高雷诺数流动理论、算法和应用的若干研究进展

在黏性流体力学的历史发展中,Navier-Stokes (NS)方程组的 简化理论、相应算法和应用构成了不同历史时期的学科前沿、核心内 容的应用热点。以此为线索,简要评述经典边界层、多层(三层)边界 层、干扰边界层、扩散抛物化(DP) NS方程诸理论、相应算法和应用的 若干研究进展;诸理论之间以及他们与实验的关系;简化湍流计算的 一点注释以及物理分析和数值模拟相结合的一些问题。     

利用低雷诺数形式的k-ε模型预测管道中的复杂湍流场

本文利用低雷诺数形式的 k-8湍流模式对先突缩再突扩的轴对称管内的复杂湍流场进行了理论预测,获得了平均速度、流函数、湍动能、湍流耗散率和平均压力场的分布曲线、对理论计算结果的特征进行了分析.     

清华大学钱学森班的理论力学教学实践

扼要介绍了给清华大学钱学森班讲授8年的 理论力学课及其升级课程``动力学与控制基础'的教学实践情况,分析了新时代学生基础差异化、教学要求多样化、学生成长需求个性化对教学的挑战,对新建课程的教学内容、体系、教材、考核等安排做了说明。     

航空专业研究生课《飞机结构强度》教学改革探析

根据《飞机结构强度》课程的特点,以及研究生教学、特别是航空航天专业研究生教学的目的,从教学体系、内容、教学方法以及考核手段的改进等几个方面,进行了《飞机结构强度》教学改革的探索,旨在实现现代教学、适应研究型大学的教学要求,提高教学质量,使学生系统地、全面地认识和掌握航空航天结构的设计思想.     

对《光弹性矩阵理论》的讨论

Theocaris 和 Gdoutos 所著《Matrix Theory of Photoelasticity》是采用矩阵理论讲述光弹性基本原理的一本著作.本文拟对该书某些不足之处进行讨论,这里包括:Poincare'球操纵的普遍情况,等效定理,三维光弹性基本方程,椭园偏振光的测定以及对该书某些错误的改正.希望通过补充讨论使光弹性矩阵理论更加完善.     

理论力学教具DIY系列(四)一个教具的受力分析与讨论

受力分析是理论力学教学中的重要内容,而多点摩擦问题是其中的难点.在目前的教学安排中,静力学有少量求解多点摩擦的习题,学生可以画图、列出方程并求解;但是在动力学中,几乎没有涉及多点摩擦的习题,而且也不要求解方程.     

用LS-DYNA辅助《材料力学》冲击问题的教学

利用显式动力分析软件LS-DYNA对小球自由落体冲击简支梁进行了数值模拟,得到了梁的位移和应力的时程曲线. 将模拟结果与《材料力学》教材中广泛采用的冲击问题简化解法的结果进行了对比. 通过观察时程曲线和冲击动画过程,可使学生对冲击问题有较深入的理解.     

基于网络的工科《理论力学》多媒体教学系统

通过对教学系统、教学媒体以及现代教育技术的分析和讨论,探索了"基于网络的多媒体教学系统",这一现代教育技术的必要性、可行性以及实现该系统的构思．     

On the low reynolds number motion of two droplets

Exact solutions are derived for the quasi steady-state creeping flow internal and external to two spherical droplets moving along their line-of-centers. Numerical results are presented, which include all previous solutions as special cases.     

An experimental investigation of moderate reynolds number flow in a T-Channel

An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20 × 20 mm inlet ID and 20 × 40 mm outlet ID) has been conducted for a Reynolds number Re range of 56–422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable to the microscale. 2-D planar particle imaging velocimetry (PIV) and laser-induced fluorescence (LIF) were used in multiple locations of the T-channel to investigate local dynamical behaviors. Steady symmetric and asymmetric flow regimes predicted in the literature, which is largely numerical, are experimentally verified. Unsteady flow regimes, which are numerically predicted to occur at higher Re but have not yet been experimentally characterized, are also examined, and real-time LIF results illuminate the evolution of unsteady structure. Experimental data of the present resolution and scope are not presently available for unsteady flow regimes. Time scales are presented for unsteady flow regimes, which are found to exhibit periodic behavior and to occur for Re  ≥ 195. An unsteady symmetrical regime is identified for Re ≥ 350 that is detrimental to mixing. Momentum fields and dynamical behaviors of all flow regimes are characterized in detail, such that published mixing trends may be better understood. Results of all experimental trials were used to construct a regime map. A symmetric topology is found to be dominant for Re from 56 to 116, when flow is steady, and 350 to 422, when flow is characterized by unsteady stagnation-point oscillation in the T-channel junction. Asymmetric flow, which is positively indicated for mixing, is dominant for Re between 142 and 298, and the fluid interface exhibits both steady (two standing vortices) and unsteady (shear-layer type roll-up) behaviors. This result is based on multiple experiments and suggests a practical operating range of 142  ≤ Re ≤ 298 where asymmetric flow is highly likely to experimentally occur. The identification of an upper limit on Re,  beyond which mixing appears negatively impacted by a more symmetrical momentum field, is practically important as pressure drops on the microscale are significant.     

Some new characteristics of the confined flow over circular cylinders at low reynolds numbers

This study summarises some new characteristics of the fluid flow over a confined circular cylinder at low Reynolds numbers. Results from both two- and three-dimensional direct numerical simulations are presented at blockage ratio between 0.1 and 0.9 and Reynolds number between 120 and 500. Floquet stability analysis of selected cases will also be presented. From the two-dimensional simulations, it is found that the fluctuating lift forces decreases with blockage ratio and becomes zero (where the flow is steady) at blockage ratio of approximately 0.7–0.8. Upon further increasing the blockage ratio to 0.9, the simulations show a dramatic increase in the fluctuating lift forces, nearly an order of magnitude greater than previously reported for an unconfined cylinder flow. It is also found that for blockage ratio of 0.5, there is a long term two-dimensional instability that becomes more prominent with increasing Reynolds number. This instability has a time scale of approximately 105 time units ($D/U_{\mathit{max}}$) at Reynolds number of 500. In addition, the transition between two- and three-dimensional flow at blockage ratios up to 0.5 is investigated. It is shown that the transition Reynolds number decreases with increasing blockage ratio. At high blockage ratio of 0.5, as we increase the Reynolds number, the transition to three-dimensional flow is shown to go from unsteady two-dimensional to steady three-dimensional before transitioning to unsteady three-dimensional flow.     

A study on the mechanism of high-lift generation by an airfoil in unsteady motion at low reynolds number

The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re=100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and , the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short “vortex street” in front of the airfoil and the “vortex street” induces a “wind”; against this “wind” the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect. The project supported by the National Natural Science Foundation of China (19725210)     

Linear approximation for the flow of a conducting gas with a high magnetic reynolds number through a channel

In this paper, we discuss the flow of a nonviscous and non-heat-conducting gas through a channel of variable cross section under the influence of a transverse magnetic field. For high magnetic Reynolds numbers, the flow is shown to consist of a core and current layers at the electrodes and at the fixed channel walls. The distributions of currents and other parameters in the core and in the current layers are found analytically, in a linear approximation. The Joule dissipation in the current layers may be more intense than that in the core. The longitudinal currents and Joule dissipation increase with increasing Hall parameter in the electrode layers. Zhigulev [1] has shown that magnetic boundary layers may form in the flow of a conducting gas when there is a high magnetic Reynolds number (R_m«1). He illustrated this situation by the shielding of a plasma flow from the magnetic fields produced near a plate which is electrically isolated from the plasma and through which a current is flowing. In an incompressible fluid, the layer thickness is proportional to R_m ^?1/2. Morozov and Shubin [2] have offered a linear-approximation treatment of the structure of the electromagnetic near-electrode layers which arise during the flow of a nonviscous plasma with a high R_m and a small “exchange” parameter ξ≈H/R_m, for flow transverse to a magnetic field and near a corrugated wall. They pointed out the possible formation of “dissipationless” near-electrode layers with thicknesses on the order of the Debye or electron Larmor radii, and a “dissipative” layer whose thickness increases along the length of the electrodes and is proportional to (R_mc_B ²/c_T ²)^?1/2, where c_B and c_T are the magnetic and thermal sound velocities. Morozov and Shubin studied the properties of dissipationless and dissipative electromagnetic layers at segmented accelerator electrodes through which a current is passing, for an arbitrary “exchange” parameter, in [2] and [3], respectively. The exchange parameter ξ was found in [4]. Such layers should also exist at solid electrodes and at the nonconducting walls of an accelerator channel. Study of the two-dimensional flow in a channel is significantly simplified when such layers are present.     

Magnetic effects on the flow with a great magnetic reynolds number by the presence free convection and mass transfer

An analysis of the effects of a magnetic field on the flow past an infinite plate by the presence free-convection and mass transfer is considered when the magnetic Reynolds number of the flow is not small. The influence of magnetic parameterM on the velocity and the skin-friction is discussed with the help of graphs, when the plate is being cooled by the free convection currents.

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Magnetische Einflüsse auf eine Strömung mit großen magnetischen Reynolds-Zahlen unter Anwesenheit freier Konvektion und Stoffübertragung

Zusammenfassung Es werden die Einflüsse eines magnetischen Feldes auf die Strömung an einer unendlichen Platte mit freier Konvektion und Stofftransport analysiert, wobei die magnetische Reynolds-Zahl der Strömung nicht klein ist. Diskutiert wird der Einfluß des magnetischen ParametersM auf die Geschwindigkeit und die Oberflächenreibung mit Hilfe graphischer Methoden, wobei die Platte durch die freie Konvektion gekühlt wird.

     

对称翼型低雷诺数小攻角升力系数非线性现象研究

采用Rogers发展的三阶Roe格式,求解非定常不可压N-S方程,时间方向为二阶精度双时间步方法, 数值模拟了对称翼型SD8020低雷诺数(Re=40000,100000)条件下,流场层流分离涡结构和升力系数随攻角的变化．同试验比较证明了数值模拟的正确性．通过对数值模拟时均化流场结果的详细分析,发现对称翼型在小雷诺数0°攻角附近出现的层流分离泡,其内部结构和演化规律都不同于经典层流分离泡模型,从而提出了一种后缘层流分离泡模型．并应用该模型对对称翼型小攻角低雷诺数流场特性以及升力系数非线性效应的形成机理进行了研究和解释．