Particle tracking velocimetry with an ant colony optimization algorithm

A new concept algorithm based on the ant colony optimization is developed for the use in 2-D and 3-D particle tracking velocimetry (PTV). In the particle matching process of PTV, the ant colony optimization is usually aimed at minimization of the sum of the distances between the first-frame and second-frame particles. But this type of minimization often goes unsuccessfully in the regions where the particles are located very close to each other. In order to avoid this flaw, a new type of minimization is attempted using a physical property corresponding to the flow consistency or the quasi-rigidity of particle distribution patterns. Specifically, the ant colony optimization is now aimed at minimization of the sum of the relaxation of neighbor particles. In the present study, the new algorithm is applied to sets of 2-D and 3-D synthetic particle images as well as the experimental images with successful results.     

Rod-like particles matching algorithm based on SOM neural network in dispersed two-phase flow measurements

A matching algorithm based on self-organizing map (SOM) neural network is proposed for tracking rod-like particles in 2D optical measurements of dispersed two-phase flows. It is verified by both synthetic images of elongated particles mimicking 2D suspension flows and direct numerical simulations-based results of prolate particles dispersed in a turbulent channel flow. Furthermore, the potential benefit of this algorithm is evaluated by applying it to the experimental data of rod-like fibers tracking in wall turbulence. The study of the behavior of elongated particles suspended in turbulent flows has a practical importance and covers a wide range of applications in engineering and science. In experimental approach, particle tracking velocimetry of the dispersed phase has a key role together with particle image velocimetry of the carrier phase to obtain the velocities of both phases. The essential parts of particle tracking are to identify and match corresponding particles correctly in consecutive images. The present study is focused on the development of an algorithm for pairing non-spherical particles that have one major symmetry axis. The novel idea in the algorithm is to take the orientation of the particles into account for matching in addition to their positions. The method used is based on the SOM neural network that finds the most likely matching link in images on the basis of feature extraction and clustering. The fundamental concept is finding corresponding particles in the images with the nearest characteristics: position and orientation. The most effective aspect of this two-frame matching algorithm is that it does not require any preliminary knowledge of neither the flow field nor the particle behavior. Furthermore, using one additional characteristic of the non-spherical particles, namely their orientation, in addition to its coordinate vector, the pairing is improved both for more reliable matching at higher concentrations of dispersed particles and for higher robustness against loss of particle pairs between image frames.     

Retrieval of aerosol size distribution using improved quantum-behaved particle swarm optimization on spectral extinction measurements

An improved quantum-behaved particle swarm optimization (IQPSO) algorithm is employed to determine aerosol size distribution (ASD). The direct problem is solved using the anomalous diffraction approximation and Lambert–Beer's Law. Compared with the standard particle swarm optimization algorithm, the stochastic particle size optimization algorithm and the original QPSO, our IQPSO has faster convergence speed and higher accuracy within a smaller number of generations. Optimization parameters for the IQPSO were also evaluated; we recommend using four measurement wavelengths and 50 particles. Size distributions of various aerosol types were estimated using the IQPSO under dependent and independent models. Finally, experimental ASDs at different locations in Harbin were recovered using the IQPSO. All our results confirm that the IQPSO algorithm is an effective and reliable technique for estimating ASD.     

Direct numerical simulation of particle dispersion in a turbulent jet considering inter-particle collisions

To investigate the behaviour of inter-particle collision and its effects on particle dispersion, direct numerical simulation of a three-dimensional two-phase turbulent jet was conducted. The finite volume method and the fractional-step projection algorithm were used to solve the governing equations of the gas phase fluid and the Lagrangian method was applied to trace the particles. The deterministic hard-sphere model was used to describe the inter-particle collision. In order to allow an analysis of inter-particle collisions independent of the effect of particles on the flow, two-way coupling was neglected. The inter-particle collision occurs frequently in the local regions with higher particle concentration of the flow field. Under the influence of the local accumulation and the turbulent transport effects, the variation of the average inter-particle collision number with the Stokes number takes on a complex non-linear relationship. The particle distribution is more uniform as a result of inter-particle collisions, and the lateral and the spanwise dispersion of the particles considering inter-particle collision also increase. Furthermore, for the case of particles with the Rosin–Rammler distribution (the medial particle size is set d₅₀ = 36.7 μm), the collision number is significantly larger than that of the particles at the Stokes number of 10, and their effects on calculated results are also more significant.     

Tomographic particle image velocimetry

This paper describes the principles of a novel 3D PIV system based on the illumination, recording and reconstruction of tracer particles within a 3D measurement volume. The technique makes use of several simultaneous views of the illuminated particles and their 3D reconstruction as a light intensity distribution by means of optical tomography. The technique is therefore referred to as tomographic particle image velocimetry (tomographic-PIV). The reconstruction is performed with the MART algorithm, yielding a 3D array of light intensity discretized over voxels. The reconstructed tomogram pair is then analyzed by means of 3D cross-correlation with an iterative multigrid volume deformation technique, returning the three-component velocity vector distribution over the measurement volume. The principles and details of the tomographic algorithm are discussed and a parametric study is carried out by means of a computer-simulated tomographic-PIV procedure. The study focuses on the accuracy of the light intensity field reconstruction process. The simulation also identifies the most important parameters governing the experimental method and the tomographic algorithm parameters, showing their effect on the reconstruction accuracy. A computer simulated experiment of a 3D particle motion field describing a vortex ring demonstrates the capability and potential of the proposed system with four cameras. The capability of the technique in real experimental conditions is assessed with the measurement of the turbulent flow in the near wake of a circular cylinder at Reynolds 2,700.     

两相流显微PIV/PTV系统的开发

开发了一个能同时测量两相流中两相速度和细颗粒尺寸分布的显微PIV/PTV系统,其硬件系统包括大功率连续激光器、显微镜、高速摄像机;软件系统由改进的球形颗粒图像识别算法、各种图像处理算法和各种先进的PIV/PTV算法组成。其中改进的圆弧识别算法能够进行更准确地进行曲线分割而能对充满噪音并相互重叠的颗粒图像给出较好的识别结果。应用该PIV系统,可以在微秒和微米数量级上捕获细颗粒/气泡图像,并能较准确地同时得到两相速度、颗粒尺寸和浓度分布。对焚香可吸入颗粒物进行了速度和尺寸的同时测量,得到了较满意的结果。     

Doppler picture velocimetry applied to hypersonics: automated DPV fringe pattern analysis using the FFT method

Doppler picture velocimetry (DPV) is a tool for visualizing and measuring the flow velocity distribution of tracer particles in a laser light sheet. A frequency sensitive Michelson interferometer, tuned for detecting the velocity distribution by the Doppler effect, visualizes the velocity information of tracer particles crossing an illuminating laser light sheet as interference fringe patterns. Many efforts have been done to evaluate best these DPV patterns, in order to obtain the frequency distribution and, by applying the Doppler formula, the velocity profile of the tracers. The first processing method, developed in 1982, relied on manual processing of the pictures by the user, due to the unavailability of suitable high performance picture processing algorithms. This drawback made DPV being considered as a rather time-consuming measurement technique with limited accuracy, compared to existing commercial velocity measurement systems (e.g. PIV). This is no more the state of the art: The new DPV analysis software, presented in this paper, allows automated processing of the interference fringe samples obtained by two images, a reference picture without frequency shift and a Doppler picture containing the frequency shift, using single beam velocimetry. Based on Fast Fourier transformation (FFT), the presented algorithm determines the corresponding velocity profile (in pseudo colours) within only a few seconds on a standard personal computer without user intervention.     

一种SPH应力修正算法及自由表面流中的应用

提出了一种SPH应力修正算法,即模型中的拉应力和压应力分别采用不同的插值核函数和状态方程来处理,改善应力稳定性问题。介绍了一种改进的Quintic核函数,用于改善模型中压应力的稳定性。通过增加钟型核函数的光滑长度,改善模型中拉应力的稳定性。采用该应力修正算法模拟了无重力条件下方形液滴的震荡变形过程,对比分析了不同算法的模拟结果。此外,为进一步验证算法的适用性,模拟了溃坝算例。研究表明,改进的Quintic型核函数明显改善了粒子聚集现象,该SPH应力修正方法可以使液滴具有更均匀的粒子分布以及更光滑的自由表面,有效改善了SPH方法中的压应力不稳定作用以及自由表面流的模拟精度。     

凹形多面体离散单元的水平集函数接触算法

多面体模型理论上可构造任意颗粒形态,然而受单元接触算法的限制,仅用于凸形颗粒材料的离散元模拟。对于具有凹形特征的多面体单元,单个接触点的搜索算法难以精确计算单元间的作用力。考虑多面体单元间存在单个或多个接触点的计算特性,本文发展了适用于凸形和凹形多面体颗粒材料的水平集函数接触算法。该方法通过点-三角形单元距离计算方法和奇-偶数判定方法建立多面体单元的零水平集函数和空间水平集函数,并对水平集函数进行三线性插值,可得到多面体单元间的单个或多个接触点。为检验水平集函数接触算法的可靠性,对球形和凹形多面体颗粒材料的堆积和倒塌过程进行离散元模拟,并分析颗粒形状对堆积密度和休止角的影响规律。     

关于弹体冲击和贯穿混凝土的三维数值模拟

利用FEM和SPH算法相结合给出弹体冲击和贯穿混凝土的三维数值模拟。将弹体划分为四面体有限元网格,混凝土划分为光滑粒子。为形成单元方便,将弹体划分成三个独立的模块并且这三个独立的模块具有拼凑功能。给出光滑粒子-有限元界面的滑移面算法。为了提高计算效率,在程序的前处理中预设界面的从属点和每个光滑粒子的关联点。计算结果与实验结果对比表明,FEM和SPH算法相结合及编制的程序能有效模拟弹体对混凝土的三维冲击和贯穿过程。同时给出在垂直冲击中二维计算与三维计算的差别。     

Investigation of a particulate flow containing spherical particles subjected to microwave heating

Microwave heating of a liquid and large spherical particles that it carries while continuously flowing in a circular applicator pipe is investigated. A three-dimensional model that includes coupled Maxwell, continuity, Navier–Stokes, and energy equations is developed to describe transient temperature, electromagnetic, and fluid velocity fields. The hydrodynamic interaction between the solid particles and the carrier liquid is simulated by the force-coupling method (FCM). Computational results are presented for the microwave power absorption, temperature distribution inside the liquid and the particles, as well as the velocity distribution in the applicator pipe and trajectories of particles. The effect of the time interval between consecutive injections of two groups of particles on power absorption in particles is studied. The influence of the position of the applicator pipe in the microwave cavity on the power absorption and temperature distribution inside the liquid and the particles is investigated as well.     

基于萤火虫群优化的空间非合作目标相对导航粒子滤波算法

研究了空间非合作目标相对导航算法,针对标准粒子滤波的重采样过程导致的粒子贫化现象及其造成的相对导航精度下降问题,分析了萤火虫优化算法的运行机制,提出一种基于萤火虫智能优化算法的改进粒子滤波算法。改进算法通过优化粒子滤波的重采样过程,使粒子群智能的向高似然区域移动,同时在低似然区域也合理保留了部分粒子,保证了粒子的多样性,提高了样本的整体质量。仿真结果表明,改进算法导航精度较标准算法提高了39.35%,达到稳定精度所需粒子数较少,有效抑制了粒子贫化问题。     

A new tracking algorithm of PIV and removal of spurious vectors using Delaunay tessellation

 A new algorithm of Delaunay Tessellation Particle Tracking Velocimetry (DT-PTV in abbreviation) is proposed for tracking particles in images of a PIV system by making use of the Delaunay tessellation (DT). The algorithm is tested by using numerically simulated particle images. The calculation results based on DT are compared with those obtained by a conventional algorithm of Binary Image Cross-correlation method (BICC). The new algorithm shows higher performance of obtaining more identical particles in two consecutive images correctly with shorter computation time even if the images contain many particles. A further application of DT to elimination of spurious vectors is also discussed. Received: 24 November 1997/Accepted: 7 August 1998     

类电磁机制算法研究与改进

针对原算法中初始种群不均匀、局部搜索随机性强及移动公式效率低等问题,提出了一种改进的类电磁机制算法。采用随机化均匀设计方法构造初始种群,将混沌搜索与试探性移动方法相结合进行局部搜索,运用改进后的公式计算粒子之间的合力,将遗传变异思想融入移动公式来更新粒子。实验结果表明,改进后的算法具有更好的收敛效果和更高的求解精度。     

The influence of particle shape,volume fraction and distribution on post-necking deformation and fracture in uniaxial tension of AA5754 sheet materials

Finite element analysis was performed over a small particle field, edge constraint plane strain post-necking model. The aim is to understand the roles of particle shape, volume fraction and distribution over the post-necking deformation and fracture of AA5754-O sheet materials. For models containing one single particle, the post-necking deformation decreases when the particle varies from circular to elliptical. The inter-particle spacing, the major parameter of distribution to determine whether a pair of particles belongs to a stringer or not, was varied for models with two particles of circular or elliptical shape. The general trend is that the post-necking deformation and fracture strains decrease with decreasing spacing between particles. There is considerable difference in terms of both fracture topographies and strains for models containing 16 particles when distributions varied from random/uniform to stringer distributions. The post-necking deformation and fracture strains monotonically decrease with particle volume fractions for models with 4–64 particles of random or stringer distribution. This indicates that the post-necking behavior for AA5754-O alloys where the matrix material is rather ductile is not solely controlled by a single or pair of particles although they may become initiation places of damage. Multiple damaging sources such as stringers or large particles can act cooperatively and speed up the damaging propagation of the material, and therefore produce small post-necking deformation and early fracture. The center clustering of particles can be beneficial for post-necking behavior and bendability of sheet materials.     

An improved SPH model for multiphase flows with large density ratios

This paper presents a new smoothed particle hydrodynamics (SPH) model for simulating multiphase fluid flows with large density ratios. The new SPH model consists of an improved discretization scheme, an enhanced multiphase interface treatment algorithm, and a coupled dynamic boundary treatment technique. The presented SPH discretization scheme is developed from Taylor series analysis with kernel normalization and kernel gradient correction and is then used to discretize the Navier‐Stokes equation to obtain improved SPH equations of motion for multiphase fluid flows. The multiphase interface treatment algorithm involves treating neighboring particles from different phases as virtual particles with specially updated density to maintain pressure consistency and a repulsive interface force between neighboring interface particles into the pressure gradient to keep sharp interface. The coupled dynamic boundary treatment technique includes a soft repulsive force between approaching fluid and solid particles while the information of virtual particles are approximated using the improved SPH discretization scheme. The presented SPH model is applied to 3 typical multiphase flow problems including dam breaking, Rayleigh‐Taylor instability, and air bubble rising in water. It is demonstrated that inherent multiphase flow physics can be well captured while the dynamic evolution of the complex multiphase interfaces is sharp with consistent pressure across the interfaces.     

基于粒子滤波的微小卫星姿态确定算法

为了提高非线性卫星姿态控制系统的滤波性能,在建立了采用磁强计及太阳敏感器的卫星姿态模型的基础上尝试了新兴的粒子滤波(PF)算法对卫星系统进行姿态估计,进而对采用矢量观测的三轴稳定卫星的姿态确定问题进行了滤波算法的实时仿真,并将四元数转换成旋转矢量引入了粒子滤波算法,最后给出了卫星模型在不同粒子数目下的滤波性能比较,并在系统初始误差较大的情况下将粒子滤波算法与EKF滤波算法进行了滤波性能的对照。仿真结果表明,粒子滤波算法对粒子数目具有明显的依赖性,但是当粒子达到一定的数目时,粒子滤波的精度以及滤波稳定性都可以得到保证,尤其是在系统初始误差较大的情况下粒子滤波算法更显示了其优于EKF算法的滤波性能。