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.     

Optical tomography applied to speckle photographic measurement of asymmetric flows with variable density

Optical tomography is applied to the speckle photographic measurement of an asymmetric flow field with variable fluid density. The convolution back projection algorithm is used for obtaining the 3-D density distribution. Noise in the experimental data is reduced by spline smoothing. The method is verified with a steady, laminar, axisymmetric helium jet exhausting vertically into the ambient air, and then applied to a non-axisymmetric helium jet for determining the helium concentration. It is found that speckle photographic recordings are very adequate for tomographic reconstruction, because they provide a high number of data points from each projection. The influence of the limited number of projections on the reconstruction quality is particularly investigated. Dedicated to Professor Dr.-Ing. J. Zierep on the occasion of his 60th birthday     

Noisy temperature field reconstruction by wavelet-expansion in OCT measurement

 A wavelet-expansion approach to reconstruct the noisy temperature field in optical computerized tomography (OCT) measurement is described. The reconstruction quality and computational efficiency of this method has been compared with some of the series expansion approach [algebraic reconstruction (ART), simultaneous algebraic reconstruction (SART) and multiplicative algebraic reconstruction (MART)]. Through computer numerical simulation, the affection of the measure errors to the reconstruction result has been studied and a sensitive equation of reconstruction error was established. Received on 22 June 2000 / Published online: 29 November 2001     

An iterative method for temperature gradient measurements in two-dimensional thermal boundary layer flows by speckle photography

An iterative method for measuring two-dimensional refractive index fields induced by convective heat transfer phenomena is presented. Starting from the so-called correction parabola, this reconstruction technique takes into account the local second derivative variation of the refractive index field. The efficiency of the method is analytically defined as a function of non-dimensional parameter and numerically investigated using a ray-tracing code for different classic index profiles and a thermal boundary layer case. Finally, as an implementation test, this algorithm is applied to an impinging jet heat transfer experiment using speckle photography measurement data set. The results show a relative dispersion of 20% compared to the parabola reconstruction when the refractive index gradient becomes more severe.     

Speckle tomography of turbulent flows with density fluctuations

The speckle tomography technique is used for reconstructing both large-scale structures in turbulent flows and the microstructure of turbulence. The technique is based on multi-projectional line-of-sight speckle photography measurements with a subsequent computer-assisted tomographic reconstruction of the interior structure of the flowfield. The large-scale structure is reconstructed using the Radon integral equation, and the microstructure is analysed using a statistical approach and a novel Erbeck-Merzkirch integral transform. Digital speckle photography and speckle tomography methods are described. Numerical simulation of the optical technique is performed using digital ray tracing through a turbulent flowfield. The methods are illustrated by the 3D "averaged" temperature fields in turbulent convective flows obtained earlier and by the recent reconstruction of 3D correlation functions of density variations in turbulent flows. Local values of turbulence (Kolmogorov) microscale are evaluated using these correlation functions and the Erbeck-Merzkirch integral transform The precision of the reconstruction and the spatial resolution achieved are analysed.     

流动显示技术及其现代发展概况

本文回顾了流动显示发展的历史,以及它在流体力学研究中所起的作用。根据获取流动信息的方式不同,流动显示技术可分为两大类型。作为近代技术发展的例子,三维非对称流干涉图的定量计算、片光技术、散斑照相术、激光诱导萤光法、多普勒干涉图象显示速度场、光电信号转换激光干涉仪、以及图象处理和计算机产生数字流谱技术均作了简要的介绍。     

Experimental and Numerical Methods for Exact Subpixel Shifting

An approach to quantifying the errors in digital image correlation (DIC) is presented using experimentally produced images. The challenge arises in creating exact subpixel shifted images in an experiment. This was accomplished via numerical binning of an ultra-high resolution image. The shifted images are then used for a preliminary analysis of 2D correlation software uncertainty and investigation of speckle pattern quality. Because it is often necessary to use numerically shifted images, for uncertainty quantification for instance, the optimum method of Fourier shifting is also presented.     

Holographic tomography by SART and it's application to reconstruction of 3D temperature distribution

Application of holographic tomography to reconstruction of 3D asymmetric temperature distribution is experimentally and numerically investigated. A new simultaneous algebraic reconstruction technique (SART) code for holographic tomography is presented. Important features of SART for reconstruction of temperature field were evaluated using numerical model functions. Data from diffuse-illumination holographic interferograms obtained at twenty different view directions and with 40° range of view angles were used to reconstruct a 3D thermal flow field by SART method. An average reconstruction error of about 5% is estimated. The ability to reconstruct the 3D field using a limited viewing range implies that SART is especially suitable for the holographic tomography of aerodynamic density field.

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Holografische Tomographie mit Simultaneous Algebraic Reconstruction Technik (SART) und ihre Anwendung zur Rekonstruktion von 3-dimensionalen Temperaturverteilungen

Zusammenfassung Die Anwendung der holografischen Tomographie zur Rekonstruktion von 3-dimensionalen asymetrischen Temperaturverteilungen wird experimentiell und numerisch untersucht. Eine neue simultane algebraische Rekonstruktionstechnik (SART) für holografische Tomographie wird beschrieben. Wichtige Eigenschaften der SART zur Rekonstruktion von Temperaturfeldern wurden unter Verwendung numerischer Modellfunktionen ausgewertet. Die numerische Analyse zeigt, daß die SART eine zuverlässigere, stabilere und wirksamere Methode zur holografischen Tomographie bietet.Die Daten von holografischen Interferogrammen mit diffuser Beleuchtung, die aus zwanzig verschiedenen Beobachtungsrichtungen und innerhalb eines Beobachtungswinkels von 40° erhalten wurden, dienten zur Rekonstruktion eines 3-dimensionalen Temperaturfeldes mit der SART-Methode. Der durchschnittliche Rekonstruktionsfehler wird mit 5% geschätzt. Die Fähigkeit 3-dimensionale Felder mit beschränkten Beobachtungswinkeln zu rekonstruieren weist die besondere Fähigkeit des SART für die holografische Tomographie von aerodynamischen Dichtefeldern nach.

     

Evaluation of finite-element calculations in a part-circular crack by coherent optics techniques

Two techniques, speckle photography and holographic interferometry, were used to test three-dimensional finite-element calculations in an internally pressurized cylinder with an external part-circular crack. Opening displacements along the crack line were measured by speckle photography. Radial displacements were obtained from holographic fringe patterns. Good agreement between experimental and numerical data is obtained. Stress-intensity factor variations along the crack front are calculated from numerical results.     

Investigation of the Cross-beam Correlation Algorithm to Reconstruct Local Field Statistics from Line-of-sight Measurements in Turbulent Flows

Optical techniques such as speckle photography and rainbow schlieren deflectometry yield path integrated measurements of deflection angle instead of the local field variable such as density, temperature and/or species concentration. Thus, a reconstruction algorithm is employed to obtain the local properties from the path-integrated measurements. Cross-beam correlation (CBC) algorithm provides the link between path-integrated and local field statistics in time-averaged axisymmetric turbulent flows. Path-integrated statistics are obtained using orthogonal light rays crossing within the turbulent flow. The algorithm assumes local isotropy and negligible correlation between points on orthogonal beams, which is valid strictly in fully turbulent flows. In this study, noise-free synthetic scalar turbulence data are generated and used to determine how different assumptions in the CBC algorithm affect the reconstruction accuracy. Results show that the reconstruction accuracy is excellent for a narrow correlation function (or small integral length scale), while significant errors are incurred in case of a wide correlation function. A procedure to improve the reconstruction accuracy of the CBC algorithm is proposed in this study.     

测量温度场分布的激光散斑照相计量术

本文讨论了激光散斑照相术在测量密度(或温度)扬中的应用。提出了激光散斑——纹影干涉仪的光学系统,可在同一瞬时得到一张散斑图和一张差分干涉图。对沿竖直加热平板自然对流的温度边界层进行了定量地测量。从散斑图和从干涉图中获得的实验结果符合极好。为了说明本方法的应用范围,对一个非稳定的波动甚大的本申灯火焰进行了测量。实验结果表示,激光散斑照相术更适于研究湍流场,因为干涉术在这种情况下已失去其定量计量的作用。     

线性规律功能梯度材料断裂行为实验研究

焦散线方法与高速摄影技术结合,对线性规律功能梯度材料的I型静、动态断裂特性进行实验研究,并利用有限元软件进行数值模拟. 首先推导了线性规律功能梯度材料静、动态裂尖的焦散线初始曲线方程和参数方程,提出通过迭代方法根据功能梯度材料的复杂焦散线方程求解应力强度因子. 为对比材料梯度对断裂行为的影响,利用重力沉降法制备了3种梯度变化规律的试件. 然后对功能梯度材料试件进行三点弯曲实验,通过CCD和高速摄影装置采集试件焦散线图像,提取各种静、动态断裂参数. 最后通过与有限元模拟结果进行对比,证明在功能梯度材料断裂实验中焦散线方法的有效性,并详细分析了不同梯度变化规律对材料静、动态断裂性能的影响.      

Analysis of Speckle Photographs by Subtracting Phase Functions of Digital Fourier Transforms

This paper presents a method for measuring displacements and strain in digital speckle photography that is an alternative to currently used correlation techniques. The method is analogous to heterodyne speckle photogrammetry wherein optical Fourier transforms were taken of individually recorded specklegrams and combined in a heterodyne interferometer where an electronic phase meter measured the phase differences between the two transforms. Here, digital photographs are recorded and Fourier transformed so that their phase functions can be subtracted and fitted to a linear function of the transform coordinates. The effect of different recording and processing parameters is investigated. It is found that incoherent speckles give better results than those formed by coherent laser light. In addition, image correlation is used to process an identical data set so that comparison of the two methods can be made.     

白光散斑剪切照相的统计分析

本文运用统计光学的理论，详细研究了白光散斑剪切照相的统计规律，导出了白光散斑剪切照相法测量位移导数场的条件，指出了物面粗糙度，光源相干性及光路对称性等对散斑条纹的影响。     

Density-sensitive whole-field flow measurement by optical speckle photography

An optical whole-field, line-of-sight method is surveyed that allows the measurement of the refractive deflection angles of light transmitted through a flow with density changes. The method uses the principles of speckle photography. In comparison to classical optical methods such as interferometry, a much higher number of data values can be obtained from one record without the need of interpolating between experimental data. This high signal density is particularly advantageous for analyzing turbulent flows with density fluctuations. Such an analysis is, so far, restricted to the existence of axisymmetric turbulence. A number of applications to this type of turbulent flow are reported, and proposals are made for future developments that will allow this speckle technique to be applied to the study of turbulent shear flow.     

基于TDLAS的层析成像技术TDLAT

可调谐二极管吸收光谱技术（tunable diode laser absorption spectroscopy,TDLAS）已成为超声速燃烧的重要测量手段之一. 为提高其空间分辨能力,需发展基于TDLAS,结合层析成像术的二维断层成像术（tunablediode laser absorption tomography, TDLAT）. 设计了一套基于6 平行光束-旋转测量的新型TDLAT 系统,吸收波长为7 185.6 cm-1 和7 444.3 cm-1 双线,采用分时-直接吸收探测策略. 重建中,使用代数重建算法,先分别反演计算两吸收线的吸收率和吸收比分布,再获得温度和浓度分布. 利用该系统,在CH₄/Air 预混平面燃烧炉上开展初步验证试验. 结果表明,TDLAT 系统可以反演出温度和浓度分布特征,反演的温度分布结果与热电偶测量值吻合较好. 进一步改进该系统,可用于超燃直连台中,测量燃烧室出口气流的温度和组分浓度分布.      

基于TDLAS的层析成像技术TDLAT简

可调谐二极管吸收光谱技术（tunable diode laser absorption spectroscopy，TDLAS）已成为超声速燃烧的重要测量手段之一. 为提高其空间分辨能力，需发展基于TDLAS，结合层析成像术的二维断层成像术（tunablediode laser absorption tomography, TDLAT）. 设计了一套基于6 平行光束-旋转测量的新型TDLAT 系统，吸收波长为7 185.6 cm^-1 和7 444.3 cm^-1 双线，采用分时-直接吸收探测策略. 重建中，使用代数重建算法，先分别反演计算两吸收线的吸收率和吸收比分布，再获得温度和浓度分布. 利用该系统，在CH₄/Air 预混平面燃烧炉上开展初步验证试验. 结果表明，TDLAT 系统可以反演出温度和浓度分布特征，反演的温度分布结果与热电偶测量值吻合较好. 进一步改进该系统，可用于超燃直连台中，测量燃烧室出口气流的温度和组分浓度分布.