基于闪光X射线照相技术的液体爆炸抛撒研究

闪光X射线照相技术相比于可见光高速摄影,具有曝光时间短、穿透能力强和能透视拍摄物体内部信息等优点。本文应用闪光X射线照相技术,对液体爆炸抛撒早期过程壳体材料变形破碎情况、液体受冲击压缩的状态等进行研究,获得了可见光高速摄影等其他测试方法不能得到的实验现象,结果表明:爆炸后液体介质存在密度不均匀区域,爆炸产物气体在不同介质中膨胀规律不同,以及冲击波通过不同介质作用于壳体时,壳体膨胀破碎机制不同。     

Quantitative flow visualization of fluidized-bed under normal- and down-flow-mode operations by neutron radiography

Heat transfer characteristics of tube-banks immersed in a fluidized-bed is dominated by the time-averaged as well as statistical characteristics of bed-material movement, especially, in the neighboring region of heat transfer tube. The neutron radiography and image processing technique have been successfully applied to the visualization of flow field and quantitative measurement of void fraction in the bed. This quantitative visualization technique is verified as a useful means in understanding the flow behavior and thus the heat transfer mechanisms. Received: 4 October 1998 / Accepted: 7 June 1999     

Full-field bubbly flow velocity measurements by digital image pulsed laser velocimetry

Digital Pulsed Laser Velocimetry (DPLV) is a full-field, two dimensional, noninvasive, quantitative flow visualization technique. The technique described here includes the novel use of direct digitization of two-phase bubbly flow images using a high resolution imaging system. The image data is stored for further analysis by new image processing and analysis software developed for flow experiments.In the technique, ten consecutive frames of data separated by a time increment of 150 ms, are recorded. Each of these ten frames contains the images of bubbles at that one instant of time. A program smooths the instantaneous image and calculates bubble parameters. Another program matches the bubbles from each of the frames into tracks of bubbles through time. This program uses a statistical technique to determine the best possible path of the bubbles.The ability of pulsed laser velocimetry to capture simultaneous and quantitative rather than qualitative information along with these image processing techniques gives the experimentalist a powerful tool to perform flow visualization and analysis.     

Three-dimensional tomographic reconstruction of unstable ejection phenomena of droplets for electrohydrodynamic jet

The electrohydrodynamic (EHD) jet is becoming increasingly popular within industrial printing areas based on phenomena induced by electrical potentials. Regardless of the physical observations of unstable ejection phenomena in regions possessing high electric potential, quantitative visualization is still necessary; no report exists exemplifying quantitative visualization. Thus, the size, shape and position of EHD droplets were reconstructed in this study using developed three-dimensional tomography methods. Two computer-synthesized phantoms for the liquid meniscus containing small satellite droplets were generated according to actual images captured by two high-speed cameras. These droplets were made in order to numerically reconstruct droplet behavior. Four three-dimensional tomography methods, such as the algebraic reconstruction technique (ART), the adaptive algebraic reconstruction technique (AART), the simultaneous iterative reconstruction technique (SIRT) and the multiplicative algebraic reconstruction technique (MART), were developed to accurately mimic droplet movement using multiple image views. Four basis functions including the cubic B-spline, cosine, o-Moms and Keys basis functions were adopted in order to improve the performance of the tomographic reconstructions. After completing a comparison of the four tomography results, the MART method in association with the cubic cosine basis function was selected as the means to significantly improve reconstruction accuracy. Additionally, it was the applied method for the reconstruction of the droplet behavior from experimental projections by two cameras.     

A quantitative laser sheet image processing method for the study of the coherent structure of a circular jet flow

A quantitative two-dimensional digital image processing technique is successfully developed to enhance qualitative flow visualization and to obtain quantitative results. The technique is applied to study some less known properties of the coherent structural interaction and evolution mechanism of a low Reynolds number circular jet flow under high level acoustic excitation. Before processing the quantitative data, many inherent errors and uncertainties of the instruments and the system are first discussed and corrected. In this research, the uniformity and the traceability of the flow marker are carefully tested, and the distortion of the imaging system and the fan-shape of the laser sheet are calibrated. Through the image processing technique, the spreading of the jet, the trajectory and the convective velocity of the vortex can be analyzed rapidly and simultaneously. By analyzing the constant jet fluid concentration contour, the mechanism of vortex roll-up and entrainment, which has been ambiguous by traditional pointwise measurements, are more solidly confirmed. Also, the detailed tearing process of the vortex and the evolution mechanism of partial pairing, which can not be clearly detected in the conventional flow visualization pictures, are made clearly visible and carefully delineated.A version of this paper was presented at the 11th Symp. on Turbulence, University of Missouri-Rolla, 17–19 Oct. 1988     

Time-resolved 3D visualization of air injection in a liquid-saturated refractive-index-matched porous medium

The main goal of this work is to implement and validate a visualization method with a given temporal/spatial resolution to obtain the dynamic three-dimensional (3D) structure of an air plume injected into a deformable liquid-saturated porous medium. The air plume develops via continuous air injection through an orifice at the bottom of a loose packing of crushed silica grains. The packing is saturated by a glycerin-water solution having the same refractive index and placed in a rectangular glass container. By using high-speed image acquisition through laser scanning, the dynamic air plume is recorded by sequential tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed in the imaging along the scanning direction. Four image processing methods are presented for the removal of these redundant pixels arising from multiple exposure. The respective results are discussed by comparing the reconstructed air plume volume with the injected one and by evaluating the morphological consistency of the obtained air plume. After processing, a 3D dynamic air flow pattern can be obtained, allowing a quantitative analysis of the air flow dynamics on pore-scale. In the present experimental configuration, the temporal resolution is 0.1 s and the spatial resolution is 0.17 mm in plane and about 1 mm out of plane of the laser sheet.     

动光弹性法中若干问题的近期研究进展

为适应解决工程中越来越多动态力学问题的需求,在近年来计算技术、高速摄影技术、图像处理技术等迅速发展的基础上,本文对动光弹性法中的若干关键问题近期的研究进展作简要叙述。内容包括：(1)新型数字式动光弹仪的研制,这种光弹仪以激光光源取代多火花放电式光源,实现了连续、实时的强光源,以高速摄影系统取代原来多个相机的拍摄系统,利用计算机软件对整个系统装置进行控制,实现了图像的优化采集,为数字化处理图像提供了有力的保障;(2)静、动光弹性材料的探索,重点介绍了动光弹模型材料的新配方和新工艺;该工艺采用硅橡胶模具,无需使用脱模剂,利用常温固化形成环氧树脂板材或块体,可精密浇注,一次到位,材料具有良好的光力学性能;(3)动光弹的工程应用,利用新型数字式动光弹仪器以及环氧树脂浇注新的工艺进行了实际工程的研究,取得了一些有价值的结果。文中以某舰船中的部件舵机和基座及海岸工程中港口关键部位柱、柱帽连接部位为实例进行了动应力分析,证明了新研制的仪器和新材料均满足试验要求,有广阔的发展前景。     

On the fragmentation characteristics of melt jets quenched in water

Experiments were carried out to investigate the characteristics of jet breakup and debris formation after melt jets fall into a subcooled water pool, which may occur in industrial processes such as the interactions of molten corium with coolant during a severe accident of light water reactors. A high-speed visualization system developed previously at KTH was used to capture the jet fragmentation phenomenon. Molten metal (Woods metal or tin) and mixture of binary oxides (WO₃-Bi₂O₃ or WO₃-ZrO₂) were employed separately as melt materials to address different breakup mechanisms (e.g., hydrodynamic vs. thermodynamic fragmentation) and material effect. Moreover, the parameters related to melt and water conditions, including superheat, jet diameter and velocity of melt as well as subcooling of water, were scrutinized for their influences on jet fragmentation characteristics. The experimental data were acquired on the melt jet fragmentation patterns, breakup length, droplet size spectrum, droplet breakup and solidification as well as debris morphology, which can be useful for validation of the codes used for the prediction of debris formation phenomena.     

Quantitative limits of thermal and fluid phenomena measurements using the neutron attenuation characteristics of materials

Temporal and spatial resolution of the neutron radiographic technique were investigated in order to apply this technique to the visualization and measurement of thermal and fluid phenomena. The temporal resolution of three imaging methods of temporally resolved neutron radiography-static neutron radiography with a pulsed neutron beam and high frame rate neutron radiography with either a pulsed or steady neutron beam-was studied. It was determined that the temporal resolution was determined by the sensitivity and light decay time of the image detector and statistical variation of neutrons, and the resolution limits of static and dynamic imaging methods were estimated to be a few microseconds and a few hundred microseconds, respectively. An image processing method was proposed to measure flow characteristics such as void fraction. By performing an error analysis to calculate the limit value of liquid film thickness that can be measured by neutron radiography, it was determined that the limit value of a rectangular channel gap or round tube diameter should be smaller than 3.25 or 4.00 mm, respectively, for measuring the void fraction of air-water flow within an error of 10%. The void fraction measuring method was experimentally confirmed by comparing the void fraction values in a rectangular duct with a 2.4-mm gap obtained by neutron radiography with those obtained by optical and conductance probe methods. It was shown quantitatively that the measurement error decreased when consecutive frames were temporally integrated.     

Liquid film characterization in horizontal,annular, two-phase,gas–liquid flow using time-resolved laser-induced fluorescence

A non-intrusive optical technique was developed to provide time-resolved longitudinal and cross-sectional images of the liquid film in horizontal annular pipe flow of air and water, revealing the interfacial wave behavior. Quantitative information on the liquid film dynamics was extracted from the time-resolved images. The planar laser-induced fluorescence technique was utilized to allow for optical separation of the light emitted by the film from that scattered by the air–water interface. The visualization test section was fabricated from a tube presenting nearly the same refractive index as water, which allowed the visualization of the liquid film at regions very close to the pipe wall. Longitudinal images of the liquid film were captured using a high-frame-rate digital video camera synchronized with a high-repetition-rate laser. An image processing algorithm was developed to automatically detect the position of the air–water interface in each image frame. The thickness of the liquid film was measured at two axial stations in each processed image frame, providing time history records of the film thickness at two different positions. Wave frequency information was obtained by analyzing the time-dependent signals of film thickness for each of the two axial positions recorded. Wave velocities were measured by cross-correlating the amplitude signals from the two axial positions. For the film cross-section observations, two high-speed digital video cameras were used in a stereoscopic arrangement. Comparisons with results from different techniques available in literature indicate that the technique developed presents equivalent accuracy in measuring the liquid film properties. Time-resolved images of longitudinal and cross-section views of the film were recorded, which constitute valuable information provided by the technique implemented.     

应用高速摄影机对泡沫铝在SHPB实验过程的变形跟踪与分析

SHPB技术虽然能方便地得出材料的动态应力应变关系,但却不能直观地揭示材料在经受冲击荷载时的变形特征,而这恰恰是泡沫金属力学性能研究的重要方面.为此,本文设计了一套SHPB-高速摄影机系统,借此系统对SHPB实验中的泡沫铝试件进行了实时的变形跟踪拍摄,然后应用图像处理软件对泡沫铝试件的动态变形进行分析并提取了位移信息,...     

Real-time image processing for particle tracking velocimetry

We present a novel high-speed particle tracking velocimetry (PTV) experimental system. Its novelty is due to the FPGA-based, real-time image processing “on camera”. Instead of an image, the camera transfers to the computer using a network card, only the relevant information of the identified flow tracers. Therefore, the system is ideal for the remote particle tracking systems in research and industrial applications, while the camera can be controlled and data can be transferred over any high-bandwidth network. We present the hardware and the open source software aspects of the PTV experiments. The tracking results of the new experimental system has been compared to the flow visualization and particle image velocimetry measurements. The canonical flow in the central cross section of a a cubic cavity (1:1:1 aspect ratio) in our lid-driven cavity apparatus is used for validation purposes. The downstream secondary eddy (DSE) is the sensitive portion of this flow and its size was measured with increasing Reynolds number (via increasing belt velocity). The size of DSE estimated from the flow visualization, PIV and compressed PTV is shown to agree within the experimental uncertainty of the methods applied.     

Three-component velocity field measurement in confined liquid flows with high-speed digital image plane holography

In the last years, several techniques have been developed for the measurement of the three velocity components in a fluid plane or volume. Techniques as stereoscopic particle image velocimetry (SPIV) or tomographic PIV need a complex set-up and present serious restrictions when applied to confined liquid flows. Other like digital holographic PIV has some limitations in the particle concentration that can be measured. In this work, high-speed digital image plane holography has been applied for the measurement of the three velocity components in a complex geometry brain aneurysm model, using a two-cavity high-speed laser, one double frame camera and normal visualization, like in regular PIV. A portable and compact system has been built for adapting the high-speed laser short coherence length to the measurement of larger areas.     

爆炸加载下金属缝隙射流定量诊断实验研究

爆轰加载下金属飞片缝隙处会有射流产生,金属缝隙射流的喷射速度可达数千米/秒,而其射流线密度只有几个mg/cm量级。采用高速摄影以及脉冲软X光照相方法对缝隙射流进行了动态观测和(半)定量测量,通过不同条件下的系列实验获得了金属射流喷射特性和射流质量随飞片材料、加载压力、缝隙宽度以及加载方式等的变化规律,通过实验数据分析拟合,初步给出了射流质量随各影响因素变化的经验型定标率模型。     

Calibrated multichannel electronic interferometry for quantitative flow visualization

Calibrated multichannel electronic interferometry, a new technique for quantitative flow visualization of transient phenomena, is discussed. This technique uses an interferometer combined with diffraction gratings to generate three phase shifted interferograms simultaneously which are used to perform multichannel phase shifting. The optical system is calibrated with no phase object present using standard piezoelectric phase shifting, and this calibration information is stored as an electro-optic hologram. The calibration information is used along with the three phase-shifted interferograms that exist with a phase object present to perform time-resolved phase shifting. Examples using natural convection and separated flows are presented.     

Vacuum transparent insulation for visualization of solidification phenomena

Experiments on solidification of aqueous solutions are sometimes carried out in tanks with transparent sidewalls to allow visualization of the flow and crystal structure. The tank sidewalls must be thermally insulated and condensation of water vapor on the outer sidewalls must be prevented for clear visualization. In this note a system is presented which is capable of eliminating the water vapor condensation, providing thermal insulation and thus facilitating continuous flow visualization of the solidification process. Some visualization photographs are presented. Received: 19 April 1999 / Accepted: 30 July 1999     

Water-walking devices

We report recent efforts in the design and construction of water-walking machines inspired by insects and spiders. The fundamental physical constraints on the size, proportion and dynamics of natural water-walkers are enumerated and used as design criteria for analogous mechanical devices. We report devices capable of rowing along the surface, leaping off the surface and climbing menisci by deforming the free surface. The most critical design constraint is that the devices be lightweight and non-wetting. Microscale manufacturing techniques and new man-made materials such as hydrophobic coatings and thermally actuated wires are implemented. Using high-speed cinematography and flow visualization, we compare the functionality and dynamics of our devices with those of their natural counterparts. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Multi-Scale Stereo-Photogrammetry System for Fractographic Analysis Using Scanning Electron Microscopy

Current systems for photogrammetry analysis rely mainly on two-dimensional visualization methods, particularly Scanning Electron Microscopy (SEM). The absence of three-dimensional information prevents the determination of important quantitative features such as local roughness and precludes a deeper comprehension of the failure mechanisms. This paper describes a new multi-scale stereo-photogrammetry system for inspection of fracture surfaces based on SEM images. The system facilitates the reconstruction of complete 3D fracture surfaces and provides interactive visualization of the multi-scale structure, thus offering better insight into fracture surfaces at different levels of detail. In particular, a new method has been developed for geometric reconstruction of a 3D textured mesh from SEM stereo images. The mesh is represented as a 3D geometric multi-resolution structure. The sampled images are represented in the form of a multi-scale hierarchical textured structure. Thus, the global shape of the sample is represented by a 3D mesh, while its micro details are represented by textured data. This multi-scale and hierarchical structure allows interactive multi-scale navigation of the 3D textured mesh. The Regions of Interest (ROI) can actually be inspected interactively at different scales by means of optical or digital zooming. Thus, the digital model can be visualized and the behavior of the 3D material can be analyzed interactively. The contributions of this research include: (a) a new 3D multi-scale reconstruction method for SEM stereo images; (b) a new visualization module for multi-scale inspection, modeling and analysis of micro-structures for a variety of materials; and (c) 3D insight into and better understanding of fracture phenomena for material micro-structures. The feasibility of the proposed method is demonstrated on samples of different materials, and a performance analysis is applied on the resulting multi-scale model. The roughness calculation was verified against roughness calculation applied to the optical profilometer.     

Visualization of high-speed gas jets and their airblast sprays of cross-injected liquid

A planar and instantaneous visualization study of high-speed gas jets and their airblast sprays was performed to qualitatively examine the different atomization performances of different gas nozzles. For the visualization of high-speed gas jets (with no liquid injected), Nd:YAG pulsed laser sheets imaged the clustered vapor molecules in the Rayleigh range (d?λ), condensed from the natural humidity during the isentropic gas expansion through a nozzle. This method visualized both underexpanded sonic gas jets from a converging nozzle (SN-Type) and overexpanded supersonic gas jets from a converging-diverging nozzle (CD-Type). When liquid is cross-injected, the same laser sheet images the spray droplets of relatively large sizes (d?λ). The present visualization results show that the SN-Type nozzle develops a wider spray than the CD-Type nozzle, quite probably because the SN-Type nozzle has a wider gas jet (in the absence of liquid) than the CD-Type. Also, the wider spray of the SN-Type nozzle lowers the probability of droplet coalescence and generates finer sprays compared to the CD-Type nozzle. These visualization results qualitatively agree with the previous quantitative finding of the different atomization characteristics of the two types of nozzles (Park et al. 1996).     

Phase discrimination method for simultaneous two-phase separation in time-resolved stereo PIV measurements

A phase discrimination method for two-phase PIV is presented that is capable of simultaneously separating the two phases from time-resolved stereoscopic PIV images taken in a particle-laden jet. The technique developed expands on previous work done by Khalitov and Longmire (Exp Fluids 32:252–268, 2002), where by means of image processing techniques, a raw two-phase PIV image can be separated into two single-phase images according to particle size and intensity distributions. The technique is expanded through the use of three new image processing algorithms to separate particles of similar size (up to an order of magnitude better than published work) for fields of view much larger than previously considered. It also addresses the known problem of noisy background images produced by high-speed CMOS cameras, which makes the particle detection and separation from the noisy background difficult, through the use of a novel fast Fourier transform background filter.