A study on impulsive sound attenuation for a high-pressure blast flowfield

The present work addresses a numerical study on impulsive sound attenuation for a complex high-pressure blast flowfield; these characteristics are generated by a supersonic propellant gas flow through a shock tube into an ambient environment. A numerical solver for analyzing the high pressure blast flowfield is developed in this study. From numerical simulations, wave dynamic processes (which include a first precursor shock wave, a second main propellant shock wave, and interactions in the muzzle blasts) are simulated and discussed. The pressure variation of the blast flowfield is analyzed to evaluate the effect of a silencer. A live firing test is also performed to evaluate four different silencers. The results of this study will be helpful in understanding blast wave and in designing silencers.     

利用光压差分技术筛选细胞的影响参数

本文采用有限体积法建立了交叉型细胞分离模型,提出了一种基于光压差分的细胞筛选仿真方法,分析微流体中细胞筛选的影响因素。基于层流、流体流动粒子追踪、波动光学理论,利用有限元分析法建立了一种交叉型光学颗粒分离模型,研究了利用光压差分技术分离细胞的各种影响因素,其中包括微粒直径,激光功率、温度、光纤直径,分析了微粒在流体中因光辐射压力作用下的偏移距离。实验结果表明:在微流体中,激光功率、细胞直径、温度(20℃)和偏移距离大体上成正比关系,光纤直径和细胞直径在大小相当的情况下光辐射压力能够达到最大值,当激光通过光纤作用于直径分别为3,8和20μm的微粒时,光纤直径为7μm或8μm时光辐射压力最大,所以选用直径为8μm的单模光纤作为一个重要的实验光学器件。所得结论为深入研究细胞筛选影响因素的数值仿真精度提供了参考与借鉴。     

A study on correlation between pressure variations and augmentation of heat transfer in acoustic fields

The present paper investigated the correlation between the acoustic pressure variations and the augmentation of heat transfer in the ultrasonic induced acoustic fields. The augmentation ratios of heat transfer coefficient were experimentally measured and were compared with the profile of the pressure distribution in the acoustic fields predicted by numerical analysis. For numerical analysis, a coupled finite element-boundary element method (coupled FE-BEM) was applied. The results of the present study reveal that the acoustic pressure is higher near two ultrasonic transducers than other points where no ultrasonic transducer was installed. The augmentation trend of heat transfer is similar with the profile of the acoustic pressure distribution. In other words, as the acoustic pressure increases, the higher augmentation ratio of heat transfer is obtained. Numerical and experimental studies clearly show that the acoustic pressure variations are closely related to the augmentation of heat transfer in the acoustic fields.     

Supersonic moist air flow with condensation in a wavy wall channel

The characteristics of Prandtl-Meyer expansion of supersonic flow with condensation along a wavy wall in a channel are investigated by means of experiments and numerical analyses. Experiments are carried out for the case of moist air flow in an intermittent indraft supersonic wind tunnel. The flow fields are visualized by a Schlieren system and the distributions of static pressure along the upper wavy wall are measured by a scanning valve system with pressure transducers. In numerical analyses, the distributions of streamlines, Mach lines, iso-pressure lines, and iso-mass fractions of liquid are obtained by the two-dimensional direct marching method of characteristics. The effects of stagnation temperature, absolute humidity, and attack angle of the upper wavy wall on the generation and the locations of generation and reflection of an oblique shock wave are clarified. Futhermore, it is confirmed that the wavy wall plays an important role in the generation of an oblique shock wave and that the effect of condensation on the flow fields is apparent.     

Investigations of the watershield effect on blast waves: One and two-dimensional study

The object of this analysis is to investigate the mitigation effects of watershield on blast waves numerically. One application of current work is to resolve the design concerns for the ammunition storage facilities. To verify the numerical procedure using a multimaterial Eulerian finite element method, the results are compared with the available experimental data for detonation in a pressured tank, and the analytical predictions for air shocks. Features of the free-field detonation process are then studied from a series of one-dimensional simulations. The magnitude of peak pressure decreases and shock arrival time increases with increasing thickness of watershield. For design analysis, the case of two-dimensional axisymmetric geometry (a vertical) right-circular cylinder) with a central charge is also considered. The full process including initial detonation, shock wave propagation and reflection from the outer rigid boundary is examined. For the explosives immersed in water, the magnitude of peak pressure becomes smaller than those in air tank without watershield. At later time period, the average gas pressure left in the air tank is about 20 bar.     

Analysis and attenuation of impulsive sound pressure in large caliber weapon during muzzle blast

Due to the supersonic speed at which propellant gas flows through the gun barrel, a high intensity impulsive sound pressure is created, which has negative effects in many respects. Therefore, the high pressure waves generated due to muzzle blast flow of tank gun during firing is a critical issue to examine. The purpose of this paper is to study and analyze this high pressure impulsive sound, generated during the blast flow. The large caliber 120 mm K1A1 tank gun has been selected especially for this purpose. An axisymmetric computational domain has been constructed by employing Spalart Allmaras turbulence model to evaluate pressure and sound level in the tank gun using Computation Fluid Dynamics technique. Approximately 90% of pressure and 20 dB of sound level have been attenuated due to use of the three baffle silencer at the muzzle end of the gun barrel in comparison to the tank gun without silencer. Also, the sound pressure level at different points in the ambient region shows the same attenuation in the results. This study will be helpful to understand the blast wave characteristics and also to get a good idea to design silencer for large caliber weapon system.     

Separation of the reservoir and wave pressure and velocity from measurements at an arbitrary location in arteries

Previous studies based on measurements made in the ascending aorta have demonstrated that it can be useful to separate the arterial pressure P into a reservoir pressure P* generated by the windkessel effect and a wave pressure p generated by the arterial waves: P = P*+p. The separation in these studies was relatively straightforward since the flow into the arterial system was measured. In this study the idea is extended to measurements of pressure and velocity at sites distal to the aortic root where flow into the arterial system is not known. P* is calculated from P at an arbitrary location in a large artery by fitting the pressure fall-off in diastole to an exponential function and assuming that p is proportional to the flow into the arterial system. A local reservoir velocity U* that is proportional to P* is also defined. The separation algorithm is applied to in vivo human and canine data and to numerical data generated using a one-dimensional model of pulse wave propagation in the larger conduit arteries. The results show that the proposed algorithm is reasonably robust, allowing for the separation of the measured pressure and velocity into reservoir and wave pressures and velocities. Application to data measured simultaneously in the aorta of the dog shows that the reservoir pressure is fairly uniform along the aorta, a test of self-consistency of the assumptions leading to the algorithm. Application to data generated with a validated numerical model indicates that the parameters derived by fitting the pressure data are close to the known values which were used to generate the numerical data. Finally, application to data measured in the human thoracic aorta indicates the potential usefulness of the separation.     

水载压力对管道超声导波信号的影响研究

为了研究水载压力对管道超声导波信号的影响,利用半解析有限元的方法分析了管道导波的频散曲线和波结构,选择 L(0,2)模态导波用于实验验证。 设计了一个可承受 30 MPa 压力的防水卡箍,对管道上的压电传感器进行封装。 将安装了防 水耐压卡箍的管道放入压力舱中进行打压循环测试,压力在 0~ 30 MPa 之间变化。 打压过程中采集无损伤及 5% 损伤的管道超 声导波信号,分析水载压力对导波信号的影响。 实验结果表明,该防水卡箍具有良好的防水和耐压性能,水载压力对 L(0,2)模 态导波信号幅值没有显著影响。     

一种磁致伸缩式超声波激发/接收传感器的研究

以研制磁致伸缩式超声波传感器为目标，分析了磁致伸缩式超声波传感器的工作原理——磁致伸缩效应及逆效应，重点讨论传感器结构设计及线圈参数选择计算，制作了磁致伸缩式超声波传感器。本文研制的传感器在脉冲激励电流的作用下，可在细丝中激发超声波，同时还可以接收细丝中传播的超声波；该传感器既可以激发扭转波，又可以激发纵波。可用于测量液位、温度、液体密度和材料的声速、弹性模量等物理量。     

基于Micro-PIV的微观流场检测技术

介绍了一种针对微观流场检测数字粒子图像测速技术,通过添加荧光显微装置,改进激光入射与照明方式、聚焦平面控制以及纳米级示踪粒子布朗运动误差消除算法等软硬件方面的技术创新,解决了微观检测的关键问题。针对微流体特征尺度小、精度要求高的特点,建立了具有自主知识产权的Micro－PIV微流场测试平台,并就几种典型微观流场进行了检测。试验结果表明该测试技术可对微米级稳态、瞬变流场进行全流场速度分布的检测,各项性能指标达到国际先进水平。     

Projection of the rotation form Navier-Stokes equation onto the half-staggered grid

A projection method for computing incompressible fluid flow is proposed. For the method, the rotation form Navier-Stokes equation (NSE), for which the velocity and the total pressure are employed, is discretized on the half-staggered, finite difference spatial grid. The total pressure couples the static pressure gradient and the convection of momentum in the continuous NSE while the half-staggered grid provides weak pressure-velocity coupling in discrete space. These two features interact synergistically for the discretized NSE to produce smooth pressure fields without additional numerical artifacts such as the momentum interpolation. The method preserves the kinetic energy at the inviscid limit condition. Numerical solutions of the decaying Taylor vortex, the inviscid Taylor vortex, the sudden expansion channel and the square-prism wake are presented.

     

A tabletop femtosecond time-resolved soft x-ray transient absorption spectrometer

A laser-based, tabletop instrument is constructed to perform femtosecond soft x-ray transient absorption spectroscopy. Ultrashort soft x-ray pulses produced via high-order harmonic generation of the amplified output of a femtosecond Ti:sapphire laser system are used to probe atomic core-level transient absorptions in atoms and molecules. The results provide chemically specific, time-resolved dynamics with sub-50-fs time resolution. In this setup, high-order harmonics generated in a Ne-filled capillary waveguide are refocused by a gold-coated toroidal mirror into the sample gas cell, where the soft x-ray light intersects with an optical pump pulse. The transmitted high-order harmonics are spectrally dispersed with a homebuilt soft x-ray spectrometer, which consists of a gold-coated toroidal mirror, a uniform-line spaced plane grating, and a soft x-ray charge coupled device camera. The optical layout of the instrument, design of the soft x-ray spectrometer, and spatial and temporal characterizations of the high-order harmonics are described. Examples of static and time-resolved photoabsorption spectra collected on this apparatus are presented.     

Fast charge-simulation procedure for determining the electron-optical properties of electrostatic systems

A charge-simulation method is outlined which allows a rapid calculation of the potential distribution of two- and some three-dimensional electrostatic systems. The potential of the true charge distribution is approximated by that of line and sheet charges, slit apertures and arrays of line charges. The trajectories of electrons moving in the resulting fields are calculated by using the Runge-Kutta method. Contrary to the well known finite element and finite difference methods, the charge-simulation method allows a fast and precise calculation of the electron trajectories. The power of the proposed method is demonstrated for two special arrangements.     

A numerical study on the triboelectrostatic separation of pvc materials from mixed plastics for waste plastic recycling

We investigate the triboelectrostatic separation of Polyvinylchloride (PVC) from mixed plastics in the laborartory scale triboelectrostatic separation system. The flow and electric fields in the precipitator are obtained from the numerical solution of finite volume method. Using these flow and electric fields, we solved the particle motion equation considering the inertia, drag, gravity and electrostatic forces acted on the particles. The particle trajectories are obtained using a Lagrangian method as a function of different important variables such as Reynolds number. Stokes number, electrostatic force, electric charge and electric field distribution, inclined angle of plane electrodes, particle rebounding, particle charge decay rate after impact on the electrode surface, etc., in order to determine the optimal design conditions. The present predicted results for the cumulative yield represent well the experimental ones.     

基于近似因子分解法的铸造充型过程数值模拟研究

目前求解流动过程中速度场、压力场最常用的方法是SOLA法,在求解过程中由于需要在连续性方程、动量方程间反复迭代,所以计算效率不高。采用近似因子分解法进行速度场、压力场求解,由于在求解过程中每一步均可采用托马斯算法求解三对角方程,所以计算简单,可有效提高计算效率。最终建立了基于有限差分法的流动场数值模拟程序,并针对Benchmark件进行了充型过程流动场数值模拟,通过对比模拟结果与实验结果,验证了计算模型的正确性。     

光纤电流传感器传感头的结构与原理

光纤电流传感器是以法拉第磁光效应为基础、以光纤为介质的新兴电力计量装置，它通过测量光波在通过磁光材料时其偏振面由于电流产生的磁场的作用而发生旋转的角度来确定被测电流的大小。传感头是光纤电流传感器最为重要和关键的部件。分析了全光纤型和混合型光纤电流传感器传感器传感头的结构和工作原理，对改进光纤电流传感器的设计，提高光纤电流传感器的性能具有重要的指导作用。     

A table for experimental study of mechanoluminescent pulse pressure transducers

The procedure and results of experimental study of mechanoluminescent pressure transducers converting mechanical stress into an optical signal are described. A prototype of the mechanoluminescent pulse pressure transducer and a mechanical shock table for laboratory study of it are presented. The developed table allows one not only to fix and evaluate the output luminous flux of the transducer, but also to quantitatively determine the mechanical loading value. Comparison of recorded oscillograms of energy-emittance pulses of the transducer and results of numerical calculations demonstrates the adequacy of the used mathematical model of mechanoluminescent conversion.     

微粗糙光学表面与多个镶嵌粒子差值散射场特性

本文基于时域有限差分算法,研究了微粗糙光学表面与多个镶嵌粒子的差值场光散射问题。将光学基片视为微粗糙光学表面,利用蒙特卡洛方法解决了光学表面存在粗糙度的问题,并将差值场散射理论加入到计算模型中,更好地分析了缺陷粒子的散射特性,将计算区域划分成上下两个半空间,建立了微粗糙光学表面与镶嵌多体粒子复合散射模型,并与矩量法计算结果比较验证了理论的有效性。运用此模型分析了入射角、镶嵌粒子尺寸、粒子间距、粒子个数等物性特征对微粗糙光学表面与镶嵌多体粒子差值散射场的影响。实验结果表明:在一定激光入射角下,以相同回波探测角度间距20°对光学表面进行测量能够有效地检测出缺陷粒子。本文结果为光学无损检测、光学薄膜、微纳米结构的光学性能设计等提供了理论依据和技术支持。     

Blast attenuation in Cymat foam core sacrificial claddings

The mechanism of air blast attenuation in a sacrificial cladding comprising a steel cover plate and a foam core is examined applying an analytical and numerical analysis. For the range of the analysed pressure pulses, the maximum magnitudes of the induced compaction velocities were usually smaller than the acoustic wave speed in the foam material and caused a non-uniform foam density distribution along the compacted region. A significant density decay measured from the proximal end of the cladding was observed within the compacted foam zone due to a rapid reduction of the compaction velocity. An analytical model was developed to reveal the characteristic features of the foam compaction under a moderate velocity impact. It was shown that waves of strong discontinuity, such as shock waves resulting from an overtaking of the stress increments cannot be initiated in the foam. It was revealed, however, that the stress wave propagation in the form of an unloading plastic wave has a significant dynamic effect on the foam compaction and energy absorption. It was concluded that a quasi-static approach cannot be used to accurately estimate the absorbed energy, at least for partially compacted foam. The predictions of the proposed analytical model are compared with other published analytical models for a foam compaction under a high and low velocity impact and with the results from the numerical simulations of the foam deformation in a finite thickness sacrificial cladding subjected to a blast loading. The numerical model was verified with blast loading tests of 50 mm thick Cymat foam claddings having a density of 253 kg/m³. The influence of the cladding characteristics on the reflected stress from the stationary end of the cladding was also analysed when using the proposed model for foam compaction.     

螺旋波纹管导波检测技术的数值仿真和试验研究

为对螺旋波纹管进行缺陷检测,研究纵向模态超声导波在螺旋波纹管中的传播特性。在Abaqus仿真软件平台中构建螺旋波纹管的计算模型,得到含有环向裂纹缺陷时的仿真结果。在此基础上,采用自主研制的电磁声换能器进行试验研究。试验所用的激励信号为经Hanning窗调制的20个振荡周期的正弦脉冲,中心频率为240 kHz,采用单一传感器在螺旋波纹管中激励和接收L(0, 2)模态导波,实现对螺旋波纹管中人工裂纹缺陷的识别,分析并计算缺陷位置测量不确定度。有限元仿真和试验结果表明,超声导波技术可以实现对螺旋波纹管裂纹缺陷的检测。