Effect of a high hydrostatic pressure on the dynamic instability of dislocation motion

The effect of a high hydrostatic pressure on the dislocation dipole vibration frequency and the forces of dynamic drag of dislocations by dislocation dipoles and of dislocation pairs by pinned dislocations is studied. Analytical expressions are obtained for the force of dynamic drag of mobile dislocation pairs by pinned dislocations and for the force of drag of isolated dislocations by dislocation dipoles in hydrostatically compressed crystals. Hydrostatic compression leads to a significant increase in these forces. This effect is most pronounced in alkali-halide crystals, where the drag force increases by a factor of 1.5–2.0.     

Dynamic steering beams for efficient force measurement in optical manipulation

An efficient and inexpensive method that uses a glass plate mounted onto a motorized rotating stage as a beam-steering device for the generation of dynamic optical traps is reported.Force analysis reveals that there are drag and trapping forces imposed on the bead in the opposite directions,respectively,in a viscous medium.The trapped bead will be rotated following the beam’s motion before it reaches the critical escape velocity when the drag force is equal to the optical trapping force.The equilibrium condition facilitates the experimental measurement of the drag force with potential extensions to the determination of the viscosity of the medium or the refractive index of the bead.The proposed technique can easily be integrated into conventional optical microscopic systems with minimum modifications.     

Effect of finite charging time on particulates in RF discharges

The effect of the finite charging time on spherical and cylindrical particulates in an RF discharge is considered. Using probe theory, analytical expressions for the charging rate under various conditions are derived and compared with numerical solutions. Scaling of the charging time with discharge parameters, in particular the electron temperature, is demonstrated. Using a one-dimensional fluid model for an rf discharge, the equilibrium electric and ion drag forces are compared for spherical and cylindrical particulates. The effect of the finite charging time on the dynamics of particulates of various sizes in the model discharge are discussed. Overall, a long cylindrical particle with the same mass as a spherical particle charges up more slowly to a larger net (negative) charge and has a significantly larger polarization. As a consequence, small cylindrical particles introduced near the electrodes are more likely to escape from the sheath region before becoming fully charged     

Micro-force measurement of drag on a small flat plate in the presence of a corona discharge

The design of a micro-force sensor suitable for the measurement of corona drag and other low velocity drag studies in a small laboratory wind tunnel facility is described. Example drag data are given for dc corona discharge generated by sharp parallel electrodes mounted on a microscope glass slide with discharge parallel to the air flow. The arrangement simulates two-dimensional flow over a flat plate useful for theoretical analysis. Measurements of free stream wind velocities in the range 0–210 cm/s with attendant drag down to 10⁻⁷ N can be detected in this facility depending on the calibration. The force sensor utilizes two strain gages mounted on a 0.127 mm stainless steel “feeler gage” in a cantilever arrangement. A bridge circuit provides sensitivities in the range 40–250 N/mV using a gravitational calibration technique. Anomalous effects from suspension wires and the interaction of electrostatic forces with the surroundings are discussed.     

The method of images for regularized Stokeslets

The image system for the method of regularized Stokeslets is developed and implemented. The method uses smooth localized functions to approximate a delta distribution in the derivation of the fluid flow due to a concentrated force. In order to satisfy zero-flow boundary conditions at a plane wall, the method of images derived for a standard (singular) Stokeslet is extended to give exact cancellation of the regularized flow at the wall. As the regularization parameter vanishes, the expressions reduce to the known images for singular Stokeslets. The advantage of the regularized method is that it gives bounded velocity fields even for isolated forces or for distributions of forces along curves. These are useful in the simulation of ciliary beats, flagellar motion, and particle suspensions. The expression relating force and velocity can be inverted to find the forces that generate a given velocity boundary condition. The latter is exemplified by modeling a cilium as a filament moving in a three-dimensional flow. The cilium velocity at various times is constructed from known data and used to determine the force field along the filament. Those forces can then reproduce the flow everywhere. The validity of the method is evaluated by computing the drag on a sphere moving near a wall. Comparisons with known expressions for the drag show that the method gives accurate results for spheres even within a distance from the wall equal to the surface discretization size.     

亚微米燃烧源颗粒物间的相互作用研究

采用微观可视化的高速摄像技术直接观察了燃烧源亚微米颗粒物间的相互作用形态,发现了亚微米颗粒间存在“吸引-旋绕-排斥”形态的相互作用。通过颗粒受力分析,认为传统所考虑的曳力、重力、库仑力、范德华力不能解释这种相互作用．根据亚微米颗粒荷电的不均匀性特征提出颗粒静电力应包括净电荷库仑力和感应偶极子间作用力两部分．感应偶极子间作用力是近程力,具有径向和周向两个方向,在颗粒比较接近的时候迅速增大,并能导致颗粒之间相互旋绕和排斥。该力与上述几种力综合起来可以很好地解释实验发现的这种颗粒相互作用形态。     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II superconductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces. The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed. The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.     

Drag force reduction on an airfoil via glow discharge plasma-based control

Glow discharge plasma on a solid surface will result in a body force which modifies the pressure distribution along the flow boundary layer, and consequently re-attaches the separated flows for reduction of a hydrodynamic drag force. In this paper, we investigate the discharge performance of various plasma-actuated electrodes in terms of their arrangements and structures. The resulting optimal configuration for the electrode separation distance was used to develop a flexible actuated panel to be mounted onto a NACA 0015 airfoil. Both uniphase and eight-phase power inputs were used to examine its drag reduction performance at various attack angles and flow velocities. Numerical calculations were also performed by including an electrostatic body force in the hydrodynamic equation. Good agreement were found between the numerical and experiment results.     

Heat transfer analysis of MHD rotating flow of Fe_3O_4 nanoparticles through a stretchable surface

The flow of a magnetite-H_2O nanofluid has been considered among two rotating surfaces,assuming porosity in the upper plate. Furthermore, the lower surface is considered to move with variable speed to induce the forced convection. Centripetal as well as Coriolis forces impacting on the rotating fluid are likewise taken into account. Adequate conversions are employed for the transformation of the governing partial-differential equations into a group of non-dimensional ordinary-differential formulas. Numerical solution of the converted expressions is gained by means of the shooting technique. It is theoretically found that the nanofluid has less skin friction and advanced heat transport rate when compared with the base fluid. The effect of rotation causes the drag force to elevate and reduces the heat transport rate. Streamlines are portrayed to reveal the impact of injection/suction.     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II super-conductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces.

The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed.

The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.     

Drag forces on inclusions in classical fields with dissipative dynamics

We study the drag force on uniformly moving inclusions which interact linearly with dynamical free field theories commonly used to study soft condensed matter systems. Drag forces are shown to be nonlinear functions of the inclusion velocity and depend strongly on the field dynamics. The general results obtained can be used to explain drag forces in Ising systems and also predict the existence of drag forces on proteins in membranes due to couplings to various physical parameters of the membrane such as composition, phase and height fluctuations.     

Fluctuation-induced electromagnetic interaction of a moving particle with a plane surface

The most general (nonrelativistic) formulas for the force of attraction to the surface and for the drag of a nonrelativistic atom moving parallel to it, as well as for the lateral and normal forces acting on a moving dipole molecule and on a charged particle (in the case of parallel and perpendicular motion), are derived for the first time in the framework of the fluctuational electromagnetic theory. The dependences of these forces on the velocity, temperature, separation, and dielectric properties of the atom and the surface are derived. The effect of the nondissipative resonance interaction between a moving neutral atom and the field of surface plasmons, as well as the possible emergence of a positive (accelerating) force acting on the atom (nanoprobe), is substantiated theoretically. The role of dynamic fluctuational forces and their possible experimental measurement when using the quartz microbalance technique and an atomic-force microscope (in the dynamic mode), as well as during deceleration of atomic beams in open nanotubes, are considered. The correctness of the obtained results is confirmed by their agreement with most of the available theoretical relations derived by other authors.     

Theory of electrodynamic levitation with a continuous sheet track — Part I

Exact analytical expressions for forces on moving rectangular current-carrying coils above and below an infinite conducting sheet track of arbitrary thickness are developed. These general expressions for the lift and drag forces acting on the excitation coils as functions of speed are investigated for normal flux, null flux and brake flux systems and discussed with the aid of numerical calculations. A concise parameter study is also made. The system velocity characteristics with constant load (technologically more relevant) are given for the first time for normal and null flux systems.     

电弧特性及其对熔池形貌影响的数值模拟

通过电弧模型与熔池模型耦合数值模拟,研究了氩弧和氦弧特性及其对SUS304不锈钢钨极惰性气体保护（TIG）焊熔池形貌的影响.通过比较氩弧和氦弧的温度轮廓线以及阳极表面电流密度和热流密度分布发现,氦弧的径向距离比氩弧收缩明显,导致更多热量传递给阳极.模拟了氩弧和氦弧下浮力、电磁力、表面张力和气体剪切力分别对熔池形貌的影响.结果表明：不论是在氩弧还是在氦弧下熔池中表面张力是影响熔池形貌的最主要驱动力.在氩弧下,影响熔池形貌的另一个重要的驱动力是气体剪切力,而氦弧下则是电磁力.由于电磁力引起的内对流运动增加了熔深,从而导致相同氧含量时氦弧下的熔深和焊缝深宽比要高于氩弧下的熔深和焊缝深宽比.随着氧含量的增加,氩弧和氦弧下的焊缝深宽比均先增加而后保持不变.焊缝深宽比的模拟结果与实验结果符合较好. 关键词： 氩弧 氦弧 电弧特性 熔池形貌     

Theory of electrodynamic levitation with a continuous sheet track — Part II

Various approximate expressions for lift and drag forces for null, normal and brake flux systems are developed from the rigorous expressions given in Part I of this work. Physical significance of the approximations used and the limits of validity are discussed. Results previously known in the literature are also discussed critically in this context.     

Dynamic Effects in a Defective System of Crystal

Works devoted to studying the interaction between dislocations and structural defects in a crystal lattice in the dynamic region of velocities are considered. Analytic expressions for the force of dynamic drag and the dynamic yield stress for a wide range of problems of dislocation dynamics are obtained using the theory of dynamic interaction between structural defects.     

煤粉颗粒所受Magnus力的数值模拟

本文采用数值模拟方法计算了煤粉颗粒所受的Ｍａｇｎｕｓ力,考察了颗粒的旋转速度、流动Ｒｅ数对Ｍａｇｎｕｓ力的影响,并且给出了相应的关系公式．通过比较煤粉颗粒在运动过程中所受到的Ｍａｇｎｕｓ力与气动力的大小,得出：当煤粉颗粒的旋转速度为１８００转／分时,其大小约为气动力的１％,因此在实际的煤粉颗粒受力分析中是完全可以忽略该力的。     

蒸汽爆炸粗混合过程中的蒸发阻力模型

在蒸汽爆炸的粗混合过程中,由于液体的快速蒸发,高温金属液滴的周围会产生一层很薄的蒸汽膜,此时液滴周围的边界层流动与没有液体蒸发时有很大的不同,因此,常温情况下的小球在连续液体中运动时的通用阻力模型在这种情况下是不适用的．本文通过受力分析,考虑了高温小球受力的分布和表面蒸发对小球周围力的影响,从阻力的基本机理上分析了蒸发状态下小球的运动阻力,分别提出了高温颗粒穿过自由表面时与其在液体中运动时的蒸发阻力模型．分析表明,当小球温度高于2500 K,特别是在靠近自由表面的区域,由于小球表面液体蒸发而产生的蒸发阻力作用非常明显．分析指出,小球的入水初速、小球表面的液体蒸发速率以及汽膜厚度都是影响小球运动阻力大小的重要因素．     

离散颗粒层被横向推移过程中的力学行为研究

研究发现,颗粒物质层被匀速推移挤压过程中,所需推移力先以线性规律增加,在某一确定点后,则会以指数规律增加.而颗粒物质是由众多离散颗粒组成的软凝聚态物质,其宏观上反映的是离散颗粒的个体性质和凝聚态物质的集体效应.颗粒与颗粒之间以及颗粒与边界之间的细观尺度接触力链的构成以及演变规律将会直接影响各种宏观受力情况,其摩擦力与挤压力便是力链的主要构成形式.围绕着定量描述细观力链特征,从而揭示力的变化规律这一目标,采用计算机模拟的方法,依照球形颗粒Hertz法向接触理论和Mindlin-Deresiewicz切向接触理论,对重力作用下不同数目的三维等径球体颗粒层的推移情况进行了离散元仿真模拟,量化分析了推移力变化规律、各摩擦力变化规律以及力链分布规律,发现摩擦力与挤压力在颗粒堆积的不同阶段对力链的构成起到了不同的主次作用,使得力链发生强弱演变,从而发现了推移颗粒物质层时推移力的变化规律及原因.这些结果有利于从力链角度揭示颗粒内部和颗粒与各边界之间的受力情况.