Xianmei Wu

Partial discharge serves as a typical indicator of insulation defects in high-voltage electrical equipment and is often accompanied by acoustic emission. The online monitoring of partial discharge via acoustic signals makes it essential to investigate the underlying mechanism of acoustic wave excitation by partial discharge. However, experimental investigation is often prohibitively expensive and struggles to capture key discharge parameters. Numerical simulation thus provides a valuable alternative for microscopic analysis. In this study, a typical needle-plane corona discharge model is employed. Based on the theory that acoustic waves are generated by gas disturbances caused by collisions between charged and neutral particles in weakly ionized gases, a numerical model for acoustic wave excitation by positive corona discharge is developed. Simulations and analyses are performed on the acoustic source characteristics and the acoustic field distribution. The results demonstrate that the spatiotemporal evolution of electron density plays a dominant role in the generation of acoustic waves during positive DC corona discharge. The characteristics of the simulated acoustic field agree well with experimental results from relevant studies, validating the effectiveness of the proposed electroacoustic coupling numerical model and providing a new tool for further research into the acoustic features of partial discharge.

Vector hydrophones play an extremely important role in marine exploration. How to reduce the size of vector hydrophones while improving their directional detection capability is a critical issue that needs to be addressed. The auditory organ of the fly Ormia ochracea represents a prime example of achieving high-resolution directional detection within a compact size range. This paper proposes a vector hydrophone that integrates an Ormia ochracea fly-inspired mechanically coupled structure with an optical fiber vibration sensing structure, offering advantages of small size and strong electromagnetic interference immunity. The hydrophone demonstrates a good response to acoustic pulse trains and can accurately demodulate acoustic waves from 1 kHz to 10 kHz. Directional response experiments show that this hydrophone can significantly amplify the time delay differences of incoming acoustic waves. At an acoustic frequency of 9.25 kHz, the time delay amplification factor reaches approximately 50 times within the range of −90° to +90°, exhibiting good cosine directionality.

Transcranial focused ultrasound (TcFUS) neuromodulation is hindered by skull-induced acoustic limitations. To enable synergistic multi-region brain stimulation, we designed non-coaxial multi-focal composite Fresnel acoustic lenses, including an overlapping Fresnel lens (OFL) and an alternating-segmented Fresnel lens (ASFL). These lenses convert planar ultrasound into multiple foci. Based on Fresnel theory, acoustic fields were analyzed via simulations and experiments, validating the generation of four non-coaxial foci (10/30 mm focal lengths) from a 1 MHz planar wave using both OFL and ASFL. The influence of lens parameters on focal pressure distribution was investigated, and morphology was quantified using a linear least-squares method. Significant differences in focal morphology and intensity between OFL and ASFL provide crucial guidance for optimizing multi-target TcFUS strategies.

π相移光纤光栅因具有较短的有效光栅传感长度，近年来成为了超声传感领域的研究热点。本研究旨在探究π相移光纤光栅作为水听器应用时对超声波的指向性特性。选取π相移光纤光栅作为超声传感单元，先基于分层介质的声传播理论计算出水中超声波入射时光纤纤芯的应变，再运用基于光学耦合模方程的传递矩阵法计算反射光谱得到光波长偏移。将角度-频率空间分为三个区域，计算了声波频率在1MHz-10MHz时不同角度下的应变结果和光波长偏移响应特性，并开展了实验研究。结果表明，理论和实验结果具有较高的一致性，π相移光纤光栅在超声波垂直光纤入射时响应最大，随声波入射方向与光纤法向间夹角的增加π相移光纤光栅的声响应先急剧下降，后在水中直达声波和光纤中导波叠加时出现极大值。此外，π相移光纤光栅的声响应随声波频率增加而降低。本研究对π相移光纤光栅在超声传感中的实际应用具有重要意义。

As a representative fluid-transporting system, fluid-conveying pipes play an essential role in many fields. For a fluid-conveying pipe system in operation, fluid pulsation in pipes contains much information about fluid flow parameters (flow velocity, fluid pressure, etc.). Therefore, the measurement of fluid pulsation is important for understanding the internal fluid flow. To use polyvinylidene fluoride (PVDF) piezoelectric film sensors to indirectly measure the pressure fluctuation of the internal fluid, we simulated a fluid-conveying pipe with PVDF piezoelectric film sensors attached to the outer pipe wall. The simulation results showed that the variation of voltage signal of PVDF, circumferential stress and strain of the pipe wall, and the pressure fluctuation of internal fluid were highly positively correlated, which proved that the PVDF piezoelectric film sensor can be applied to indirectly measure the pressure fluctuation of internal fluid. We also studied the influences of flow velocity pulsation and mechanical vibration caused by the pipeline pump during operation. It is found that the flow velocity pulsation had little influence on the measurements of the variation of circumferential stress and strain of the pipe wall and the internal fluid pressure fluctuation. When both ends of the pipe were fixed by hoops, mechanical vibration had little influence on the measurement of the variation of circumferential stress and strain of the pipe wall as well as the fluid pressure fluctuation. Finally, simulation results were verified by experiments.

Time-of-flight (ToF)-based 3-D target localization is a very challenging topic because of the pseudo-targets introduced by ToF measurement errors in traditional ToF-based methods. Although the influence of errors in ToF measurement can be reduced by the probability-based ToF method, the accuracy of localization is not very high. This paper proposes a new 3-D target localization method, Iterative Maximum Weighted Likelihood Estimation (IMWLE), that takes into account the spatial distribution of pseudo-targets. In our method, each pseudo-target is initially assigned an equal weight. At each iteration, Maximum Weighted Likelihood Estimation (MWLE) is adopted to fit a Gaussian distribution to all pseudo-target positions and assign new weight factors to them. The weight factors of the pseudo-targets, which are far from the target, are reduced to minimize their influence on localization. Therefore, IMWLE can reduce the influence of pseudo-targets that are far from the target and improve the accuracy of localization. The experiments were carried out in a water tank to test the performance of the IMWLE method. Results revealed that the estimated target area can be narrowed down to the target using IMWLE and a point estimate of target location can also be obtained, which shows that IMWLE has a higher degree of accuracy than the probability-based ToF method.

In this paper, a microscale high-frequency ultrasonic transducer was prepared by combining traditional planar ultrasonic phased-array technology and micro processing technology. The piezoelectric ceramic material PZT was used as the functional material of the transducer. The number of the arrays was 72, the width of each array was 50 μm, the pitch of each array was 70 μm, and the length of each array was 3 mm. The PZT chip was finely ground to a thickness of 130 μm and could reach a frequency of 10 MHz. The experimental platform of micron-scale precision was set up for a beam-forming lateral sound field test and imaging experiment to validate the theoretical analysis. The echo imaging test showed that a mold with a feature size of about 400 μm could be imaged well.

The nonlinear spring model combined with dislocation dipole theory was applied to describe the acoustic nonlinearity during the fatigue process in metals. The spring stiffness changes with fatigue degree. For the early stage, spring stiffness approaches infinity, and the heavier nonlinearity mainly results from the increase of dislocation density. Further fatigue leads to the occurrence of micro-cracks, during which spring stiffness begins to decrease. Abundant micro-crack sprouting accelerates the crack’s expansion, and spring stiffness drops quickly, which causes the obvious decline in the transmitted harmonic amplitudes. Solutions obtained from the nonlinear wave equation with dislocation terms were added into the spring model. Varying spring stiffness was chosen for simulating the fatigue process. Then, nonlinear harmonic variation during this process was observed, which was classified into three stages: (I) the early dislocation fatigue stage; (II) the micro-crack sprouting stage; (III) the crack expansion stage. Nonlinear acoustic measurements were carried out on an aluminum alloy specimen during its fatigue process until cracks could be seen clearly. Harmonic variations in experiments can also be classified into the same three stages as the numerical results, which provides a theoretical and experimental reference for fatigue evaluation in metals using the nonlinear acoustic method.

The propagations of one-dimensional nonlinear acoustical waves are studied numerically and experimentally in this paper. The finite element method (FEM) is used to simulate the propagations of nonlinear acoustic waves. The FEM equation of one-dimensional nonlinear acoustic wave is derived according to the theory of nonlinear acoustics. A three-dimensional matrix appearing in the nonlinear FEM equation represents the nonlinear part of the nonlinear acoustic equation and indicates the complex propagation characteristics of nonlinear acoustic waves. However, there is no corresponding matrix in the linear FEM equation. The matrix correlates with the nonlinear properties of propagating waves such as wave distortion, high order harmonic wave generation and transformation of energy from basic frequency to high order harmonic frequency, etc. Then, an FEM program is coded to compute the propagations of the one-dimensional nonlinear acoustic waves. The results show that the nonlinear acoustic waves are distorted obviously during propagation. After fast Fourier transform processing the original wave signal, the basic frequency signals and high order harmonic signals both appear in the frequency-region signals. To prove the correctness of the FEM results, nonlinear acoustic experiments in water are carried out under different conditions. In the first experiment, the distance between the transmitting and receiving transducers is kept unchanged, but the transmitting transducer is excited with different energies. So with propagation distance fixed, the influences of different exciting energies on the nonlinear properties of acoustic waves are obtained from this experiment. In the second experiment, with the exciting energy fixed, the influences of different propagation distances on the nonlinear properties of acoustic waves are obtained by changing the distance between the transmitting and receiving transducers. Then the numerical results and the experimental results are compared and analyzed carefully. The result shows that the waveforms and the spectra of simulated nonlinear waves are in good agreement with those of experimental signals. These results prove the correctness of the proposed numerical method. It is also noticed that the propagation properties of basic frequency wave and the second order harmonic waves are different. The amplitude of basic frequency wave decreases gradually, but the amplitude of second order harmonic wave first increases and then decreases after propagating some distance. The amplitude of the second harmonic wave changes with propagation distance and energy of the input source amplitude. The relationship between the amplitude of second harmonic wave and the propagation distance is numerically fitted. We find a fitting equation of the relation between high order harmonic acoustic wave and propagation distance, which also brings clear physical meaning for the high order harmonic waves. Finally, the properties of nonlinear acoustic wave propagation in solid are preliminarily discussed. This study provides theoretical and experimental evidence for the nonlinear acoustic wave traveling in liquid.