation of the vibration due to moving loads using pile rows embedded in a poroelastic half-space is investigated in this study. Based on Biot's theory and integral transform method, the free field solution for a moving load applied on the surface of a poroelastic half-space and the fundamental solution for a harmonic circular patch load applied in the poroelastic half-space are derived first. Using Muki and Sternberg's method and the fundamental solution for the circular patch load as well as the obtained free field solution for the moving load, the second kind of Fredholm integral equations in the frequency domain describing the dynamic interaction between pile rows and the poroelastic half-space is developed. Numerical solution of the frequency domain integral equations and numerical inversion of the Fourier transform yield the time domain response of the pile–soil system. Comparison of our results with some known results shows that our results are in a good agreement with existing ones. Numerical results of this study show that velocity of moving loads has an important impact on the vibration isolation effect of pile rows. The same pile row has a better vibration isolation effect for the lower speed moving loads than for the higher speed ones. Also, the optimal length of piles for higher speed moving loads is shorter than that for lower speed moving loads. Moreover, stiff pile rows tend to produce a better vibration isolation effect than flexible pile rows do.     

Analyses of contact forces and vibration response for a defective rolling element bearing using an explicit dynamics finite element model

This paper provides insights into the physical mechanism by which defect-related impulsive forces, and consequently, vibrations are generated in defective rolling element bearings. A dynamic nonlinear finite element model of a rolling element bearing with an outer raceway defect was numerically solved using the explicit dynamics finite element software package, LS-DYNA. A hypothesis was developed to explain the numerical noise observed in the predicted vibrations and contact forces, and the noise frequencies were analytically estimated. In-depth analyses of the numerically estimated dynamic contact forces between the rolling elements and the raceways of a bearing, which are not measured in practice, and have not been reported previously, are presented in this paper. Several events associated with the traverse of the rolling elements through the outer raceway defect are elaborated, and the impulsive force generating mechanism is explained. It was found that the re-stressing of the rolling elements that occurs near the end of a raceway defect generates a burst of multiple short-duration force impulses. The modelling results also highlight that much higher contact forces and accelerations are generated on the exit of the rolling elements out of defect compared to when they strike the defective surface. A bearing with a machined outer raceway defect was tested in a controlled experiment; the measured acceleration response compared favourably with the numerically modelled acceleration results, thereby, validating the low- and high-frequency characteristics of the de-stressing and re-stressing of the rolling elements, respectively.     

结构声辐射的振动模态分析和声辐射模态分析研究

基于辐射声功率的二次型表达式,采用有限元法、Rayleigh积分和边界元法对结构声辐射进行了振动模态分析和声辐射模态分析研究。振动模态间的耦合对辐射声功率影响的研究表明:结构各阶振动模态自身对结构辐射声功率的贡献是增大结构的辐射声功率,而振动模态间的耦合可能会增大结构辐射声功率,也可能会减小结构辐射声功率,或对辐射声功率没有影响。而且,当振动模态间的耦合作用对辐射声功率的影响不大时,采用振动模态控制可取得较好的减振降噪双重控制效果。将混合的Helmholtz积分方程方法用广义逆引入到三维复杂结构声辐射分析的声辐射模态公式中,解决了特征频率下解不唯一问题。还研究了正方形封闭空间结构声辐射模态的辐射效率和形状,并对结构声辐射的振动模态控制和声辐射模态控制进行了讨论。     

Dephasing of a qubit due to quantum and classical noise

The qubit (or a system of two quantum dots) has become a standard paradigm for studying quantum information processes. Our focus is decoherence due to interaction of the qubit with its environment, leading to noise. We consider quantum noise generated by a dissipative quantum bath. A detailed comparative study with the results for a classical noise source such as generated by a telegraph process, enables us to set limits on the applicability of this process vis à vis its quantum counterpart, as well as lend handle on the parameters that can be tuned for analysing decoherence. Both Ohmic and non-Ohmic dissipations are treated and appropriate limits are analysed for facilitating comparison with the telegraph process.     

水下噪声音色属性回归模型及其在目标识别中的应用

通过对声音的主观评价与客观分析而建立的主观感受数学模型,在许多领域都有重要的应用. 本文采用多元线性回归分析手段对水下噪声音色属性建立回归模型,提取音色特征并改善水下目标的识别效果. 首先,在前期水下噪声音色属性主观评价实验的基础上,将构成音色属性空间的5个成分的评价分值作为回归分析中的因变量,提取大量与听觉感知相关的听觉特征作为自变量;然后,通过相关分析和改进的逐步筛选法,挑选出反映音色属性的“最优”自变量子集;最后,利用向后剔除回归分析和水下目标识别实验,确定适当的音色模型,并通过假设检验证明该线性模型不仅正确有效,而且能改善水下目标识别效果. 关键词： 音色 多元线性回归 主观评价