自然循环过冷沸腾流动不稳定性的实验研究

本文以氟里昂作工质，对自然循环过冷沸腾流动不稳定性进行了实验研究．实验过程中发现自然循环系统内可能发生高频脉动和低频脉动二种类型的过冷沸腾流动不稳定性．通过实验研究揭示了这二种类型流动不稳定性的发生机理，证实高频脉动属于声波型脉动，低频脉动属于密度波型脉动．通过实验得出了判断系统稳定性的界限，并使用积分方程无因次分析方法得出了预测密度波型流动不稳定性的经验公式．     

Loschmidt echo of a central spin coupled to an XY spin chain: Role of three-site interaction

Effects of two types of three-site interaction, i.e., XZX+YZY and XZY−YZX, on Loschmidt Echo (LE) of a central spin coupled to an XY spin chain are studied. The dynamical evolution behaviors of the LE are investigated analytically and numerically. The XZX+YZY type of three-site interaction (α₁) can shift the critical points of the magnetic field λ. At the critical points |λ−α₁|=1, the decay of the LE is enhanced. The role of the XZY−YZX type of three-site interaction (α₂) depends on its strength. In some specific intervals, α₂ can remarkably delay the decay of the LE.     

Fracture of diamond coatings by high velocity sand erosion

This paper describes a study of the behaviour of diamond coatings when subjected to solid particle erosion from sand particles. The coatings were deposited by chemical vapour deposition (CVD) onto tungsten substrates and tested using a high velocity air–sand erosion test facility. The erosion tests were conducted using particle impact velocities of between 33 and 268 m/s. Examination of the eroded test specimens showed that the principal damage features were circumferential cracks and pin-holes. Comparison with Hertz impact theory revealed that the measured circumferential crack diameters were more than double the predicted Hertzian contact diameter. Moreover, a trend of increasing circumferential crack diameter with coating thickness, which is not predicted by Hertz, was found. Instead, the crack diameters showed good agreement with those predicted by the theory of stress wave reinforcement, which is more commonly associated with liquid impact damage of brittle materials. During impact, the bulk compression and shear waves are reflected at the rear surface of debonded regions of the coating to return to the front surface and reinforce the Rayleigh surface wave, which generates a tensile stress. Where this stress exceeds the local tensile strength of the coating, a ring of cracks surrounding the area of impact is created. The results from the present study therefore suggest that stress wave reflection is responsible for the formation of the cracks at locally debonded regions of the coating. This hypothesis was supported by images acquired using scanning acoustic microscopy, which showed that circumferential cracks and pin-holes were only found on areas of the coating that had become delaminated by multiple particle impacts during the erosion tests.     

基于并行蚁群算法的长基线定位方法*

为了降低各个误差源对水声目标导航定位精度的影响,该文将水声目标导航定位问题抽象为带约束条件的非线性优化问题,并论证了最优化表达式参数求解过程与降低误差源干扰的过程具有同一性;设计了并行蚁群算法求其最优解。海试数据处理结果表明,该方法具有收敛速度快、解稳定和定位精度高等优点,能有效地降低各个误差源对水声目标导航定位精度的影响。     

多层介质中超声相控阵声场仿真*

由于良好的声束偏转与聚焦特性,超声相控阵已经广泛应用于多层固体介质的缺陷检测。当超声束经过多层介质时,由于反射、透射以及模式转换的存在,多种声束存在于这种结构中,使得声场分析变得复杂。为了提高多层介质检测的准确性,有必要对超声声场的分布规律进行深入地了解。该文结合高斯声束等效点源模型以及射线追踪法,给出了相控阵声源在多层固体介质中激发声场的仿真方法,并且模拟计算了一维线型相控阵在楔块-铝-黄铜-钢四层固体介质中的辐射声场。通过对不同延时法则的计算,实现了声波在这种复杂介质中的偏转与聚焦,进而研究了不同焦点处聚焦声场的分布。结果表明:相控阵方法能使聚焦点处的声场幅值增大,能量集中,提高了检测分辨率;不同聚焦点处声场聚焦效果不同,实际检测时应根据检测区域结构及位置特点,合理放置相控阵换能器。与瑞利积分法的比较表明,该文的仿真方法适用于多层介质相控阵声场的计算。     

Study on Transient Properties of Levitated Object in Near-FieldAcoustic Levitation

A new approach to the study on the transient properties of the levitated object in near-field acoustic levitation (NFAL) is presented. In this article, the transient response characteristics, including the levitated height of an object with radius of 24 mm and thickness of 5 mm, the radial velocity and pressure difference of gas at the boundary of clearance between the levitated object and radiating surface (squeeze film), is calculated according to several velocity amplitudes of radiating surface. First, the basic equations in fluid areas on Arbitrary Lagrange-Euler (ALE) form are numerically solved by using streamline upwind
petrov galerkin (SUPG) finite elements method. Second, the formed algebraic equations and solid control equations are solved by using synchronous
alternating method to gain the transient messages of the levitated object and gas in the squeeze film. Through theoretical and numerical analyses, it
is found that there is a oscillation time in the transient process and that the response time does not simply increase with the increasing of velocity
amplitudes of radiating surface. More investigations in this paper are helpful for the understanding of the transient properties of levitated
object in NFAL, which are in favor of enhancing stabilities and responsiveness of levitated object.     

周期性轴对称凹槽结构隔声特性数值模拟

提出一种在钻铤上切割周期性轴对称凹槽方式构建隔声体,消除钻铤模式波的方法.结果表明,其隔声效果与凹槽深度H、长度L、凹槽数目N、凹槽截面形状以及钻铤壁厚有关.选择矩形截面凹槽、减小钻铤壁厚、增加凹槽深度都可有效提高隔声体的隔声效果,该结构最高可获得约36 dB的声衰减,基本满足随钻声波测井仪器对隔声效果要求.     

Finite element modeling of acoustic scattering from an encapsulated microbubble near rigid boundary

This article proposes a finite element model (FEM) for predicting the acoustic scattering from an encapsulated microbubble near rigid boundary. The validity of the model is first examined by comparing the acoustic nonlinear response of a free microbubble with that obtained by the Church model. Then this model is used to investigate the effect of the rigid boundary on acoustic scattering signals from microbubble. The results indicate that the resonance frequency decreases while the oscillation amplitude increases as the microbubble approaches the rigid boundary. In addition, the fundamental component of the acoustic scattering signal is enhanced compared with that of the free microbubble.     

Acoustic dipole radiation model for magnetoacoustic tomography with magnetic induction

An acoustic dipole radiation model for magnetoacoustic tomography with magnetic induction (MAT-MI) is pro-posed,based on the analyses of one-dimensional tissue vibration,three-dimensional acoustic dipole radiation and acoustic waveform detection with a planar piston transducer.The collected waveforms provide information about the conductiv-ity boundaries in various vibration intensities and phases due to the acoustic dipole radiation pattern.Combined with the simplified back projection algorithm,the conductivity configuration of the measured layer in terms of shape and size can be reconstructed with obvious border stripes.The numerical simulation is performed for a two-layer cylindrical phantom model and it is also verified by the experimental results of MAT-MI for a tissue-like sample phantom.The proposed model suggests a potential application of conductivity differentiation and provides a universal basis for the further study of conductivity reconstruction for MAT-MI.     

Adaptive and optimal detection of elastic object scattering with single-channel monostatic iterative time reversal

In active sonar operation,the presence of background reverberation and the low signal-to-noise ratio hinder the detection of targets.This paper investigates the application of single-channel monostatic iterative time reversal to mitigate the difficulties by exploiting the resonances of the target.Theoretical analysis indicates that the iterative process will adaptively lead echoes to converge to a narrowband signal corresponding to a scattering object’s dominant resonance mode,thus optimising the return level.The experiments in detection of targets in free field and near a planar interface have been performed.The results illustrate the feasibility of the method.