基于科赫边界模型的流动换热数值分析

基于分形理论简述了科赫边界模型,采用数值模拟的方法分析了稳定流下科赫边界模型(i=1)温度场、速度场和压力场的分布特点,并与振荡流下科赫边界模型的流动换热情况做对比分析。表明:在不同入口速度的工况下,K1模型的流场呈现流道中间温度低、速度大、压力高,流道边缘温度高、速度小、压力低的现象,且振荡流具备强化换热能力。     

速度剪切流中圆柱体绕流特性的数值模拟

采用由速度梯度、圆柱半径以及圆柱中心平面上的来流平均速度定义的无量纲剪切参数β来描述速度剪切的强度。在雷诺数Re=60-1000范围内,运用三维直接数值模拟(DNS)和大涡模拟(LES)两种方法对速度剪切流中的圆柱体的绕流特性进行了数值模拟。研究了驻点、高速和低速两侧分离点、圆柱表面压力分布以及不稳定尾流结构随剪切参数的变化及其对雷诺数的依赖性,从而得到了剪切流中圆柱体的气动力以及脱落特性,并对其机理进行了详细探讨。     

不同结构均质阀内流场的数值模拟与对比研究

根据相关实验数据,应用商用CFD软件Fluent对Stansted均质阀、射流均质阀、倒Z环均质阀内部流场的压力和速度等参数进行数值模拟研究.结果表明:任何一种均质阀流场内,压力骤减均发生在流道最窄处的始端附近,在流道最窄段均质压力沿流道释放;压力梯度不能唯一决定均质破碎效果;在流道最窄段的始端位置,速度变化与压力骤变...     

叶顶吸气效应抑制叶片振动的研究

吸气可以改变叶片周围的气流特性,多用来控制叶片吸力面流动分离。将吸气应用于叶片减振,通过模型实验研究了吸气对叶片振动的影响,改变吸气位置和吸缝长度,测量了不同条件下的叶片振幅并作比较。数值模拟计算了不同位置、不同速度吸气对叶片周围流场的影响,重点分析了叶顶间隙气流速度的变化。数值计算在一定程度上解释了实验结论,二者所得的规律统一。最终结果表明:叶片正上方或上方偏来流侧吸气有明显减振效果,模型实验条件下,振幅最大降低了27%。     

扰动情况下双油槽圆形轴瓦滑动轴承性能分析

采用FLUENT软件及其提供的气穴模型,计算了双油槽圆形轴瓦滑动轴承的油膜压力分布及气穴存在区域大小和位置,并同其他边界条件进行了对比;讨论了进油压力对轴承的流场和承载能力的影响;利用自行改进的动网格技术对轴颈扰动速度对流场和油膜力的影响进行了非稳态计算。计算结果表明,进油压力对油膜力的影响较大,油膜压力受轴颈扰动速度的影响很大,油膜力与轴颈扰动速度明显呈非线性关系,利用改进的动网格技术可以很好地对轴承的动特性进行分析。论文为今后大扰动条件下转子-轴承系统的非线性动特性分析打下了基础,为滑动轴承的设计提供了一种理论参考依据     

不同多相流模型在航行体出水流场数值模拟中的应用

基于VOF和Mixture两种均相流模型,并结合输运方程类空化模型、k-ε湍流模型和动网格技术,针对圆柱形航行体出水过程进行了数值模拟,获得了无空化、带空泡两种状态下航行体出水过程中的多相流物理景象和表面压力变化历程。根据计算结果分析了VOF模型、Mixture模型在多相流界面捕捉、压力计算等方面的异同,并给出了两种模型的适用范围,在理论研究和工程应用上都具有重要意义。     

基于大涡模拟考虑叶片停机位置大型风力机风振响应分析

为研究停机状态下不同叶片位置对大型风力机塔架-叶片体系风振响应的影响,以某3 MW大型水平轴三叶片风力机为研究对象。基于大涡模拟(LES)方法对叶片八个不同停机位置下的风力机体系流场和气动力性能进行了数值模拟,并通过与国内外实测数据对比验证了该方法的有效性;结合有限元法对考虑叶片不同停机位置的风力机塔架-叶片耦合模型进行了动力特性和风振响应时域分析。主要结论为:不同叶片停机位置对风力机塔架绕流特性和气动力分布影响显著,塔架中上部风振响应受叶片不同停机位置的影响最大,尤其是迎风面和背风面的脉动位移均方差响应较大,相应最大值出现在工况八下塔顶350°位置;最大塔底弯矩出现在工况二的环向330°位置;工况五下三叶片叶尖顺风向位移响应峰值2.5 m;研究表明在进行大型风力机抗风设计时应考虑不同叶片停机位置的影响。     

箱梁断面静风力系数的CFD数值模拟

用计算流体力学(CFD)方法模拟了桥梁跨中断面周围的风场特征,不仅能得到流场的压力、速度和涡旋的分布,还提取了箱梁断面的三分力系数。分别采用不同密度网格划分对主梁断面进行数值模拟,并将数值模拟结果与风洞试验值进行比较,选取最合理的网格划分方法。然后,对某一大跨度桥梁跨中断面的箱梁模拟了从-5°至+5°共11个整数度风攻角工况的三分力系数。并将数值模拟结果与风洞试验进行了对比,给出了不同攻角下的压强和速度分布,验证了采用CFD技术模拟桥梁三分力系数方法的可行性与可靠性。     

PET薄膜挤出成型有限元模拟和阻流分析

设计了厚度为0.12mm的聚对苯二甲酸乙二醇酯(PET)薄膜衣架式挤出流道,使用有限元软件模拟了聚合物熔体的流动规律,获得了流道内部的压力场、速度场和温度场的分布,分析了阻流设计对流场的影响。研究表明,流道结构对压力分布影响显著,对速度、温度分布影响不明显;流道阻流部分尺寸较小的改变,会引起流道内部熔体压力较大的变化,易导致挤出成型过程的不稳定,从而显著影响产品质量。此外,挤出流量对流场压力分布影响很小。     

爆炸驱动多层球形破片初速场分析

为准确计算多层球形破片在爆炸驱动下的初速场,通过对装药结构的等效分析,基于Gurney假定和相邻层颗粒之间力和力的波动量等概率传递假定,忽略排列方式引起的孔隙率变化,应用动量和能量守恒建立了破片初速场的理论计算模型。该模型反映了炸药参数和破片的密度、层数和直径等因素对破片初速的影响;针对典型的爆炸驱动前向多层破片模型,用LS-DYNA3D非线性有限元程序对多层钨球破片的爆炸驱动过程进行数值模拟,开展了相关验证试验并分析了理论计算值与试验误差产生的原因,分析讨论了不同球形破片直径和不同破片层数下破片初速的变化情况。结果表明:理论计算值与数值模拟及试验结果吻合较好;随着相同直径破片层数的增大,破片初速减小,相邻层间破片的速度差值更大;层数相同时,随着破片直径的减小,破片初速增大,但相邻层数破片的速度差值更小。     

Research on the underlying mechanism behind abrasive flow machining on micro-slit structures and simulation of viscoelastic media

In this study, the machining mechanism of abrasive flow machining (AFM) microstructures was analyzed in depth according to the transmission morphology and rheological behaviors of the abrasive media. The transmission morphology demonstrated the excellent combination of the polymer melt with abrasive grains at the interface, indicating that the polymer melt, combined with the uniform distribution of the polymer chains, could exert a harmonious axial force on the abrasive grains. Based on the rheological behavior analysis of the abrasive media, for example, the stress relaxation and moduli of storage and loss, a machining mechanism model was established incorporating the effect of microplastic deformation and continuous viscous flow, which was further verified by the grooves along the flow direction. In addition, the PhanThien-Tanner (PTT) model combined with a wall slipping model was employed to simulate the machining process for the first time here. The value of the simulated pressure (1.3 MPa) was similar to the measured pressure (1.45 MPa), as well as the simulated volumetric rate (0.011 4 mL/s) to the measured volumetric rate (0.067 mL/s), which further proved the validity of the simulation results. The flow duration (21 s) derived from a velocity of 1.2 mm/s further confirmed the residual stretched state of the polymer chains, which favored the elasticity of the abrasive media on the grains. Meanwhile, the roughly uniform distribution of the shear rate at the main machining region exhibited the advantages of evenly spread storage and loss moduli, contributing to the even extension of indentation caused by the grains on the target surface, which agreed with the mechanism model and machined surface morphology.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00395-0     

An Analytical Model for Material Removal in Abrasive Jet Machining for Brittle Materials

Optical microscopy has indicated that the abrasive particles used in abrasive jet machining have sharp edges with the shapes similar to cone or pyramid. Moreover, microscopic examination of cross section of work samples eroded by AJM shows that, for brittle materials, material removal is due to intersection and propagation of cracks produced by adjacent impacting particles on target surface. An analytical model has been developed based on the above observations for predicting the material removal in abrasive jet machining process. The model also suggests the critical value of mass flow rate which has been substantiated experimentally. In addition, the influence of velocity and mass flow rate of abrasive particles on material removal rate is briefly described.     

Parametric optimization of magnetic‐field‐assisted abrasive flow machining by the Taguchi method

Some hybrid‐machining processes have been developed in the recent past with a view to devising composite machining processes, which are able to overcome the limitations of one process with the help of advantageous features of another similar process. The present paper identifies the parameters of abrasive flow machining (AFM) that significantly affect the material removal when a magnetic field is applied around the workpiece. The Taguchi method has been adopted for studying the effect of magnetic‐field‐assisted AFM parameters, individually, on the abrasion rate of work materials. Optimization of the process parameters has been carried out for the purpose of off‐line monitoring of the process. Copyright © 2002 John Wiley & Sons, Ltd.     

磨料水射流对脆性材料的冲蚀研究

磨料水射流技术作为一种特种加工技术，具有无刀具接触、无热影响区和加工范围广等优势，在众多领域得到应用。为了探究磨料水射流对脆性材料的冲蚀效果，构建和设计了磨料水射流外流场冲蚀仿真模型与磨料水射流冲蚀实验。以30 mm×50 mm的喷嘴外流场域为计算域，建立磨料水射流冲蚀仿真模型，并分析射流冲蚀过程中压力分布、水与磨料的速度分布及它们在射流中心线上的衰减规律。通过对氧化铝陶瓷材料的冲蚀实验，分析工艺参数对冲蚀孔径的影响，并结合仿真结果对比分析了射流束宽度与冲蚀孔径的关系。结果表明：水的速度随着喷嘴距离的增大而减小且分布范围变宽，射流宽度呈线性增大，磨料速度随喷嘴距离的增大而减小且分布范围基本不变；射流中心线上水的速度与磨料速度呈三段式衰减，水的第1段速度衰减段长度比磨料的长，但水的第2段速度衰减段长度比磨料的短；射流束能量的有效利用部分逐渐减小，但在15~25 mm的靶距范围内其有效利用部分较稳定，为40%；冲蚀孔径随喷嘴距离增大呈线性增大。研究结果为磨料水射流切割、铣削及抛光加工的参数选择提供实验依据，同时为磨料水射流加工过程仿真提供参考。     

Parametric Optimization of Centrifugal Force-Assisted Abrasive Flow Machining (CFAAFM) by the Taguchi Method

Extrusion honing, known as abrasive flow machining (AFM), deburrs, polishes, and radiuses surfaces and edges by flowing an abrasive-laden media over these areas. The process is particularly used on internal shapes that are difficult to process by other nonconventional machining processes. Because abrasive action occurs only in areas where the flow is restricted, tooling is used to direct the media to the appropriate areas. Like other nonconventional machining processes, AFM has the limitation of lower material removal rates. The application of centrifugal force (by using rotating rectangular rod inside the hollow workpiece) has been explored for the productivity enhancement of the process. This article reports that centrifugal force enhances the material removal rate (MRR) and improves the scatter of surface roughness (SSR) value in AFM. It outlines the development of a system that determines sets of viable process parameters for a new process called centrifugal force-assisted abrasive flow machining (CFAAFM). Cylindrical workpieces of brass are used for the experiment. During the experiments, parameters, such as rotational speed of rectangular rod, extrusion pressure, and grit size, were varied to explore their effect on material removal and scatter of surface roughness. Taguchi's parameter design strategy has been applied to investigate the effect of process parameters on the MRR and SSR values.     

不同雷诺数条件下静止管道车环状缝隙流场数值模拟

为了探究雷诺数对于静边界环状缝隙流场的影响,采用Fluent软件对雷诺数为115513、154018、192522三种工况下静止管道车环状缝隙流场进行了数值模拟,并通过物理试验进行验证,结果表明:同一雷诺数条件下,在环状缝隙入口处流速最大,之后沿管道车车身方向,环状缝隙流流速呈现先减小后增大的变化趋势,且环状缝隙流流速大于管道内水流的平均流速;各横断面环状缝隙流场均以管道中心呈同心圆分布,即半径相同的各点环状缝隙流流速值大致相同,且从管道车壁面到管壁面方向环状缝隙流流速呈现二次抛物线的变化形式;不同雷诺数条件下,管道内压强值沿程变化趋势大致相同,管道车上游位置处压强沿程呈现逐渐降低的变化趋势,环状缝隙内部和管道车下游位置处的压强值沿程呈现先升高后降低的变化趋势;随着雷诺数的增加,环状缝隙流流速值与压强值和涡量值均呈现出逐渐增大的变化趋势;数值模拟结果同试验结果基本吻合,两者所得环状缝隙流流速最大相对误差不超过7%,表明通过数值模拟来研究管道车环状缝隙流场的方法是可行的。     

Mechanism of material removal in the magnetic abrasive process and the accuracy of machining

In this paper, we focus on the magnetic abrasive process as a sizing process and present a theory which explains the out-of-roundness error phenomenon based on force analysis of the material removal mechanism. The theory is independent of the size and material of the workpiece and the design of the magnetic abrasive machine tool. The theory results are verified experimentally. The limiting accuracy of the magnetic abrasive process, the machining time required to achieve specified accuracy of the workpiece, and the material removal rate are determined.     

Parametric optimization of abrasive water-jet machining processes using grey wolf optimizer

Abrasive water-jet machining (AWJM) is a hybrid advanced machining process, which can be economically applied to machine almost any kind of material. It employs a high velocity waterjet to propel abrasive particles through a nozzle on the workpiece surface for material removal. The machining performance of AWJM process naturally depends on its several control (input) parameters, like water pressure, nozzle diameter, jet velocity, abrasive concentration, nozzle tip distance etc., which have also predominant effects on its responses, i.e., material removal rate, surface roughness, overcut, taper etc. In this paper, a new evolutionary algorithm, i.e., grey wolf optimizer (GWO), a technique based on the hunting behavior of grey wolves, is applied for finding out the optimal parametric combinations of AWJM processes. The main advantage of this algorithm is that it does not accumulate towards some local optima, and the presence of a social hierarchy helps it in storing the best possible solutions obtained so far. The derived results using GWO exhibit a significant improvement in the response values as compared to the previous attempts for parametric optimization of AWJM processes while applying other algorithms.     

Rotary ultrasonic machining— a new cutting process and its performance

In conventional ultrasonic machining (USM), brittle materials are machined by using ultrasonic impacts on the workpiece, through a medium of abrasive slurry. In this paper a new cutting process that resulted due to introduction of an additional parameter, namely the rotation of the workpiece during the machining, is presented. This may be called ‘rotary ultrasonic machining’. The material removal rates (MRR) in rotary USM are up to four times those in conventional USM. The MRR increases with increase in speed of rotation of workpiece. An explanation for the superior performance of rotary USM is presented. The performance of rotary USM as a function of static load, abrasive grain size, concentration of abrasive slurry, diameter of tool and ratio of diameters of hollow tools, is studied and the parameters are optimized for minimum machining time or maximum material removal rate. Comparisons are made with conventional USM.     

Finite element analysis of single-particle impact in abrasive water jet machining

This contribution presents an explicit finite element analysis (FEA) of a single abrasive particle impact on stainless steel 1.4301 (AISI 304) in abrasive water jet (AWJ) machining. In the experimental verification, the shapes of craters on the workpiece material were observed and compared with FEA simulation results by means of crater sphericity. The influences of the impact angle and particle velocity were observed. Especially the impact angle emerged as a very suitable process parameter for experimental verification of FEA simulation, where crater sphericity was observed. Results of the FEA simulation are in good agreement with those obtained from the experimental verification. The presented work gives the basis for further FEA investigation of AWJ machining, where influences such as particles rotation and other process parameters will be observed.