轿车乘员舱内气流对气动阻力影响的数值模拟

为揭示轿车侧窗开启程度和乘员舱内布置对气动阻力的影响规律,建立1∶1阶背式英国汽车研究协会(Motor Industry Research Association,MIRA)标准模型;基于FLUENT,在30 m/s风速下采用可实现k-ε湍流模型对不同侧窗开度的模型进行三维稳态数值模拟,得到气动阻力因数随侧窗开启程度的增大而增大的变化趋势.在侧窗全开时,改变舱内布置,得到气动阻力因数随假人个数的变化规律;对比不同情况下模型的流场分布发现,当考虑乘员舱内气流时,气动阻力的大小不仅与进气量有关,而且受舱内流场分布的影响.     

Influence of hydrogen concentration on Fe2O3 particle reduction in fluidized beds under constant drag force

The fixed-gas drag force from a model calculation method that stabilizes the agitation capabilities of different gas ratios was used to explore the influence of temperature and hydrogen concentration o...     

Study of hydrodynamic characteristics of particles in liquid–solid fluidized bed with modified drag model based on EMMS

Flow behavior of solid phases is simulated by means of Eulerian–Eulerian in a liquid–solid fluidized bed with modified drag model based on energy-minimization multi-scale (EMMS) method. The modified EMMS drag coefficient is characterized by the treatment of the particle-rich dense phase and the liquid-rich dilute phase as the two interpenetrating continua. It was shown that the modified EMMS drag coefficient can predict reasonably the solid concentration profiles in a liquid–solid fluidized bed. The distributions of solid velocity, granular temperature and granular pressure are predicted. The phenomenon of back-mixing near the wall is found in the liquid–solids fluidized beds.     

有限长波浪形圆柱绕流数值模拟

为研究一端固定在壁面上,另一端为自由端的有限长波浪形圆柱的减阻抑振效果,对有限长波浪形圆柱进行了计算.首先,采用大涡模拟数值模型的方法,对雷诺数为3 900时的有限长直圆柱和不同波长、波幅组合后的12种有限长波浪形圆柱进行了计算;其次,针对计算结果进行后处理,得到并比较了不同组合形式下的有限长波浪形圆柱的升、阻力系数大...     

含聚丙烯酰胺的管道螺旋流流动特性试验

为了解聚丙烯酰胺(PAM)对管道螺旋流流动特性的影响,搭建管道试验系统和制作多导叶式局部起旋装置,使用5孔直头球形测针和1套水压力测压系统,测量含PAM管道螺旋流的管道边壁静水压强和测针内动水压强,分析管道螺旋流流速分布、输固能力和压力特性。结果表明:加入少量PAM(100 mg/L)之后,管道螺旋流的流速分布相比清水发生变化,管道螺旋流发展过程(产生—发育成型)延长67%,管道螺旋流的输固能力增大79.1%～91.5%,阻力损失降低18.0%～23.8%。     

Hydrodynamic optimization of a triswach

A new methodology for hydrodynamic optimization of a Tri SWACH is developed, which considers not only the positions of the side hulls but also the shape of the side hulls. In order to account for the strong near-field interference effects between closely-spaced multihulls, an integrated hydrodynamic computational tool that consists of a potential-flow based simple CFD tool and an Euler/RANS/Navier-Stokes based advanced CFD tool has been further developed and integrated into a practical multiobjective hydrodynamic optimization tool. The other components of this hydrodynamic optimization tool consist of a hull shape representation and modification module and an optimization module. This enhanced multi-objective hydrodynamic optimization tool has been applied to the hydrodynamic design optimization of the Tri SWACH for reduced drag by optimizing the side hulls only. A new methodology is developed to optimize side hull forms so that the Tri SWACH has a minimal drag for a wide speed range and for various side hull positions. Two sets of the side hulls are developed and used for the design of two optimal Tri SWACH models. Model tests are carried out for two optimal Tri SWACH models at Webb Institute for validations. Substantial drag reductions have been obtained for a wide range of speed.     

MATLAB 软件在电机与拖动课程教学中的应用

针对电机与拖动课程中的概念抽象、数学推导复杂,其工作特性变化多的特点,本文将 MATLAB 仿真软件应用在本课程的教学中。通过两个仿真实例表明,应用 MATLAB 仿真软件可以使课堂中许多抽象的问题形象化、具体化,帮助学生更好地理解教学内容中的重点和难点,充分调动学生学习的积极性,保证教学效果和教学质量。     

An investigation of yarn snarling and balloon flutter instabilities in ring spinning

Abstract

This article reports theoretical and experimental investigation on yarn snarling and balloon fluttering in ring spinning. Yarn snarling and balloon fluttering affect yarn breakage in ring spinning. The theoretical model has incorporated the tangential component of air drag on a ballooning yarn, which was ignored in previous models. The results show that yarn snarling happens in the balloon when the ratio of yarn length in the balloon to balloon height is greater than a specific value that depends on the yarn type and count. Yarn tension experiences an obvious change before and after yarn snarling. The balloon flutter appears between normal balloons while the balloon loops are changing. Fluttering balloon shapes that oscillate periodically between two and three loop configurations as yarn tension varies periodically have also been observed experimentally.     

Energy cost and mechanical efficiency of riding a human-powered recumbent bicycle

When dealing with human-powered vehicles, it is important to quantify the capability of converting metabolic energy in useful mechanical work by measuring mechanical efficiency. In this study, net mechanical efficiency (η) of riding a recumbent bicycle on flat terrain and at constant speeds (v, 5.1–10.0 m/s) was calculated dividing mechanical work (w, J/m) by the corresponding energy cost (C_c, J/m). w and C_c increased linearly with the speed squared: w = 9.41 + 0.156 · v²; C_c = 39.40 + 0.563 · v². η was equal to 0.257 ± 0.0245, i.e. identical to that of concentric muscular contraction. Hence, i) η seems unaffected by the biomechanical arrangement of the human–vehicle system; ii) the efficiency of transmission seems to be close to 100%, suggesting that the particular biomechanical arrangement does not impair the transformation of metabolic energy in mechanical work. When dealing with human-powered vehicles, it is important to quantify mechanical efficiency (η) of locomotion. η of riding a recumbent bicycle was calculated dividing the mechanical work to the corresponding energy cost of locomotion; it was practically identical to that of concentric muscular contraction (0.257 ± 0.0245), suggesting that the power transmission from muscles to pedals is unaffected by the biomechanical arrangement of the vehicle.