An efficient CFD-based method for aircraft icing simulation using a reduced order model

In-flight icing is a critical technical issue for aircraft safety and, in particular, the droplet impingement areas on aircraft surfaces must be investigated for anti-/de-icing devices. As a step toward the prediction of droplet impingement on aircraft, an Eulerian-based droplet impingement code that provides collection efficiency for air flows around an airfoil containing water droplets is developed. A computational fluid dynamics (CFD) solver was also developed to solve the clean airflow. Then, a proper orthogonal decomposition (POD) method, a reduced order model (ROM), that optimally captures the energy content from a large multi-dimensional data set is utilized to efficiently predict the collection efficiency and ice accretion shapes on an airfoil following the mean volume diameter, liquid water contents and angle of attacks. As a result it is shown that the collection efficiency and ice shapes were in good agreement with the simulated and predicted results.     

TIG电弧辅助熔滴沉积增材制造SiCp增强铝基复合材料中的熔池动力学与颗粒迁移行为

基于计算流体力学方法,考虑电弧力、碳化硅颗粒(SiC_p)增强相与液态铝合金基体相之间的相互作用以及表面张力等因素,建立了Si C_p增强铝基复合材料钨极氩弧焊(TIG)电弧辅助熔滴沉积增材制造中的三维瞬态熔池行为数值模型,通过与堆积试样的横截面形貌、Si C_p分散状态实验结果对比,验证了熔池行为数值模型的有效性。通过数值模拟,揭示了堆积过程熔池峰值温度演变规律、熔滴冲击诱导的熔池动力学行为、熔池流态对Si C颗粒迁移行为的影响。结果表明,堆积过程涉及熔滴冲击、合并、铺展、熔池回弹4个阶段;冲击点附近出现重熔,熔滴冲击造成熔池中心区域产生明显的V字形凹陷,熔池边缘形成冠状隆起;在熔体拖拽力的作用下Si C颗粒更多的分布于堆积层两侧;熔滴冲击引起的熔池内局部高压以及熔池底部对流,抑制了Si C颗粒向熔池底部沉降。     

Numerical simulation of droplet formation in a micro-channel using the lattice Boltzmann method

This study investigates droplet formation in a micro-channel using the lattice Boltzmann (LB) method. A cross-junction micro-channel and two immiscible, water and oil phase fluids, were used to form the micro-droplets. Droplets are formed by the hydrodynamic instability on the interface between two immiscible fluids when two immiscible fluids are imported simultaneously in a cross-junction micro-channel. The Shan & Chen model, which is a lattice Boltzmann model of two-phase flows, is used to treat the interaction between immiscible fluids. The detailed process of the droplet formation in the cross-junction micro-channel was illustrated. The results of the droplet formation by the LBM predicted well the experimental data by PIV (particle image velocimetry). The effect of the surface tension and the flow rate of water phase fluid on the droplet length and the interval between droplets was also investigated. As the surface tension increased, the droplet length and the interval between droplets were increased. On the other hand, when we increased the flow rate of the water phase fluid under the condition of the fixed oil-phase fluid flow rate, the droplet size was increased while the interval between droplets was decreased.     

Computations of droplet impingement on airfoils in two-phase flow

The aerodynamic effects of leading-edge accretion can raise important safety concerns since the formulation of ice causes severe degradation in aerodynamic performance as compared with the clean airfoil. The objective of this study is to develop a numerical simulation strategy for predicting the particle trajectory around an MS-0317 airfoil in the test section of the NASA Glenn Icing Research Tunnel and to investigate the impingement characteristics of droplets on the airfoil surface. In particular, predictions of the mean velocity and turbulence diffusion using turbulent flow solver and Continuous Random Walk method were desired throughout this flow domain in order to investigate droplet dispersion. The collection efficiency distributions over the airfoil surface in simulations with different numbers of droplets, various integration time-steps and particle sizes were compared with experimental data. The large droplet impingement data indicated the trends in impingement characteristics with respect to particle size; the maximum collection efficiency located at the upper surface near the leading edge, and the maximum value and total collection efficiency were increased as the particle size was increased. The extent of the area impinged on by particles also increased with the increment of the particle size, which is similar as compared with experimental data.     

A study of a nano-sized water droplet’s transition from a flat surface to a pillared surface

The ability to control the hydrophobicity of a surface is of importance to many industries. The dynamic behavior of nano-sized water droplets moving from a flat surface to a pillared surface using molecular dynamics simulations was investigated. Simulations were carried out in two steps. In the first computational step, the initial group of water molecules reached equilibrium on a flat graphite surface. In the second computational step, a constant force was applied to the water droplet and the motion of the water droplet was evaluated as it moved from the flat surface to the pillar-type surface. The movement of the water droplet could be grouped into three different categories and depended on the pillar height and the magnitude of the applied force. The results showed the strongest body force with a pillar height of 6 graphite layers allowed most of the water molecules to move along the top of the pillars. In conclusion, a strong force and pillar height approximately half of the droplet height displayed the best transition from a flat surface to a pillared surface.

     

外加高频磁场下电弧快速成形过程的电磁-流体耦合数值模拟

外加电磁作用是改善电弧快速成形零件组织和性能的有效方式之一。为了揭示高频磁场对熔池传热、对流和形态的影响机理,采用有限元电磁计算和有限体积流体分析耦合的方法,建立电磁场、熔池温度场和流体流动场分析的三维模型,分析工件和熔池中高频电磁力/热的分布特征,研究高频电磁力与表面张力、电弧力以及熔滴冲击共同作用下的熔池表面动态变形,对比分析有/无外加高频磁场情况下熔池温度分布和流体流动模式上的差异,并由此预测外加高频磁场对凝固组织和熔池形态的改变。结果表明,高频电磁力驱动熔池流体在垂直焊接方向的平面内形成单漩涡旋转对流,有利于熔断枝晶细化晶粒,熔池表面形状向远离线圈一侧倾斜,熔宽增大。金相和焊道横截面测试证实了上述模拟结果。     

Research on formation and stability of keyhole in stationary laser welding on aluminum MMCs reinforced with particles

The formation and stability of keyhole in stationary laser welding on aluminum metal matrix composites reinforced with particles are studied using a numerical simulation. The interaction between molten pool and reinforcement particles is evaluated by using the particle–fluid coupling model in the numerical simulation. In order to study the effect of different volume fractions of particles on the keyhole stability and fluid flow inside the molten pool, keyhole formation process, variation of free surface, temperature distribution, and fluid flow are calculated numerically, respectively. The calculation results show that the keyhole is stable at the beginning under different conditions and then the protrusion occurs inside the keyhole with increasing calculation time. The flow behavior of molten pool affected by particles and forces acting on the surface could explain the forming of humps inside the keyhole, which directly cause the variation of the keyhole. As the volume fraction of TiB₂ particles increases, the keyhole is more likely to be instable and the oscillation occurs at an earlier time. Fluctuations of the surface tension and recoil pressure due to the uneven distribution play an important role in the instability of the keyhole.     

Experimental method to analyze the oil mist impingement over an insert used in MQL milling process

This paper proposes an original method to determine the oil mist impingement over an insert based on different machining configurations (rotation velocities) and different inner canalization designs, without material removal, on milling tool. The oil mist spray is determined using different holders as an insert. Glass plate and blotting paper are used and evaluated in this work to get as much information as possible, based on micro–macro scale observations. The imprint size of the oil mist spray distribution, the particle sizes and the fluid film formation have been studied to determine the effects of the different configurations on the oil mist impingement behavior. The droplet impingements and distributions on the glass plate are analyzed under a 3-D profilometer (microscopic scale). On the blotting paper, the distributions are analyzed with pictures of the oil marks (macroscopic scale). This original method gives quite fast particle distributions that can be used in industry. The different experimental results give large information about the oil mist behavior sprayed on the glass plate. Increasing rotation velocities increases the oil mist amount in the cutting area. Increasing the canalization orientation design gives a better focused spray. The rake angle has strictly no influence on the oil mist spray distribution. But sputtering effects have been highlighted and should be avoided to keep MQL efficiency.     

The effect solid particle lubricant contamination on the dynamic behavior of compliant journal bearings

The proposed work concerns a theoretical and numerical investigation of the effect of solid particle contamination of lubricant oils on the static and dynamic characteristics of a finite length compliant journal bearing operating under isothermal conditions with laminar flow. In the present investigation, we use simple models based on the Einstein's mixture theory, which is characterized by the presence of suspended rigid particles in a fluid. Using the classical assumptions of lubrication, a Reynolds equation is derived and solved numerically by the finite difference method. The displacement field at the fluid film bearing liner interface due to pressure forces is determined using the elastic thin layer model. The results obtained show that the presence of suspended rigid particles in the lubricating oil (solid contamination) has significant effects on the hydrodynamic performance characteristics such as the pressure field, friction force, flow rate, elastic surface deformation as well as stability maps of the rotor‐bearing system (critical mass and whirl frequency) especially at high volumetric concentration.     

熔化极气体保护焊焊接过程的分析

以流体动力学、电磁理论及VOF法(Volume of Fluid Method)为基础建立了熔滴过渡的动态模拟模型,模型中考虑电磁收缩力、表面张力、等离子流力的影响。利用建立的模型模拟了熔滴的形成、长大及脱离过程,计算了电流对过渡熔滴尺寸及频率的影响,计算结果与实验结果吻合较好。计算了不同阶段的熔滴中的流场,并利用计算的流场分析了熔滴的脱离机制。     

圆管式压电喷头的液体分配

针对用于喷墨打印的圆管式压电喷头建立了计算模型,并且根据它的驱动特点选择了合适的边界条件。介绍了仿真软件针对自由表面流动问题的计算原理。然后,以乙二醇水溶液为例,计算了压电喷头分配该溶液的分配过程;利用液滴成像系统获取了不同时刻的液滴图像,验证了建立的模型和数值算法的正确性。最后,计算了压电喷头在不同输入位移、不同黏度以及不同表面张力下的液体分配过程。仿真结果显示:液体分配性能与激励位移密切相关,在72.5mN/m的表面张力作用下,10nm的输入位移很难分配黏度为4.0mPa·s的液体,而15nm的输入位移在分配黏度为4.0mPa·s液体时却能够产生卫星液滴。因此,对于某种液体寻找一个合适的激励条件非常重要,过小的激励产生不了液滴,过大的激励则会产生较大甚至多个卫星液滴;增大黏度会延缓或阻滞液滴形成过程,增大表面张力却能加快液滴形成过程。本文的计算方法对于研制新式喷头或者研究喷头的喷射能力均具有指导意义。     

Study on droplet formation with surface tension for electrohydrodynamic inkjet nozzle

Droplet ejection from an electrohydrodynamic (EHD) inkjet nozzle depends on many factors such as an onset voltage, liquid conductivity, surface tension, etc. Since the surface tension has an influence on the contact angle between the nozzle surface and the liquid droplet, the surface tension change should be investigated for the control of the droplet ejection. In this study, surfactant, which can weaken the surface tension force, was used to analyze the effect of the surface tension. Furthermore, hydrophobic coating of the nozzle surface was considered as another factor for the droplet ejection. Also, a flow visualization technique was developed to observe the droplet formation and ejection from the EHD inkjet nozzle by various surface tension values.     

近壁面超声空化微射流对微细颗粒的破碎作用

针对微细粉体“团聚”导致粉磨极限的问题,在液相环境下利用超声的分散和空化冲击作用,开展了近壁面超声空化微射流对微细颗粒破碎作用的研究。通过理论计算空化微射流冲击微细颗粒破碎的有效作用范围,从空化泡溃灭速度的角度分析了超声频率、声压幅值、介质尺寸等主要参数对微射流强度的影响;结合物料质量浓度、介质尺寸、介质面积和功率等影响因素的微细颗粒超声空化破碎正交试验,并利用SEM观测粉体形貌,分析了颗粒中位粒径D50、10%体积累积粒径D10和比表面积（SSA）等分布特性。参数组合优化后获得了粒径小于800目的微细颗粒,破碎率高达79.35%;粒径大于10 000目的极细颗粒产率高达12.84%。从提高微细颗粒破碎率的角度,发现介质面积是主要影响因素,功率次之,其次为介质尺寸和物料质量浓度。试验结果与理论研究成果基本一致,表明优化超声空化微射流参数与增加介质壁面面积等方法可有效提高微细颗粒的破碎率。     

Determination of particle impacts and impact energy in the erosion of X65 carbon steel using acoustic emission technique

An in-situ acoustic emission (AE) monitoring technique has been implemented in a submerged jet impingement (SIJ) system in an effort to investigate the effect of sand particle impact on the degradation mechanism of X65 carbon steel pipeline material in erosion conditions.A detailed analysis of the acoustic events' count rate enabled the number of impacts per second to be quantified for a range of flow velocities (7, 10, 15 m/s) and solid loadings (0, 50, 200, 500 mg/L) in a nitrogen-saturated solution at 50 °C. The number of impacts obtained from acoustic signals showed a strong agreement with theoretical prediction for flow velocities 7 and 10 m/s. A deviation between practical readings and theory is observed for flow velocity of 15 m/s which may be due to error from detected emissions of multiple rebounded particles.Computational fluid dynamics (CFD) was used in conjunction with particle tracking to model the impingement system and predict the velocity and impact angle distribution on the surface of the sample. Data was used to predict the kinetic energy of the impacts and was correlated with the measured AE energy and material loss from gravimetric analysis. The results demonstrate that AE is a useful technique for quantifying and predicting the erosion damage of X65 pipeline material in an erosion–corrosion environment.     

A numerical study on the entry problem of an elastic body using a level set method and a monolithic formulation

A finite element method (FEM) based on a level set method and a monolithic formulation was investigated for the analysis of the entry problem of an elastic body. Both the free surface tracking and the interaction of an entry body with fluid flow should be considered in an entry problem. Free surface tracking was achieved using a level set method, in which advection and reinitialization equations for the level set variable are discretized using the least-squares FEM (LSFEM). The coupling of the motion of the elastic body with fluid flow was conducted using a monolithic mixed finite element formulation. The arbitrary Lagrangian Eulerian method was used for the movement of a grid because the downward motion of the body is dominant in the entry problem. Since a mixed finite element was used, a linear basis function that belongs to the H¹ space was used for velocity and level set variables on each sub-element (T_h), and a linearly interpolated pressure variable that belongs to the L² space was adopted on each element (T_2h). The level set method based on the LSFEM was verified by solving some free surface tracking problems. Then, the finite element formulation for the entry problem was validated by comparing it with existing experimental results. Finally, the entry of an elastic body was compared with that of a rigid body.     

接口前处理在光机系统动力学分析和面形优化中的应用

为了解决光机系统动力学分析与面形优化过程中由于光机接口程序后处理方式所引起的计算数据量大或接口处理失效的问题,提出将光机接口处理过程移到有限元前处理中,并进行了光机系统的集成分析与优化。首先,为了解决标准Zernike多项式在环形离散点域内的非正交性,引入了节点面积加权因子和环域Zernike多项式。提出了通过对镜面施加均匀压强求节点支反力的方式求取节点面积加权因子的计算方法。然后采用最小二乘法推导出镜面刚体位移和拟合镜面变形的Zernike多项式系数与镜面各节点变形量之间的线性关系式。最后,编写接口程序将这些线性表达式以多点约束(MPC)的方式导入到有限元模型中,在前处理过程中完成系统的光机接口处理过程。通过对非稳态风载引起的某1.2m地基望远镜视轴抖动和液压whiffletree支撑下的主镜镜面高阶变形量进行结构动力学随机响应分析,验证了光机前处理方法对解决光机系统动力学问题的有效性。此外,还以镜面面形为优化目标对1.2m轻量化主镜的镜体结构和尺寸进行敏感度分析,证明了光机前处理方法可以有效地简化镜面面形的优化分析过程。     

A Granular motion simulation by discrete element method

A granular system is defined as a group of tiny particles; they interact with each other by collisions and elastic force. To analyze granular dynamics, conventional methods based on continuum mechanics are not applicable, so new simulation methods are needed. Recently, thanks to improvement in computing technology, the discrete element method (DEM) is being focused on, in which equations of motion are built on each particle and the behavior of all particles is analyzed by solving those equations. In this paper, a computer program has been developed to analyze particle dynamics by using the discrete element method. As examples, the particle packing process and mono-component non-contact development process in a laser printer are simulated. It is seen that the particle motions in the processes are well described by the program.     

Calculation of fuel spray impingement and fuel film formation in an HSDI diesel engine

Spray impingement and fuel film formation models with cavitation have been developed and incorporated into the computational fluid dynamics code, STAR-CD. The spray/wall interaction process was modeled by considering the effects of surface temperature conditions and fuel film formation. The behavior of fuel droplets after impingement was divided into rebound, spread and splash using the Weber number and parameter $K(\sqrt {We\sqrt {Re} } )$ . The spray impingement model accounts for mass conservation, energy conservation, and heat transfer to the impinging droplets. The fuel film formation model was developed by integrating the continuity, momentum, and energy equations along the direction of fuel film thickness. Zero dimensional cavitation model was adopted in order to consider the cavitation phenomena and to give reasonable initial conditions for spray injection. Numerical simulations of spray tip penetration, spray impingement patterns, and the mass of film-state fuel matched well with the experimental data. The spray impingement and fuel film formation models have been applied to study spray/wall impingement in high-speed direct injection diesel engines.     

A novel polishing method for single-crystal silicon using the cavitation rotary abrasive flow

Traditional low-pressure abrasive flow polishing can produce highly smooth surfaces, but the efficiency of this method is too low for polishing of hard-brittle materials parts. This paper proposes a novel cavitation rotary abrasive flow polishing (CRAFP) method. The energy generated from the cavitation bubble collapse is used to increase the kinetic energy of the abrasive particles in the low-pressure abrasive flow and the motion randomness of the abrasive particles near the wall; thereby, the efficiency and quality of low-pressure abrasive flow polishing are improved. The CRAFP mechanism was first introduced, and then the characteristics of the CRAFP process were investigated using computational fluid dynamics (CFD)-based abrasive flow simulation. Subsequently, a single-crystal silicon wafer polishing test was carried outperformed to verify the validity of the CRAFP method. The polishing results were compared with those of the traditional low-pressure abrasive flow polishing method. After 8 h of polishing using the CRAFP method and the traditional low-pressure abrasive flow polishing method, the surface roughness of the workpiece decreased to7.87 nm and 10.53 nm, respectively. Furthermore, by starting at similar initial roughness values, the polishing time required to reduce the roughness to 12 nm was 3.5 h and 6 h, respectively. The experimental results demonstrated that CRAFP can satisfy the surface requirements of single-crystal silicon (R_a < 12 nm) and exhibit high polishing efficiency and good quality.     

A micromechanics analysis of the material removal mechanisms in the cutting of ceramic particle reinforced metal matrix composites

In previous investigations on the cutting of ceramic particle reinforced metal matrix composites using the finite element (FE) method, the particles are usually considered to be rigid. This is inconsistent with the actual situation and thus makes the FE predictions less practical. This paper proposes a micromechanics model to investigate the material removal mechanisms by considering the elasticity and fracture of the particles. It was found that the interaction position of a particle relative to the cutting edge greatly influences the particle and matrix fracture, particle-matrix debonding, surface integrity and cutting forces. There are two particle cracking mechanisms. One is caused by the direct contact of the cutting edge with the particle, and the other is due to the indirect tool-particle interaction through matrix. When the cutting path is below a particle, a smooth surface without subsurface damage can be achieved. When it is passing through a particle, particle fracture mostly occurs. If it is above a particle, subsurface damage is dominated by particle-matrix debonding. Cutting force peaks once the cutting edge is in contact with a particle and the cutting edge is easier to be damaged when the tool is passing through the upper part of the particle.