Simulating gas–liquid multiphase flow using meshless Lagrangian method

A multiphase flow model has been established based on a moving particle semi‐implicit method. A surface tension model is introduced to the particle method to improve the numerical accuracy and stability. Several computational techniques are employed to simplify the numerical procedure and further improve the accuracy. A particle fraction multiphase flow model is developed and verified by a two‐phase Poiseuille flow. The multiphase surface tension model is discussed in detail, and an ethanol drop case is introduced to verify the surface tension model. A simple dam break is simulated to demonstrate the improvements with various modifications in particle method along with a new boundary condition. Finally, we simulate several bubble rising cases to show the capacity of this new model in simulating gas–liquid multiphase flow with large density ratio difference between phases. The comparisons among numerical results of mesh‐based model, experimental data, and the present model, indicate that the new multiphase particle method is acceptable in gas–liquid multiphase fluids simulation. Copyright © 2014 John Wiley & Sons, Ltd.     

Modelling effects of degree of crystallinity on mechanical behavior of semicrystalline polymers

Viscoplasticity theory based on overstress (VBO) which is one of the unified state variable theories is extended to account for crystallinity ratio () on mechanical behavior of semicrystalline polymers. The modifications on VBO are done considering the semicrystalline polymeric materials somewhat as a composite material since it consists of amorphous and crystalline phases. Amorphous and crystalline phase resistances are arranged in two different analog models: amorphous stiffness and flow are in parallel and series with crystalline phase. Apart from many existing work in the literature, not only uniaxial loading are modeled but also creep and relaxation behaviors are simulated for a hypothetical material. It is shown that when amorphous and crystalline phase resistances acting in parallel are considered in the model, creep, relaxation and uniaxial loading and unloading behaviors can be simulated well using the modified VBO. In addition, uniaxial compression loading and unloading behavior of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) and creep behavior of polytetrafluoroethylene (PTFE) with different crystallinity ratios are simulated using the proposed VBO model where amorphous and crystalline phases are parallel. Simulation results are compared to the experimental data by Kurtz et al. (2002) and Sun et al. (2005) [Kurtz, S.M., Villarragaa, M.L., Herra, M.P., Bergström, J.S., Rimnacc, C.M., Edidin, A.A., 2002. Thermomechanical behavior of virgin and highly crosslinked ultra-high molecular weight polyethylene used in total joint replacements. Biomaterials 23, 3681–3697; Sun, H., Cooke, R. S., Bates, W. D., Wynne, K.J., 2005. Supercritical CO₂ processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance. Polymer 46, 8872–8882] respectively and good match with experimental data is obtained.     

Prediction of Gear Dynamics Using Fast Fourier Transform of Static Transmission Error*

ABSTRACT

An analytical computer simulation procedure for dynamic modeling of low-contact-ratio spur gear systems is presented. The procedure computes the gear static transmission error and uses a Fast Fourier Transform (FFT) to generate its frequency spectrum at various tooth profile modifications. The dynamic loading response of an unmodified (perfect involut) gear pair is compared with that of gears with profile modifications. Correlations are found between several profile modifications and the resulting dynamic loads. An effective error, obtained from frequency domain anal     

高温高应变率下材料拉伸性能测试的一种新方法

为了实现高温环境下材料高应变率动态拉伸实验技术,将分离式Hopkinson杆直接拉伸装置中试样与拉杆的螺纹连接形式变成楔形连接形式,并加装了气动同步装置系统。这样,在对试样加高温时,能使靠近试样的入射和透射杆端处于较低温度。当撞击管向传递法兰运动时,气动同步装置瞬间拖动透射杆和试样,使两者之间的间隙为零,此时沿入射杆传递的入射波同时对试样拉伸加载。经实验验证,此方法可以有效实现材料高温高应变率拉伸加载。     

结构静态拓扑重分析的迭代组合近似方法

提出一种拓扑修改的静态重分析的迭代组合近似方法. 这种方法基本上是两步法. 首先，将新增加的自由度通过Guyan缩减方法凝聚到原始自由度上，形成凝聚方程. 其次， 用迭代组合近似方法求出原始自由度的近似位移，从而求出原结构自由度的位移. 新增加自 由度的位移可以通过恢复得到. 通过板结构的加筋布局优化设计的数值例子表明， 该方法对拓扑修改较大时仍可得到满意的结果.     

An elliptic blending differential flux model for natural,mixed and forced convection

Several modifications are introduced to the Elliptic Blending Differential Flux Model proposed by Shin et al. (2008) to account for the influence of wall blockage on the turbulent heat flux. These modifications are introduced in order to reproduce, in association with the most recent version of the EB-RSM, the full range of regimes, from forced to natural convection, without any case-specific modification. The interest of the new model is demonstrated using analytical arguments, a priori tests and computations in channel flows in the different convection regimes, as well as in a differentially heated cavity.     

Modeling nanostructural surface modifications in metal cutting by an approach of thermodynamic irreversibility: Derivation and experimental validation

Performance and operational safety of many metal parts in engineering depend on their surface integrity. During metal cutting, large thermomechanical loads and high gradients of the loads concerning time and location act on the surfaces and may yield significant structural material modifications, which alter the surface integrity. In this work, the derivation and validation of a model of nanostructural surface modifications in metal cutting are presented. For the first time in process modeling, initiation and kinetics of these modifications are predicted using a thermodynamic potential, which considers the interdependent developments of plastic work, dissipation, heat conduction and interface energy as well as the associated productions and flows of entropy. The potential is expressed based on the free Helmholtz energy. The irreversible thermodynamic state changes in the workpiece surface are homogenized over the volume in order to bridge the gap between discrete phenomena involved with the initiation and kinetics of dynamic recrystallization and its macroscopic implications for surface integrity. The formulation of the thermodynamic potential is implemented into a finite element model of orthogonal cutting of steel AISI 4140. Close agreement is achieved between predicted nanostructures and those obtained in transmission electron microscopical investigations of specimen produced in cutting experiments.     

Influence of cycle time distribution on coating uniformity of particles in a spray fluidized bed by using CFD-DEM simulations

Cycle Time Distribution (CTD) plays a critical role for determining uniformity of particle coating in spray fluidized beds. However, the CTD is influenced by both geometrical structure and operating conditions of fluidized bed. In this study, a spray fluidized bed of coating process is simulated by a comprehensive Computational Fluid Dynamics-Discrete Element Model (CFD-DEM). To achieve different behaviors of CTD, some modifications are designed on a pseudo-2D internally circulating fluidized bed, which traditionally composes of a high-velocity upward bed and low-velocity downward bed. These modifications include making the air distributor slope and/or laying a baffle in the downward bed. First, the CTD and evolution of particle size distribution under different bed structures are compared. The CTD directly influences the coating uniformity. By making the particles flowing along a parallel direction in the downward bed through the geometrical modifications, the CTD becomes narrower and the coating uniformity is significantly improved. Second, under the optimized bed structure, the influence of operating conditions on the coating uniformity is studied. Properly increasing the fluidization gas velocity and the fluidization gas temperature and reducing the liquid spray rate can improve the coating uniformity.     

Optimization of straight-sided spline design

Spline connection of shaft and hub is commonly applied when large torque capacity is needed together with the possibility of disassembly. The designs of these splines are generally controlled by different standards. In view of the common use of splines, it seems that few papers deal with splines and the subject of improving the design. The present paper concentrates on the optimization of splines and the predictions of stress concentrations, which are determined by finite element analysis (FEA). Using different design modifications, that do not change the spline load carrying capacity, it is shown that large reductions in the maximum stress are possible. Fatigue life of a spline can be greatly improved with up to a 25% reduction in the maximum stress level. Design modifications are given as simple analytical functions (modified super elliptical shape) with only two active design parameters and the designs are practical realizable. Some generality is found in the design parameters.     

Development of DDES and IDDES Formulations for the k-ω Shear Stress Transport Model

Modifications are proposed of two recently developed hybrid CFD strategies, Delayed Detached Eddy Simulation (DDES) and DDES with Improved wall-modeling capability (IDDES). The modifications are aimed at fine-tuning of these approaches to the k-ω SST background RANS model. The first one includes recalibrated empirical constants in the shielding function of the SA-based DDES model which are shown to be suboptimal (not providing the needed level of elimination of the Model Stress Depletion (MSD)) for the SST-based DDES model. For the SST-IDDES variant, in addition to that, a simplification of the original SA–based formulation is proposed, which does not cause any visible degradation of the model performance. Both modifications are extensively tested on a range of attached and separated flows (developed channel, backward-facing step, periodic hills, wall-mounted hump, and hydrofoil with trailing edge separation).     

小长径比弹丸加装大展弦比尾翼的气动特性研究

为了提高小长径比弹丸射击质量,设计了一种大展弦比张开式尾翼,采用AUSM+格式、SST(shearstress transport)湍流模型和隐式算法(lower-upper symmetric Gauss-Seidel implicit method, LU-SGS),求解三维RANS 方程,对前体形状完全相同,不同展弦比的3 种尾翼弹进行了数值模拟,得到了三者在马赫数1.5~3.5 下的气动力特性的差异,分析其原因,并给出了不同展弦比张开式尾翼的适用范围. 计算结果表明:C型弹的升阻比较B 型弹在1.5 马赫数区域附近增加了7% 以上,当马赫数达到2.5 以上时,A 型弹的升阻比大于B 型弹和C 型弹,在3.5 马赫数区域附近A 型弹的升阻比较B 型弹增加了5.4% 以上. 3 种弹丸的俯仰力矩系数随着马赫数的增大而负向减少,且减少的趋势随着展弦比的增加而增大. A 型弹、B 型弹、C 型弹的静稳定裕度的变化范围分别为4%~20.3%,8.5%~23.2%,11.4%~25.6%.     

Methods of analytical mechanics in the constrained adaptive identification problems

The application of analytical mechanics methods to the problem of adaptive identification is discussed. The estimated constant parameters satisfy nonlinear equations considered as parametric constraints. Some modifications of identification algorithms are proposed to satisfy such constraints.     

Finite element modelling of stratified flow in estuaries and reservoirs

Experiences relating to the application of finite element models for laterally averaged stratified flow are discussed and modifications to the basic approach are suggested that alleviate these difficulties. An example problem is used to demonstrate the revised model and to make a preliminary assessment of the hydrostatic pressure assumption when applied to reservoir analysis.     

Results of the Development and Problems of System Computer Technologies in Aviation

The possibilities of using modern computer technologies in the aircraft industry and in aircraft operation under severe conditions are considered. The discrete vortex method, together with its modifications, is shown to be a useful tool. The results obtained by the author and his school are reviewed.     

Das unterschiedliche Schwellverhalten eines extrudierten Schmelzestranges bei Variation des Relaxationsspektrums

The die swell of viscoelastic fluids after leaving a die depends not only on the geometry, temperature effects and the mass flow rate, but also on the specific properties of the media. Among these specific properties is the relaxation spectrum which determines the relaxation of stresses in the fluid. It is this relaxation that produces a die swell. This paper investigates theoretically the manner in which modifications of the relaxation spectrum affect the viscosity curve and the die swell behaviour. In particular it is shown which modifications of the spectrum have a large influence on the die swell and which are relatively unimportant. It is also shown how the time intervals in which the main die swell occurs dependend on the shear rates in the die and which parts of the relaxation spectrum are responsible for the size of the die swell.     

Improvement of cavitation mass transfer modeling based on local flow properties

This paper presents and studies the effect of two modifications to improve cavitation mass transfer source term modeling for transport equation based models by considering local flow properties. The first improvement is by creating an analogy between the phase change time scale and turbulence time scale, and have the model to automatically adjust mass transfer rate based on the flow. This will alleviate the manual calibration of model parameter that is often necessary in presently used models. The second modification introduces an influence of shear stress on the liquid rupture in flows relevant for hydromachinery. This relates to that the pressure threshold, which represents the criteria of when phase change occurs, is normally taken as the value relevant for a fluid at rest, but is in reality affected by the flow conditions.To demonstrate the effect of the model modifications, the three-dimensional, fully turbulent, cavitating flow around the Delft Twist11 foil is simulated. The suggested modifications are implemented in and evaluated using the Sauer mass transfer model, with simulations performed with an incompressible implicit LES flow model. The pressure distribution across different sections of the foil, lift force, and cavitation behavior, such as generation, separation, and collapse processes, are studied and compared with the experimental data. The comparison shows the capability of the presented model to improve the prediction of the complex physics of the cavitation around the Twist11 foil, compared with using only the original Sauer mass transfer model.     

Evolution of jets effusing from inclined holes into crossflow

The turbulent flow structure and vortex dynamics of a jet-in-a-crossflow (JICF) problem, which is related to gas turbine blade film cooling, is investigated using the particle-image velocimetry (PIV) technique. A cooling jet emanating from a pipe interacts with a turbulent flat plate boundary layer at a Reynolds number Re_∞ = 400,000. The streamwise inclination of the coolant jet is 30° and two velocity ratios (VR = 0.28, VR = 0.48) and two mass flux ratios (MR = 0.28, MR = 0.48) are considered. Jets of air and CO₂ are injected separately into a boundary layer to examine the effects of the density ratio between coolant and mainstream on the mixing behavior and consequently, the cooling efficiency. The results show a higher mass flux ratio to enlarge the size of the recirculation region leading to a more pronounced entrainment of hot outer fluid into the wake of the jet. Furthermore, the lateral spreading of the coolant is strongly increased at a higher density ratio. The results of the experimental measurements are used to validate numerical findings. This comparison shows an excellent agreement for mean velocity and higher moment velocity distributions.     

壳体初始形状对球形容器液压胀形结果的影响

借助于数值模拟研究了锥台形壳体液压胀形过程中可能出现的质量问题,以及壳体的初始形状对成形结果的影响,在此基础上提出了锥台形壳体的改进方案,以避免壳体的起皱,并最终获得预期的球形容器或椭球形容器.     

A new approach for improving ballistic performance of composite armor

An experimental investigation of the ballistic performance of composite armor with geometric modifications was carried out. The armor was simulated using polymeric materials. Four different geometric modifications were incorporated into the front plate of the armor, and two different adhesives were considered in this study. High-speed photography was employed to observe the real-time evolution of impact damage and to obtain the projectile penetration history. The nature and extent of damage for each modification and adhesive was estimated by postmortem inspection of the impacted armor and was compared to that obtained in unmodified armor of equal weight. The results of the study indicate that the geometric modifications after the nature and extent of damage significantly compared to conventional composite armor. The strong adhesive causes tearing of the back plate, whereas the compliant adhesive results in extensive delamination without any back plate damage. The modifications assist in spreading the damage laterally away from the impact site, thus distributing the load onto a larger area of the back plate. Calculations using a one-dimensional theoretical model also conclude that geometrical modifications improve the ballistic performance of the armor.