A computational simulation study on the acoustic pressure generated by a dental endosonic file: Effects of intensity,file shape and volume

One of the uses of ultrasound in dentistry is in the field of endodontics (i.e. root canal treatment) in order to enhance cleaning efficiency during the treatment. The acoustic pressures generated by the oscillation of files in narrow channels has been calculated using the COMSOL simulation package. Acoustic pressures in excess of the cavitation threshold can be generated and higher values were found in narrower channels. This parallels experimental observations of sonochemiluminescence. The effect of varying the channel width and length and the dimensions and shape of the file are reported. As well as explaining experimental observations, the work provides a basis for the further development and optimisation of the design of endosonic files.     

Numerical modelling of acoustic pressure fields to optimize the ultrasonic cleaning technique for cylinders

Fouling build up is a well-known problem in the offshore industry. Accumulation of fouling occurs in different structures, e.g. offshore pipes, ship hulls, floating production platforms. The type of fouling that accumulates is dependent on environmental conditions surrounding the structure itself. Current methods deployed for fouling removal span across hydraulic, chemical and manual, all sharing the common disadvantage of necessitating halting production for the cleaning process to commence. Conventionally, ultrasound is used in ultrasonic baths to clean a submerged component by the generation and implosion of cavitation bubbles on the fouled surface; this method is particularly used in Reverse Osmosis applications. However, this requires the submersion of the fouled structure and thus may require a halt to production. Large fouled structures such as pipelines may not be accommodated. The application of high power ultrasonics is proposed in this work as a means to remove fouling on a structure whilst in operation. The work presented in this paper consists of the development of a finite element analysis model based on successful cleaning results from a pipe fouled with calcite on the inner pipe wall. A Polytec 3D Laser Doppler Vibrometer was used in this investigation to study the fouling removal process. Results show the potential of high power ultrasonics for fouling removal in pipe structures from the wave propagation across the structure under excitation, and are used to validate a COMSOL model to determine cleaning patterns based on pressure and displacement distributions for future transducer array design and optimization.     

基于COMSOL Multiphysics对Cu2S量子点的表面等离激元共振模拟研究

Cu₂S量子点作为一种新型半导体纳米材料以其特有的化学及光学性质而越来越被人们所关注. 本文通过应用COMSOL Multiphysics4.2a这一基于有限元法的模拟软件, 运用其中的电磁波模块, 结合Maxwell电磁波理论对影响Cu₂S量子点产生表 面等离激元共振的条件做出了模拟分析, 运用Kretschmann棱镜耦合系统建立了理论模型, 通过改变量子点的粒径、有机溶剂的折射率、入射光的波长以及对量子点进行包裹等方式, 模拟出在不同条件下Cu₂S量子点产生表面等离激元共振信号的情况, 为Cu₂S量子点在表面等离激元共振传感中的实际应用提供了理论依据和参考. 关键词： COMSOL Multiphysics 2S量子点')" href="#">Cu₂S量子点 表面等离激元共振 有限元法     

Comparison of sonochemiluminescence images using image analysis techniques and identification of acoustic pressure fields via simulation

One common method to determine the existence of cavitational activity in power ultrasonics systems is by capturing images of sonoluminescence (SL) or sonochemiluminescence (SCL) in a dark environment. Conventionally, the light emitted from SL or SCL was detected based on the number of photons. Though this method is effective, it could not identify the sonochemical zones of an ultrasonic systems. SL/SCL images, on the other hand, enable identification of ‘active’ sonochemical zones. However, these images often provide just qualitative data as the harvesting of light intensity data from the images is tedious and require high resolution images. In this work, we propose a new image analysis technique using pseudo-colouring images to quantify the SCL zones based on the intensities of the SCL images and followed by comparison of the active SCL zones with COMSOL simulated acoustic pressure zones.     

聚焦区域高声强声场分布的测量方法研究

常用的声场分布测量是采用水听器扫描声场的方法,该方法对于声能量密度较大的声场难以测量,因为在这种情况下声振幅比较大,水听器在这种声场中呈现非线性或遭到破坏。设计了一种用辐射压力测量高声强声场分布的方法,该方法利用一根微细管,直接测量声场的冲流压力,通过对声场进行扫描测量可以得到高声强声场压力分布。从理论上分析了这种测量方法的可行性,对测量基本要求及实验装置做了阐述。实验结果证实:该方法可以用来测量高声能密度声场压力分布;测量结果与水听器测量结果基本吻合;测量方法存在测量盲区。     

双驱动电动换能器电磁场和声-结构耦合场分析

电动换能器的设计涉及电磁场和声-结构耦合问题,本文利用COMSOL多物理场分析软件提供的该两种分析模块进行组合,以电磁理论为基础推导出了适合COMSOL数值计算的公式,把两种耦合场的分析结合起来,利用电磁场分析过程计算所需要变量,代入声-结构耦合场进行分析,得到声场分布情况。文章也大致介绍了COMSOL的有限元建模和该两种模块的使用方法。本文对电动换能器结构如何优化不做讨论。     

Simulation of the spatial distribution of the acoustic pressure in sonochemical reactors with numerical methods: A review

Numerical methods for the calculation of the acoustic field inside sonoreactors have rapidly emerged in the last 15 years. This paper summarizes some of the most important works on this topic presented in the past, along with the diverse numerical works that have been published since then, reviewing the state of the art from a qualitative point of view. In this sense, we illustrate and discuss some of the models recently developed by the scientific community to deal with some of the complex events that take place in a sonochemical reactor such as the vibration of the reactor walls and the nonlinear phenomena inherent to the presence of ultrasonic cavitation. In addition, we point out some of the upcoming challenges that must be addressed in order to develop a reliable tool for the proper designing of efficient sonoreactors and the scale-up of sonochemical processes.     

Numerical modelling of acoustic cavitation threshold in water with non-condensable bubble nuclei

Numerical modelling of acoustic cavitation threshold in water is presented taking into account non-condensable bubble nuclei, which are composed of water vapor and non-condensable air. The cavitation bubble growth and collapse dynamics are modeled by solving the Rayleigh-Plesset or Keller-Miksis equation, which is combined with the energy equations for both the bubble and liquid domains, and directly evaluating the phase-change rate from the liquid and bubble side temperature gradients. The present work focuses on elucidating acoustic cavitation in water with a wide range of cavitation thresholds (0.02–30 MPa) reported in the literature. Computations for different nucleus sizes and acoustic frequencies are performed to investigate their effects on bubble growth and cavitation threshold. The numerical predictions are observed to be comparable to the experimental data in the previous works and show that the cavitation threshold in water has a wide range depending on the bubble nucleus size.     

Field emission pressure sensors with non-silicon membranes

We report on the fabrication of cold cathode emitter and the design parameter simulation of a functional field emission-based pressure sensor. This device comprises a membrane made of metallic compound acting as the anode in front of a flat cold cathode emitter. First, the mechanical deflection of a diaphragm under selected input pressures is modeled. The current density distribution in the deflected diaphragm is then calculated using realistic field emission characteristics from fabricated sulfur doped boron nitride (S-BN) cold cathode device. The total current output was found by integrating the measured current density of the fabricated electron emitter device over the entire diaphragm area of the membrane as function of external pressure. The results show that conventional silicon membranes would pose problems when implemented in a real field emission device, and show how the use of unconventional materials (i.e., TiN) can help overcome these problems.     

Interaction of two bubbles with distortion in an acoustic field

Expression of the secondary Bjerknes force of two bubbles is obtained by considering the distrotion of two bubbles. The secondary Bjerknes forces in different acoustic fields are simulated, and the influence factors are analyzed and discussed. It is shown that the distortion of a bubble has an important influence on the interaction of two bubbles. The strength and even the directions of the secondary Bjerknes force of two bubbles with distortion differ considerably from the predictions of the sherical symmetry theory. The results show that when two bubbles oscillated stably in an acoustic field, the secondary Bjerknes force of two bubbles with distortion is several times more than that of two spherical bubbles in the same condition. The secondary Bjerknes force of two bubble with distortion has more interaction distance than that of two spherical bubbles. The secondary Bjerknes force of two bubbles with distortion depends on the distance of two bubbles, the shape mode of two bubbles, the equilibrium radii of two bubbles and the driving acoustic filed. The nonspherical distortion effects of the secondary Bjerknes has an importance on understanding the structure formation of bubbles and evolution process of bubble group in an acoustic field.     

A comparative study on the modeling of sound pressure field distributions in a sonoreactor with experimental investigation

In sonochemical reactors the effect of emerging cavitation bubbles has significant influence on the amplitude and structure of the developing sound field. Calculations show that the damping parameter and the phase velocity may, depending on the pressure amplitude, change by several orders of magnitude. For example, the sound velocity in water comes to 1500 ms⁻¹, whereas in a bubbly liquid it may decrease to 20 ms⁻¹, which is much below the velocity of sound in air (about 340 ms⁻¹). In this paper, a method of calculating the time dependent three-dimensional pressure field in sonochemical reactors of various shapes is presented. It takes into account inhomogeneous distributed wave parameters which are a function of the spatial depending pressure amplitude. The modeled results are then compared with experimentally measured values of a certain kind of reaction vessel. The agreement is found to be satisfactory.     

Enhancement of oscillation amplitude of cavitation bubble due to acoustic wake effect in multibubble environment

Acoustic cavitation occurs in ultrasonic treatment causing various phenomena such as chemical synthesis, chemical decomposition, and emulsification. Nonlinear oscillations of cavitation bubbles are assumed to be responsible for these phenomena, and the neighboring bubbles may interact each other. In the present study, we numerically investigated the dynamic behavior of cavitation bubbles in multi-bubble systems. The results reveal that the oscillation amplitude of a cavitation bubble surrounded by other bubbles in a multi-bubble system becomes larger compared with that in the single-bubble case. It is found that this is caused by an acoustic wake effect, which reduces the pressure near a bubble surrounded by other bubbles and increases the time delay between the bubble contraction/expansion cycles and sound pressure oscillations. A new parameter, called “cover ratio” is introduced to quantitatively evaluate the variation in the bubble oscillation amplitude, the time delay, and the maximum bubble radius.     

Nanocomposite cerium oxide polymer matching layers with adjustable acoustic impedance between 4 MRayl and 7 MRayl

A new class of materials for ultrasonic matching layers is presented. The materials consist of nanoscale cerium oxide particles in an epoxy functionalized organic inorganic hybrid polymer matrix. The cerium oxide agglomerates to particles with 20 nm diameters. The content of particles in the polymer matrix could be increased to 75 wt.% which corresponds to 37 vol.%. The most technical important piezoelectrical ceramics have an acoustic impedance of about 30 MRayl, to improve coupling into water or biological tissue with an acoustic impedance of about 1.5 MRayl a matching layer should have an acoustic impedance of about 6.8 MRayl. With a filling degree of 75 wt.% the new composite material reaches an acoustic impedance of 7 MRayl. The materials are synthesized by a hydrolytic condensation combined with polymerization. This way of synthesis allows the use of organic solvents to adjust the viscosity of the sol and the application of different coating techniques. Ultrasound transducers (100 MHz) were built to test the new matching layers and an increase of the voltage signal amplitude of about 100% could be detected.     

常压微波等离子体炬装置的模拟与设计

 介绍了一台低成本的常压微波等离子体炬设备,给出了该设备构造及喷嘴的设计思路,分析了各种气体的非磁化微波等离子体的击穿电场强度,数值求解了设备中矩形TE103谐振腔中的电磁场分布,应用高频电磁场模拟分析软件HFSS优化了喷嘴在波导中的具体位置,并对优化后喷嘴周围的电场分布进行了模拟。模拟结果表明：微波输入有效功率为500 W,喷嘴伸出矩形波导1 mm时,喷嘴尖端处的电场强度在1.2×10⁶ V·m^-1以上,远大于氩气的击穿电场强度,更易于等离子体炬的激发。实验结果证明了模拟结果的正确性和装置的有效性。     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

常压微波等离子体炬装置的模拟与设计

介绍了一台低成本的常压微波等离子体炬设备,给出了该设备构造及喷嘴的设计思路,分析了各种气体的非磁化微波等离子体的击穿电场强度,数值求解了设备中矩形TE103谐振腔中的电磁场分布,应用高频电磁场模拟分析软件HFSS优化了喷嘴在波导中的具体位置,并对优化后喷嘴周围的电场分布进行了模拟。模拟结果表明：微波输入有效功率为500 W,喷嘴伸出矩形波导1 mm时,喷嘴尖端处的电场强度在1.2×106 V·m-1以上,远大于氩气的击穿电场强度,更易于等离子体炬的激发。实验结果证明了模拟结果的正确性和装置的有效性。     

A transformation based method to design ultrasound array

In medical ultrasound imaging, the desired lateral field distribution at each focal distance can be obtained by optimal apodization. On the other hand, the lateral field is a function of focal distance. Hence, finding the optimal apodization is a very arduous process. To overcome this, we have introduced a suboptimal method by which optimal apodization can be calculated in any distance through a nonlinear transformation by the knowledge of the optimal one at a distance. This transformation is established on a fact that the lateral field distribution at focal distance can be expressed as the Fourier transform of a nonlinear function of the aperture weighting, instead of direct expression as the Fourier transform of the above. We have applied this method to map the apodization which obtains the desired beam pattern into the apodization which maintains the same properties on the lateral field distribution. For example, applying this method on a 50-elements λ/2 spaced linear array with length D has resulted in apodization that is optimal at distances D or D/2 by precision better than 9%. This method is useful especially in optimization problems, having no explicit constraint on the main lobe width, such as minimizing the sidelobe levels or minimizing main lobe width constrained to a predetermined value of sidelobe level. However, as the results show, this technique provides acceptable results in other cases.     

Circular motion of submillimeter-sized acoustic bubbles attached to a boundary by high-speed image analysis

The circular motion of submillimeter-sized bubbles attached to a boundary in an 18.5 kHz ultrasonic field are investigated experimentally by high-speed photography and image analysis. It is found that the vibration of gas bubbles with diameters of 0.2–0.4 mm is between spherical radial vibration and regular surface fluctuation. Different from the circular motion of suspended bubbles in water, the circular motion of gas bubbles attached to a boundary presents some new characteristics. These bubbles attached to a boundary (wandering bubbles) will rotate around a fixed bubble array (holding bubbles). Both the wondering bubbles and holding bubbles are “degas” bubbles. The primary Bjerknes force acting on wandering bubbles in the acoustic wave field and the secondary Bjerknes force between the wandering bubbles and the holding bubbles strongly affects the circular motion. The circling and residence behavior of gas bubbles is described and analyzed in detail, which is helpful to understand and improve industrial applications such as ultrasonic cleaning, sonochemical treatment, aeration and cavitation reduction.     

Acoustic crystals with Dirac-like cone dispersion mapped to effective zero-index acoustic materials

Different two-dimensional (2D) acoustic crystals are investigated and the conditions under which these acoustic crystals can be mapped to effective zero-index materials are discussed. Acoustic crystals with Dirac-like points can only be mapped to effective zero-index materials when all three of the following conditions are met simultaneously: (i) the Dirac-like point is formed by monopole and dipole moments, (ii) the Dirac-like point is at a low enough frequency, and (iii) in the extended states above the Dirac-like point frequency, there is only one propagating band. The difference between the optical and acoustic regime is the change in the refractive index near the Dirac point frequency.     

一个解决声纳系统同频干扰的新思路

多平台联合作战将是未来海上作战的重要形式之一,多部主动声纳的同时存在使声兼容问题十分突出。由于友舰直达波经历的是单程传播,其强度往往比经历了双程传播的目标回波大的多。通过波形设计可以获得相关系数小的声纳波形,虽然可以从一定程度上缓解声兼容问题,但由于获得的增益有限,并不能从根本上解决这一问题。本文通过对常规检测器的修正,引入能量归一化的概念,有效地消除了能量对检测器的影响,从而解决了这一问题。同时本文还提出了与现有声纳信号处理体系相兼容的频域实现方法,使该算法在工程上的实现成为可能。计算机仿真证明,这一方法是正确有效的。