Measurement of the normal acoustic impedance using beamforming method

A beamforming technique is introduced to measure the normal acoustic impedance at both normal and oblique incidence in a free field. In the proposed method, microphone array signals are decomposed into incident and reflected waves using an adaptive nulling algorithm, which is a type of beamforming algorithm. The acoustic impedance can then be calculated from the ratio of these two signals. To obtain better results, the pressure vector commonly used in array signal processing is replaced with the transfer function vector between each microphone, and the white Gaussian noise is suppressed by a wavelet shrinkage technique. For an accurate experimental setup, the incident and reflected angles are estimated by the multiple signal classification method with spatial smoothing. The experiments conducted in a semi-anechoic room show that the proposed method is efficient and accurate in measuring the normal acoustic impedance of sound-absorbing materials under a free field condition. This paper was recommended for publication in revised form by Associate Editor Hong Hee Yoo Jongcheon Sun received his B.S. degree in Mechanical Engineering from Korea University, Korea, in 2002. He is currently in the Unified Master’s and Doctor’s course at the School of Mechanical and Aerospace Engineering at Seoul National University, Korea. His research areas are acoustic holography and beamforming.     

Analysis of wave reflection of a stenotic vessel blood pressure wave using the lattice Boltzmann method and impedance boundary condition

Pressure wave produced by a stenotic vessel was analyzed in terms of the stiffness index, augmentation index and reflection index. An understanding of blood pressure wave reflection is key to developing non-invasive and easy-to-use diagnostic methods. The purpose of this study was to use computational fluid dynamics to analyze blood pressure waves and understand blood pressure wave reflections in stenotic vessels. The lattice Boltzmann method was used with the impedance boundary condition for the blood pressure waves. Variations in blood pressure wave parameters according to different degrees of stenosis were analyzed, in addition to fluid dynamic properties, including flow energy loss. We found that blood pressure wave reflection is related to flow energy loss from stenosis.

     

Hybrid prediction method of load-carrying capacity considering boundary condition of expansion joint sleeve and rotation angle of spindle

Journal of Mechanical Science and Technology - Taking the double-cone expansion joint sleeve (EJS) as an example, this paper proposes a hybrid prediction method of load-carrying capacity...     

基于表面阻抗边界的变压器杂散损耗计算方法

表面阻抗法是基于电磁波理论并将导体表面阻抗作为一种边界条件进行有限元求解的方法,它具有求解规模小、计算效率高等优势。首先借助工程电磁场数值计算软件Magnet,使用有限元法和表面阻抗法对该模型进行仿真,然后基于TEAM Problem 21基准族中的P21-B模型搭建了变压器杂散损耗测试系统,用于测量在不同频率电流激励条件下的变压器杂散损耗。针对现有测量方法的不足,引入温度系数对测量结果进行修正,并采用改进线圈损耗法以得到更准确的杂散损耗测量结果。通过与实验值对比,验证了有限元法与表面阻抗法计算结果的准确性。从计算规模和时间角度分析,表面阻抗法远远小于有限元法,大大节省了计算资源。     

Reconstruction of the complex reflectance function in acoustic microscopy

We describe the theory and practical implementation of an iterative computer algorithm (an extension of the Gerchberg Saxton algorithm, originally developed for electron microscopy) for enabling an estimate of the complex reflectance function of a material to be reconstructed from measured values of only the magnitude of the response of an acoustic microscope (i.e. without the phase of the transducer voltage). Results are presented for eight materials measured with a spherical lens (at 320 MHz) and five with a cylindrical line-focus lens (at 210 and 228 MHz).     

Effect of boundary condition history on the symmetry breaking bifurcation of wall-driven cavity flows

A symmetry breaking nonlinear fluid flow in a two-dimensional wall-driven square cavity taking symmetric boundary condition after some transients has been investigated numerically. It has been shown that the symmetry breaking critical Reynolds number is dependent on the time history of the boundary condition. The cavity has at least three stable steady state solutions for Re=300-375, and two stable solutions if Re>400. Also, it has also been showed that a particular solution among several possible solutions can be obtained by a controlled boundary condition.     

支撑条件对钢轨模态参数影响研究

钢轨的振动形态直接影响着铁路轮轨的接触状态,不同的支撑条件对钢轨的振动有着较大的影响.本文应用模态分析和有限元理论研究了钢轨不同支撑条件对钢轨固有频率和振型的影响.应用环境激励下模态参数识别方法对钢轨在不同支撑条件下的模态参数进行了识别.针对四种支撑方式,分别得到了模态参数,通过理论和试验分析表明,不同的支撑方式对轨道模态参数有较大影响.这一结论为合理铺设轨道提供依据.     

Analysis of the piston ring lubrication with a new boundary condition

Research on the inlet and outlet lubricational characteristics of the piston ring have been intense and ongoing. Many researchers had considered that the entire surface of the ring was enveloped in an oil film, but much experimental research has discovered that not all the entire surface was soaked. To consider a partially lubricated ring, the following conditions are presupposed; oil starvation is applied to the inlet region and the open-end assumption to the outlet region. This algorithm confirms flow continuity and permits the nadir of the pressure to go down to the saturation pressure. Using these new boundary conditions, the actual effective width participating in ring lubrication is determined and the minimum film thickness and flow rate for the ring pack can be calculated. The effective width is expected to be about 20 to 30 percent of the whole width of the ring, and the minimum film thickness is less than the result obtained by using the Reynolds cavitation boundary condition.     

Application of the generalized boundary condition to solving thermal friction problems

A new generalized boundary condition is advanced to describe the nonideal thermal contact in the tribosystem body-counterbody with high heat conductivity. The solutions of one-dimensional problems of nonstationary heat conductivity in half-bounded and bounded bodies with a generalized boundary condition are presented. The applicability of this condition to simulation of the contact temperature is exemplified by the polymer-metal tribosystem.     

EMP径向滑动轴承计入边界滑移的热弹流分析

弹性金属塑料瓦（ＥＭＰ）径向滑动轴承是一种新型的轴承,轴瓦材料的特殊性使其热弹变形远大于普通金属瓦轴承,同时它所特有的边界滑移现象,对改善径向滑动轴承的润滑性能有较为明显的优越性。建立了计入边界滑移情况后对轴承３Ｄ热弹流分析的教学模型,并给出列,对其润滑机理进行了初步的分析。     

An isotropic magnetic-field transducer based on the giant magnetic impedance effect

A comparative analysis of the giant magnetoimpedance effect and the results of a mathe- matic simulation are presented. This simulation was used for optimizing the topology of a wide-angle magnetic transducer equipped with a sensitive element that consists of two crossed Fe₃Co₆₇Cr₃Si₁₅B₁₂ amorphous ribbons that form different angles between them.     

Effects of shear plane boundary condition on stress loading in orthogonal machining

The finite element method is used to investigate the effect of shear plane boundary conditions on the stress field in the workpiece during machining. The balances of force and moment on both the shear plane and the tool rake face are used to calculate the unknown boundary condition, i.e. normal stress distribution on the shear plane. The results of finite element analysis show that the normal stress distribution on the shear plane does not significantly affect the value of the stresses in the workpiece under the tool tip. The length of the shear plane is found to be a major parameter governing the stress field in the workpiece in machining, confirming a previous experimental study.     

Mathematical modeling of pressure characteristics of the deflector-jet pilot stage considering boundary layer flow

The performance of a deflector-jet servo valve significantly depends on the pressure characteristics of its pilot stage. The complex flow field within the deflector-jet pilot stage presents enormous difficulty and challenge in establishing its mathematical model. In this work, according to the energy conversion characteristics of the flow field, the flow process within the pilot stage is divided into five phases for modeling: inside-pipe flow, first jet, first pressure recovery, secondary jet, and secondary pressure recovery. To better reveal the intrinsic operating mechanism of a deflector-jet pilot stage, the boundary layer is introduced into the model. Computational fluid dynamic (CFD) simulation and experiment are conducted to validate the developed mathematical model at different supply pressures. The result demonstrates that the velocity distribution of the jets and the pressure characteristics of the pilot stage are significantly influenced by the boundary layer. The boundary layer flow within the V-groove has a greater impact on the pressure characteristics than that within the nozzle. Verified by the CFD simulation and experiment, the developed model can accurately predict the velocity distribution of the jets and pressure characteristics. At different supply pressures, the maximum relative error between the theoretical and experimental results of the dimensionless pressure characteristic is 5.36%.     

On the effort to discriminate the principal function of tribofilm on friction under the boundary lubrication condition

Friction coefficient under boundary lubrication conditions is affected by many factors. In order to study the friction characteristic of tribofilm, friction coefficient of tribofilm formed on steel disk under the boundary lubrication conditions was separately measured under dry condition using a laboratory-made micro-tribometer to eliminate the hydrodynamic lubrication action of fluid. This paper introduces the approaching method to understand the friction characteristic of tribofilm. In this paper, sliding speed dependency of friction exerted by tribofilm was observed that could not be interpreted by the classical boundary lubrication mechanisms. The additives having long alkyl chain showed strong sliding speed dependency of friction.     

A study of the tribological behaviour of piston ring/cylinder liner interaction in diesel engines using acoustic emission

The novel use of non-intrusive acoustic emission (AE) measurements to provide information pertaining to the interaction between piston rings and cylinder liners in a range of diesel engines is investigated in this paper. In doing so, this technique is shown to offer a new method of investigation into this important interface in engine operation.AE generated during normal engine operation is known to consist of contributions from a number of different sources such as injector and valve activity. A recent finding has been the identification of AE signals associated with the ring/liner interface which presents the opportunity for in-service monitoring. This work discusses the possible AE source mechanisms, such as asperity contact, lubricant flow and/or blowby, through reference to a number of tests on motored and in-service small HSDI diesel engines and large, 2-stroke, marine diesel engines. The influence of various factors such as engine speed, load and lubrication is considered.     

Dissipated energy and boundary condition effects associated to dry friction on the dynamics of vibrating structures

In turbomachines, dry friction devices (under platform dampers, shrouds, and tie-wire) are usually introduced to reduce resonant responses of bladed disks. Dry friction between rubbing elements induces a highly nonlinear dynamic behaviour which flattens the frequency response functions. It is clear that such behaviour requires an optimisation process to find the optimum parameters that lead to the minimum forced response amplitudes. However, different interpretations still remain concerning the explanation of the physical origin of this type of flattening. The most common one is based on dissipated energy. In this case, heat resulting from the relative frictional motion between contacting surfaces is supposed to bring sufficient dissipation to flatten response functions. On the other hand, a different approach considers that a decrease in vibrational amplitudes is explained by changes in boundary conditions induced by a stick/slip behaviour. In this study, a single degree-of-freedom system is used and analysed both in time and in frequency domains (Harmonic Balance Method) in order to show the contribution of respectively energy dissipation and change of contact state on peak levels.     

Reconstruction of electrical impedance tomography (EIT) images based on the expectation maximum (EM) method

Electrical impedance tomography (EIT) calculates the internal conductivity distribution within a body using electrical contact measurements. The image reconstruction for EIT is an inverse problem, which is both non-linear and ill-posed. The traditional regularization method cannot avoid introducing negative values in the solution. The negativity of the solution produces artifacts in reconstructed images in presence of noise. A statistical method, namely, the expectation maximization (EM) method, is used to solve the inverse problem for EIT in this paper. The mathematical model of EIT is transformed to the non-negatively constrained likelihood minimization problem. The solution is obtained by the gradient projection-reduced Newton (GPRN) iteration method. This paper also discusses the strategies of choosing parameters. Simulation and experimental results indicate that the reconstructed images with higher quality can be obtained by the EM method, compared with the traditional Tikhonov and conjugate gradient (CG) methods, even with non-negative processing.     

边界条件对衡器性能影响问题的探讨

一些问题随着衡器载重量的增加,出现在大型衡器的使用中。刘九卿老师在其编著的《电阻应变式称重传感器》一书中,介绍了多种称重传感器的边界条件问题。那么,大型衡器是否也存在边界条件的影响问题呢？这就是本文要探讨的核心。     

The effect of abradant particle comminution on the intensity of three-body abrasion

An attempt has been made to understand why testing involving abradant particles leads to inconsistent results, despite identical test conditions. The work undertaken involved experiments on a rubber wheel abrasion tester (RWAT), and found comminution of the abradant to be the omitted factor: careful examination of the abradant particles after each test revealed that, even for the lowest load, the particles did not remain unbroken. The results obtained showed that at least part of the variation between tests carried out under nominally identical conditions can therefore be ascribed to ignoring the effects of size reduction. The paper concludes by recommending some changes in standard procedure.     

角度偏差对过盈联接强度的影响

在分析母线角度偏差引起配合长度上压力变化的基础上。提出在母线角度不同时计算圆锥过盈联接圆周力矩的敷学模型,论述母线角度偏对相对圆周力矩的影响,为圆锥过盈联接的设计提供了重要的理论参考依据。