Numerical simulation of single particle acceleration process by SPH coupled FEM for abrasive waterjet cutting

The existing numerical simulations of hydrodynamic characteristics of abrasive waterjet in a cutting head were mainly based on Eulerian grid or arbitrary Lagrange–Eulerian grid method to establish computational fluid dynamics models. However, using these two methods, the abrasive and water were premixed and given an identical initial velocity, which were different from the mixing and acceleration processes of abrasive in the cutting head. This paper presents a more suitable numerical model that the abrasive particle enters into the mixing chamber in a low velocity and is accelerated in the focus tube by a high-speed waterjet from the orifice. In order to model this mixing-and-acceleration process of abrasive and high-speed waterjet, the smooth particle hydrodynamics (SPH) coupled finite element method (FEM) is adopted, in which SPH particles are used to model the high-speed waterjet to adapt its extremely large deformation and FEM is applied to model the discrete abrasive particle, cutting head, and workpiece. As a result, evolution of abrasive and waterjet velocities along focus tube is analyzed; trajectory of single abrasive particle in focus tube is sighted; the relationships between abrasive particle velocities and different water pressures are described; the rule of outlet velocities of abrasive particle vs. dimensionless ratio of diameter is conducted; depth of penetration caused by single abrasive particle impact is obtained. The current model is validated by the existing theoretical and experimental data.     

Abrasive Waterjet Machining Simulation by Coupling Smoothed Particle Hydrodynamics / Finite Element Method

In dealing with abrasive waterjet machining(AWJM) simulation,most literatures apply finite element method(FEM) to build pure waterjet models or single abrasive particle erosion models.To overcome the mesh distortion caused by large deformation using FEM and to consider the effects of both water and abrasive,the smoothed particle hydrodynamics(SPH) coupled FEM modeling for AWJM simulation is presented,in which the abrasive waterjet is modeled by SPH particles and the target material is modeled by FEM.The two parts interact through contact algorithm.Utilizing this model,abrasive waterjet with high velocity penetrating the target materials is simulated and the mechanism of erosion is depicted.The relationships between the depth of penetration and jet parameters,including water pressure and traverse speed,etc,are analyzed based on the simulation.The simulation results agree well with the existed experimental data.The mixing multi-materials SPH particles,which contain abrasive and water,are adopted by means of the randomized algorithm and material model for the abrasive is presented.The study will not only provide a new powerful tool for the simulation of abrasive waterjet machining,but also be beneficial to understand its cutting mechanism and optimize the operating parameters.     

Analysis of Hagen-Poiseuille flow using SPH

This paper shows how to formulate the transient analysis of 2-dimensional Hagen-Poiseuille flow using smoothed particle hydrodynamics (SPH). Treatments of viscosity, particle approximation and boundary conditions are explained. Numerical tests are calculated to examine effects caused by the number of particles, the number of particles per smoothing length, artificial viscosity and time increments for 2-dimensional Hagen-Poiseuille flow. Artificial viscosity for reducing the numerical instability directly affects the velocity of the flow, though effects of the other parameters do not produce as much effect as artificial viscosity. Numerical solutions using SPH show close agreement with the exact ones for the model flow, but SPH parameter must be chosen carefully. Numerical solutions indicate that SPH is also an effective method for the analysis of 2-dimensional Hagen-Poiseuille flow.     

内置周期挡板的T-型微混合器

设计了一种流道内布置周期挡板结构的高效T-型微混合器来提高微流控系统的混合效率。该微混合器结构简单,周期布置的挡板可以有效地缩短流体混合所需的流道长度和时间,混合效率高。安排了正交实验组,利用计算流体力学软件ANSYS CFX研究了流道结构参数对混合效果的影响。采用静态田口分析法对数值模拟结果进行分析。结果表明:流道结构参数对混合效果的相对影响程度排列如下:挡板攻角(θ)流道高度(H)挡板宽度(L)相邻混合单元之间距离(D)。根据结构参数对混合效果的影响程度,得出研究参数范围内的最优组合为:θ=75°,H=0.4 Wm,L=0.7 Wm,D=0.6 Wm(这里Wm为流道宽度,等于200μm)。实验显示,结构参数符合最优参数组合的微混合器的混合效果提升显著,雷诺数Re=54时即可实现完全混合(混合指标M95%)。文中研究了流道结构对进出口压降的影响,结果显示,攻角θ对进出口压降的影响趋势在不同雷诺数下相同,参数H,D亦如此。     

主辅通道型微混合器的设计与制作

为了实现微量液体的快速均匀混合,设计了一种PDMS双层结构的新型微混合器。研究了混合器的制作方法以及几何尺寸和Re数对混合的影响。依据Fick第一定律介绍了主辅通道型微混合器的设计原理;采用有限元方法对不同几何尺寸及Re数下混合器中液体的速度流场及浓度场进行了数值模拟;数值分析显示,随着主辅通道出口宽度比的减小,通道长度的增加和雷诺数的减小,混合器的混合率增加。最后,依据仿真结果制作了主辅通道深度比为0.71,出口宽度比为1,通道长度为9mm的微混合器并进行了去离子水和红墨水的混合实验。实验结果表明:当Re5时,设计的混合器能实现液体的快速混合,并且混合率随着Re的减小而增大,基本满足低Re数下微量液体快速均匀混合的要求。     

不可压流体自由表面流动的SPH数值模拟

根据SPH方法的原理提出了一套模拟流体自由表面流动的方法,场变量及其导数通过核函数插值求取,不需进行差分,也不需要网格,时间积分采用分数步长法,避免了不可压条件带来的压力计算不稳定问题。对流项通过粒子的运动求解,完全消除了数值扩散和自由表面模糊问题。可以模拟飞溅、融合等复杂自由表面现象,并对水坝坍塌这一典型自由表面流动问题进行了模拟,模拟结果与试验吻合良好。     

光滑粒子动力学方法及其应用

光滑粒子动力学(Smoothed Partic le Hydrodynam ics,SPH)方法是近年来得到广泛发展和应用的无网格方法的一个重要分支,它是一种纯Lagrangian方法。本文对现有的光滑粒子动力学方法进行了综述,介绍了该方法的理论基础、连续介质守恒方程、方法稳定性的改善等,重点阐述了边界条件的处理,并给出了SPH方法的算例。最后,介绍了SPH方法的最新进展状况。     

SPH method applied to high speed cutting modelling

The purpose of this study is to introduce a new approach of high speed cutting numerical modelling. A Lagrangian smoothed particle hydrodynamics (SPH)-based model is carried out using the Ls-Dyna software. SPH is a meshless method, thus large material distortions that occur in the cutting problem are easily managed and SPH contact control permits a “natural” workpiece/chip separation. The developed approach is compared to machining dedicated code results and experimental data. The SPH cutting model has proved is ability to account for continuous to shear localized chip formation and also correctly estimates the cutting forces, as illustrated in some orthogonal cutting examples. Thus, comparable results to machining dedicated codes are obtained without introducing any adjusting numerical parameters (friction coefficient, fracture control parameter).     

基于SPH算法的驾驶室底部结构对爆炸冲击波响应数值仿真

针对地雷等简易爆炸装置在车辆驾驶室底部非接触爆炸问题,引入无网格光滑粒子流体动力学(Smoothed particle hydrodynamics,SPH)算法模拟爆炸冲击波作用下车辆底部结构的响应。以四边约束靶板为研究对象,分析靶板在爆炸冲击下的能量、应力变化和破坏形态,通过与传统的任意拉格朗日欧拉(Arbitrary Lagrangian-Eulerian,ALE)固流耦合分析法和经验公式计算结果对比,验证SPH算法应用于处理此类问题的可行性;利用SPH算法对爆炸冲击波作用下驾驶室底部结构进行数值仿真,分析车辆底部的油箱、电瓶支架、驾驶室地板、车架等关键结构的冲击响应,并与试验做出对比验证。仿真结果表明,基于SPH算法的爆炸仿真分析具有精度较高、建模简单、耗费计算资源少等优势,能够应用于爆炸冲击波作用下驾驶室底部板壳结构的响应研究,并为驾驶室底部结构抗爆炸设计提供参考。     

Concentration measurement of a mixture using an infrared transceiver

Microreaction technologies are employed in various applications like space propulsion, atomization, drug delivery and in lab-on-chip applications. Mixing index measurement in micro reactors is essential to assess the performance of the proposed micro mixer. A major roadblock in the design of a micromixer is its assessment and determination of mixing index during the operation due to the complex flow characteristics and mass transfer that occur during the mixing of fluids or reactants. Sensor design is thus vital in analyzing the performance of micro mixers during operation. Infrared sensing is a versatile flow visualization technique which has been effectively used previously to characterize two phase flows in conventional macro channels. The present work develops a robust multiphysics simulation framework and provides a comprehensive characterization of an infrared sensor to perform online assessment of micromixers during their operation. This study considers the case of an optimized passive micromixer described in several literatures and models the microfluidic mixing between silica nanoparticles and distilled water at various concentrations. Further coupling was done by applying the physics of ray optics to understand the performance of an infrared sensor. The variation in infrared sensor output with the mixing index is presented based on the multiphysics coupling. With this analysis, the characteristic output of an infrared sensor can be inferred for any practical microfluidic mixing and microreactor application, thus helping the instrumentation design for online performance assessment of micromixers.     

超声微流混合器内流场试验

制作超声微流混合器,该混合器混合腔直径6 mm,深60 μm,振动膜厚和压电层厚210 μm,测量辅助立管高度2 cm。计算该混合器不同的振动模态,利用粒子图像技术测量实际振动模态图和流场图,分析模态计算结果与试验的误差原因。试验结果表明,混合器在超声激励下腔体内存在平面旋状结构;环流边界处有明显地微粒汇聚现象;一阶模态振动会产生净压差。最后得到压差的高度与压电驱动电压的近似关系。     

RGB色彩模型在气动微混合芯片中的应用

研究提出一种基于气动薄膜的微混合器及基于数字图像的混合效率量化分析方法。给出了利用RGB(Red,Green,Blue)色彩模型、灰度转换模型和方差数学模型对微混合腔内不同试剂的混合程度量化分析方法,并利用聚二甲基硅氧烷(PDMS)材料和软刻蚀方法对微混合芯片进行封装。采用实验研究方式使微混合腔内两种不同颜色试剂充分混合,利用混合程度量化分析方法对微混合腔内混合效率进行量化分析,并与自然对流混合效率进行对比。与传统计算混合效率的方法相比,基于RGB色彩模型的混合效率量化方法更简单、直观、有效和方便。     

Investigation of subsurface damage considering the abrasive particle rotation in brittle material grinding

This paper presents the feasibility study of potential application of recently developed surface defect machining (SDM) method in the fabrication of silicon and similar hard and brittle materials using smooth particle hydrodynamics (SPH) simulation approach. Simulation study of inverse parametric analysis was carried out to determine the Drucker-Prager (DP) constitutive model parameters of silicon by analysing the deformed material response behaviour using various DP model parameters. Indentation test simulations were carried out to perform inverse parametric study. SPH approach was exploited to machine silicon using conventional and surface defect machining method. To this end, we delve into opportunities of exploiting SDM through optimised machining quality, reduced machining time and lowering cost. The results of the conventional simulation were compared with the results of experimental diamond turning of silicon. In the SPH simulations, various types of surface defects were introduced on the workpiece prior to machining. Surface defects were equally distributed on the top face of the workpiece. The simulation study encompasses the investigation of chip formation, resultant machining forces, stresses and hydrostatic pressure with and without SDM. The study reveals the SDM process is an effective technique to manufacture hard and brittle materials as well as facilitate increased tool life. The study also divulges the importance of SPH evading the mesh distortion problem and offer natural chip formation during machining of hard and brittle materials.     

基于SPH法的土壤切削刀具三维数值模拟及优化

为优化刀具结构,减小土壤切削的功耗,采用大型动力学软件LS—DYNA中的光滑粒子流体动力学（SPH）求解器,结合MAT147材料模型就松土刀具对土壤的切割过程进行了三维动态仿真。建立了反旋凿切的数值模拟模型;并结合正交试验法对松土铲耕作条件进行了仿真。仿真结果表明：利用SPH方法对土壤材料进行大应变、高应变率的变形计算是有效的;定性地得到了耕宽与耕深的关系,且结合相关理论将可以更全面合理地对松土铲刀具结构进行优化。     

Characteristics of optimization algorithms applied to the electrode design of a magnetohydrodynamic micromixer

To reduce the time needed for the analysis and diagnosis in a lab-on-a-chip, the sample and reagent inside a micromixer should be efficiently blended within a short time. In this paper, we propose an efficient mixing method based on the magnetohydrodynamic effect, which is driven by Lorentz force. In our development of the proposed mixing system, we optimize the dimensions of five electrodes located at the bottom of the batch-type micromixer in order to shorten the mixing time of the sample and reagent. The optimization algorithms are thus verified as useful tools, enabling a rapid mixing in the micromixer.     

Material deformation and removal due to single particle impacts on ductile materials using smoothed particle hydrodynamics

Smoothed particle hydrodynamics (SPH) was used to simulate the impact of single angular particles on Al6061-T6 targets, and the implications for solid particle erosion were discussed. The results of the simulations were verified by comparison to measurements obtained from impact experiments performed using a gas gun which was specifically designed to accelerate angular particles without disturbing their orientation with respect to the target. Both the simulations and the experiments showed that an increase in impact angle and initial orientation of the particle altered the deformation mechanism of the target material, as noted by other investigators. For impact angles close to normal, a significant amount of target material was extruded and piled up at the edge of the impact craters, due to the limited strain hardening of Al6061-T6. However, for certain combinations of incident parameters, the particle machined the surface and a chip was removed. With appropriate constitutive and failure parameters, SPH was demonstrated to be suitable for simulating all of the relevant damage phenomena, including crater formation, material pile-up and chip separation.     

Evaluation of surrogate models for optimization of herringbone groove micromixer

Surrogate models have been applied to shape optimizations of a micromixer with the aim of assessing the performance of the models. The surrogate models considered include polynomial response surface approximation, Kriging, and radial basis neural network. In addition, a weighted average model based on global error measures is constructed. A mixing index at the exit of the micromixer is used as the objective function. The mixing index is calculated based on Navier-Stokes equations. Two cases of optimization, one with two design variables and the other with three design variables, have been tested. The design variables are selected among the ratio of the groove depth to channel height, the angle of groove, and the ratio of groove width to groove pitch. D-Optimal design generated sampling points are used for sampling. It is found that although the weighted average model does not predict the best optimal point, it does show consistent and reliable performance.     

Numerical simulations of impact flows with incompressible smoothed particle hydrodynamics

A 2D incompressible smoothed particle hydrodynamics (SPH) method is implemented to simulate the impact flows associated with complex free surface. In the incompressible SPH framework, pressure Poisson equation (PPE) based on the projection method is solved using a semi-implicit scheme to evaluate the correct pressure distribution. In this procedure, the PPE comprises the divergence-free velocity condition and density-invariance condition with a relaxation parameter. To test the accuracy and efficiency of the proposed incompressible SPH method, it was applied to several sample problems with largely distorted free surface, including 2D dam-break over horizontal and inclined planes with different inclination angles, as well as the water entry of a circular cylinder into a tank. We mainly focused on the time history of impact pressure on various positions of the solid boundary and temporal evolution of free surface profiles. The results showed reasonably good agreement with experimental data. However, further improvement is needed for extremely high impact flow.     

Investigation of mixing and simulation of an electroosmotic micromixer

An investigation of mixing in a micromixer structure with a novel designed chamber and some ribs in it is carried out through the use of COMSOL Multiphysics software. By designing a new chamber structure, some ribs with considered dimensions, four microelectrodes and using computer simulation, acceptable mixing results are achieved. The performance of the micromixer and pressure requirement with different rib dimensions, with and without microelectrodes is compared. The design under continuous flow conditions using computer simulation is analyzed. By the generation of eddies, flow deviation and electroosmotic effect, mixing performance is enhanced. In spite of obstacles, to have better performance, the pressure need for rapid mixing is decreased. In the final stage, the effects of the AC voltage and its frequency on mixing quality were surveyed. Advantages of this design are easy fabrication, admirable mixing performance, and high integration capability.     

Processing capabilities of micro ultrasonic machining for hard and brittle materials: SPH analysis and experimental verification

Micro ultrasonic machining (micro-USM) is an unconventional micromachining technology that has capability to fabricate high aspect ratio micro-holes, intricate shapes and features on various hard and brittle materials. The material removal in USM is based on brittle fracture of work materials. The mechanical properties and fracture behaviour are different for varied hard and brittle materials, which would make a big difference in the processing capability of micro-USM. To study the processing capability of USM and exploit its potential, the material removal of work materials, wear of abrasive particles and wear of machining tools in USM of three typical hard and brittle materials including float glass, alumina, and silicon carbide were investigated in this work. Both smoothed particle hydrodynamics (SPH) simulations and verification experiments were conducted. The material removal rate is found to decrease in the order of glass, alumina, and silicon carbide, which can be well explained by the simulation results that cracking of glass is faster and larger compared to the other materials. Correspondingly, the tool wear rate also dropped significantly thanks to the faster material removal, and a formation of concavity on the tool tip center due to intensive wear was prevented. The SPH model is proved useful for studying USM of different hard and brittle materials, and capable of predicting the machining performance.