Reprint of : Transient dynamics of spin-polarized injection in helical Luttinger liquids

We analyze the time evolution of spin-polarized electron wave packets injected into the edge states of a two-dimensional topological insulator. In the presence of electron interactions, the system is described as a helical Luttinger liquid and injected electrons fractionalize. However, because of the presence of metallic detectors, no evidences of fractionalization are encoded in dc measurements, and in this regime the system does not show deviations from its non-interacting behavior. Nevertheless, we show that the helical Luttinger liquid nature emerges in the transient dynamics, where signatures of charge/spin fractionalization can be clearly identified.     

Settling and fluidization of non Brownian hard spheres in a viscous liquid

A mean field approach is used to estimate the energy dissipation during the homogeneous sedimentation or the particulate fluidization of non Brownian hard spheres in a concentrated suspension of infinite extent. Depending on inertial screening and the range of the hydrodynamic interactions, the effective buoyancy force is determined either from the average suspension density in a Stokes flow or from the fluid density in the turbulent flow regime. An energy balance then yields a settling or fluidization law depending on the particle Reynolds number in reasonable agreement with the Richardson and Zaki correlation and recent experimental results for particle settling or fluidization. We further estimate the energy dissipation in the turbulent boundary layers around the particles to precise the Reynolds number dependence of the hindered settling function in the intermediate flow regime. Received 22 February 1999 and Received in final form 14 June 1999     

间断有限元方法在弹尾超音速喷流计算中的应用

采用间断有限元方法对超音速无粘喷流流动进行数值模拟.将二维双曲守恒方程的间断有限元方法发展到轴对称Euler方程,并就某导弹尾部超音速伴随射流进行数值计算.计算结果与实验照片反映的流动特征吻合较好,与高精度、高分辨率TVD格式的计算结果相比,间断有限元方法的计算结果在轴线反射点附近具有较高的分辨率,表明该方法对激波具有较强的捕捉能力,在激波阵面上不会产生振荡或抹平间断现象.     

An efficient and general numerical method to compute steady uniform vortices

Steady uniform vortices are widely used to represent high Reynolds number flows, yet their efficient computation still presents some challenges. Existing Newton iteration methods become inefficient as the vortices develop fine-scale features; in addition, these methods cannot, in general, find solutions with specified Casimir invariants. On the other hand, available relaxation approaches are computationally inexpensive, but can fail to converge to a solution. In this paper, we overcome these limitations by introducing a new discretization, based on an inverse-velocity map, which radically increases the efficiency of Newton iteration methods. In addition, we introduce a procedure to prescribe Casimirs and remove the degeneracies in the steady vorticity equation, thus ensuring convergence for general vortex configurations. We illustrate our methodology by considering several unbounded flows involving one or two vortices. Our method enables the computation, for the first time, of steady vortices that do not exhibit any geometric symmetry. In addition, we discover that, as the limiting vortex state for each flow is approached, each family of solutions traces a clockwise spiral in a bifurcation plot consisting of a velocity-impulse diagram. By the recently introduced “IVI diagram” stability approach [Phys. Rev. Lett. 104 (2010) 044504], each turn of this spiral is associated with a loss of stability for the steady flows. Such spiral structure is suggested to be a universal feature of steady, uniform-vorticity flows.     

The effective stiffness of an embedded graphene in a polymeric matrix

Modeling the real sizes of an embedded graphene and the surrounding polymer of a representative volume element in a molecular dynamics simulation is a tedious task. The less computational limitations made the continuum-based method a good candidate for modeling of nanocomposites. However, having a good knowledge of mechanical properties of the embedded graphene in a polymeric matrix is a challenge for employing a continuum-based method. Since the applied stress on the graphene/epoxy nanocomposites has not been directly transferred to the embedded graphene, it brings the following question to mind. Is the stiffness of the embedded graphene different from that of the isolated one? To answer to this question, a model was developed by combining the molecular dynamic simulation and the finite element method to calculate the stiffness of an embedded graphene in a polymeric matrix. The results show that the longitudinal stiffness of the embedded graphene is different from that of the isolated graphene and is a function of its length. The use of this relationship in the micromechanical method leads to consider the nanosize effect in macroscale. The results were compared with some available experimental data to validate the model.     

The HyperCASL algorithm: A new approach to the numerical simulation of geophysical flows

We describe a major extension to the Contour-Advective Semi-Lagrangian (CASL) algorithm [D.G. Dritschel, M.H.P. Ambaum, A contour-advective semi-Lagrangian numerical algorithm for simulating fine-scale conservative dynamical fields, Quart. J. Roy. Meteorol. Soc. 123 (1997) 1097–1130; D.G. Dritschel, M.H.P. Ambaum, The diabatic contour advective semi-Lagrangian algorithm, Mon. Weather Rev. 134 (9) (2006) 2503–2514]. The extension, called ‘HyperCASL’ (HCASL), uses Lagrangian advection of material potential vorticity contours like CASL, but a Vortex-In-Cell (VIC) method for the treatment of diabatic forcing or damping. In this way, HyperCASL is fully Lagrangian regarding advection. A grid is used as in CASL to deal with ‘inversion’ (computing the velocity field from the potential vorticity field).     

水滴烧蚀多脉冲激光推进性能

 用实时的推力测试方法研究了水滴烧蚀模式多脉冲TEA CO2激光推进的推进性能。用纹影法研究了伴随水滴烧蚀产生的激波等流场变化过程。多脉冲激光推进的比冲和冲量耦合系数等性能参数随激光脉冲重复频率的变小和脉冲数目的增加而逐渐下降。与激光传输相反和相同方向的激波传播最大速度分别为10 km/s和7 km/s。与纹影法结果同步获得的推力曲线表明：汽化过程对推力的形成过程贡献最大,激波也对推力的形成过程有一定贡献。     

Conformational stability of the tetrameric de novo designed hexcoil-Ala helical bundle

We have performed molecular dynamics method to investigate the conformational stability of the homotetramer form of HexCoil-Ala (PDB Code 3S0R). The previous experiments showed that the chains tend to form tetramer structures. The system was simulated in explicit water model at several temperatures by using isobaric-isothermal ensemble to better understand the behaviour of each monomer and its tetramer form. It was observed that central residues of each monomer have highly helical percentages in comparison with the termini residues. As the temperature increased, these percentages decreased, and bend-like configurations came into being due to the fact that the C-and N-terminals of the monomer were getting closer. When free energy landscapes of HexCoil-Ala were calculated by using the distance between Leu-Zipper and Ala-Coil interface, it was seen that the assemblies of monomers were very strong. What's more, the average values obtained from them were very close to the native case between 300?K and 350?K. It was also observed that the direct salt bridge forming between the residues E8 with R25 in the other chains plays a significant role for keeping tetramer structure. Consequently, our results are in better agreement with the results of experimental observations.     

Snapshot POD analysis of transient flow in the pilot stage of a jet pipe servo valve

The spatial evolution of a turbulent flow in the pilot stage of a jet pipe servo valve at the inlet pressure and deflection angle of the jet pipe is investigated using a large eddy simulation (LES). The pressure of the same flow field is measured by a high frequency dynamic pressure sensor in the experiments and is compared with the results of the LES, as well as their root-mean-square (RMS) and fast Fourier transform (FFT) results. The results of experiments and LES are in good agreement, indicating that LES is able to predict the flow dynamics. Velocity datasets based on LES are utilised to conduct the snapshot proper orthogonal decomposition (snapshot POD) technique. The snapshot POD analysis results of the first 4 modes show a full ability to directly visualise details of the coherent structures. The influences of the inlet pressure and deflection angle of the jet pipe are also discussed. Under different inlet pressures, the velocity eigenfunctions of the first mode are similar, while the locations and strengths of the vortices in high modes are different. The Lamb-Oseen vortices that affect the trajectory of jet streams are observed in the vicinity of the entrances of receiver channels only in the first mode, and several spindly vortices appear in the region of ?5?y/n?相似文献   

水滴烧蚀多脉冲激光推进性能

用实时的推力测试方法研究了水滴烧蚀模式多脉冲TEA CO2激光推进的推进性能。用纹影法研究了伴随水滴烧蚀产生的激波等流场变化过程。多脉冲激光推进的比冲和冲量耦合系数等性能参数随激光脉冲重复频率的变小和脉冲数目的增加而逐渐下降。与激光传输相反和相同方向的激波传播最大速度分别为10 km/s和7 km/s。与纹影法结果同步获得的推力曲线表明：汽化过程对推力的形成过程贡献最大,激波也对推力的形成过程有一定贡献。     

螺旋上升对自激发锯齿型双壁碳纳米管振荡行为的影响

运用分子动力学方法模拟了锯齿型双壁碳纳米管体系的振荡行为,其中旋转的内管施加了不同大小的螺旋上升长度.不同于以前关于扶手椅型碳纳米管的工作(Zeng Y H,et al.2016 Nanotechnology 27 95705),锯齿型的内管在施加了不同大小的螺旋上升长度之后,其管壁结构会产生畸变或缺陷.模拟过程中,锯齿型内管在施加一定的旋转速度以后保持自由,而固定的外管为无任何缺陷的理想锯齿型碳纳米管.分子动力学模拟结果显示锯齿型内管的轴向振荡行为与内管施加的螺旋上升长度密切相关.内管的振荡频率随着内管螺旋上升长度的增加而增加.但当内管的螺旋上升长度较大时,由于螺旋上升所引起的内管缺陷结构造成整个内管的破裂,从而导致其无法进行稳定的轴向振荡.模拟结果还显示,对于无螺旋上升的理想锯齿型碳管,虽然其轴向振荡效果非常微弱,但却可以作为一种具有恒定旋转频率的旋转致动器.此外,对螺旋上升长度为0.5 nm的内管在不同温度下的振荡性能进行了模拟分析,结果表明内管振荡的幅度随温度的升高而相应地增加,但当温度超过一定的临界值后,内管不能保持稳定的振荡.     

Dynamic response of turbulent swirling flames to acoustic perturbations

The dynamic response of a turbulent, perfectly premixed flame, stabilized by means of an aerodynamic flameholder, to an upstream acoustic perturbation of the approaching flow is investigated by means of experimental and analytical tools, and simulated through a large eddy simulation of the reacting flow. It is found that the main contribution to the unsteady heat release rate is due to the fluctuation in area of the flame front, which in turn is strongly influenced by the corresponding response of the flow field to the acoustic perturbation. Numerical data show that perturbing a swirling flow that undergoes vortex breakdown results in a strong displacement of the breakdown position along its axis, while its outer part only weakly responds to the perturbation. This results in a translational motion of the flame's anchoring point, which ultimately leads to an unsteady variation of the flame area and, therefore, of the amount of heat released. This unsteady heat release mechanism can be described in a way similar to that used for characterizing the dynamic behaviour of ducted flames, stabilized by means of a bluff-body flameholder; differently from these models, however, the anchoring point of the flame can now fluctuate freely in space, and the time delay of the system is no longer identified with the travelling time of a perturbation of the flame element along it, but is now associated with the oscillation of the breakdown position. Controlling the interaction between breakdown and acoustics should allow for obtaining optimal flame dynamics, so as to limit and possibly avoid the occurrence of strong pulsation peaks whenever the combustion device is operated in an acoustically closed system.     

Asymptotics of unit Burger number rotating and stratified flows for small aspect ratio

Rotating and stably stratified Boussinesq flow is investigated for Burger number unity in domain aspect ratio (height/horizontal length) δ<1 and δ=1. To achieve Burger number unity, the non-dimensional rotation and stratification frequencies (Rossby and Froude numbers, respectively) are both set equal to a second small parameter ?<1. Non-dimensionalization of potential vorticity distinguishes contributions proportional to (?δ)⁻¹, δ⁻¹ and O(1). The (?δ)⁻¹ terms are the linear terms associated with the pseudo-potential vorticity of the quasi-geostrophic limit. For fixed δ=1/4 and a series of decreasing ?, numerical simulations are used to assess the importance of the δ⁻¹ contribution of potential vorticity to the potential enstrophy. The change in the energy spectral scalings is studied as ? is decreased. For intermediate values of ?, as the flow transitions to the (δ?)⁻¹ regime in potential vorticity, both the wave and vortical components of the energy spectrum undergo changes in their scaling behavior. For sufficiently small ?, the (δ?)⁻¹ contributions dominate the potential vorticity, and the vortical mode spectrum recovers k⁻³ quasi-geostrophic scaling. However, the wave mode spectrum shows scaling that is very different from the well-known k⁻¹ scaling observed for the same asymptotics at δ=1. Visualization of the wave component of the horizontal velocity at δ=1/4 reveals a tendency toward a layered structure while there is no evidence of layering in the δ=1 case. The investigation makes progress toward quantifying the effects of aspect ratio δ on the ?→0 asymptotics for the wave component of unit Burger number flows. At the lowest value of ?=0.002, it is shown that the horizontal kinetic energy spectral scalings are consistent with phenomenology that explains how linear potential vorticity constrains energy in the limit ?→0 for fixed δ.     

Reflection of neutrons from a helical system

Analytical expressions have been derived for the reflection and transmission of neutrons from magnetic mirrors with helical magnetization. The reflection curves with and without neutron spin flip are presented. The resonance properties of helical systems have been analyzed.     

金属钨中氦行为的分子动力学模拟

采用分子动力学方法模拟了氦在金属钨中的扩散聚集行为. 首先,建立了氦与钨原子间相互作用势,短程部分采用ZBL势形式,长程部分采用从头算法数据,实现了两者之间的平滑连接. 通过计算氦在钨中不同间隙位的形成能发现,单个氦原子更易存在于金属钨中的四面体间隙位,这与最新的研究成果是一致的. 在400-1200 K的温度范围内,考察了氦原子在金属钨中的扩散行为,获得了扩散迁移能,其值介于实验值和从头算法结果之间. 最后,研究了氦的聚集行为,从能量的角度考察了氦团簇形成初期的生长机理. 研究发现,在氦团簇形成初期,氦团簇对氦的结合能随着氦团簇的生长有逐渐增大的趋势,说明氦团簇吸收氦的能力逐渐增强. 关键词： 氦扩散 氦团簇 辐照损伤 分子动力学模拟     

Molecular simulations of electroosmotic flows in rough nanochannels

A highly efficient molecular dynamics algorithm for micro and nanoscale electrokinetic flows is developed. The long-range Coulomb interactions are calculated using the Particle–Particle Particle–Mesh (P³M) approach. The Poisson equation for the electrostatic potential is solved in physical space using an iterative multi-grid technique. After validation, the method is used to study electroosmotic flow in nanochannels with regular or random roughness on the walls. The results show that roughness reduces the electroosmotic flow rate dramatically even though the roughness is very small compared to the channel width. The effect is much larger than for pressure driven flows because the driving force is localized near the walls where the charge distribution is high. Non-Newtonian behavior is also observed at much lower flow rates. Systematic investigation of the effect of surface charge density and random roughness will help to better understand the mechanism of electrokinetic transport in rough nanochannels and to design and optimize nanofluidic devices.     

Oscillatory motion of a viscoelastic fluid past a stretching sheet

Summary The boundary layer flow of a viscoelastic fluid (Walters' B model) past a stretching sheet is considered for investigation. The solution of the equation of motion is obtained using the power series method. The effects of the viscoelastic parameterk ₁ on the flow have been investigated. It is also found that the effects of unsteadiness in the wall velocity and skin-friction are appreciable.     

II. Recirculation flow induced by a bubble stream rising in a viscous liquid

The recirculation flow induced by the rising motion of a bubble stream in a viscous fluid within an open-top rectangular enclosure is studied. The three-dimensional volume averaged conservation equations are solved by a control-volume method using a hybrid finite differencing scheme to describe the liquid phase hydrodynamics. The momentum exhange between the bubbles and the liquid phase is modeled with a source term equals to the volumetric buoyancy force acting on the gas in the bubble stream. The volumetric buoyancy force accounts for in line interactions between bubbles through the average gas volume fraction in the gas liquid column which depends on the size and the rising velocity of bubbles. The fluid flow within an open-top rectangular enclosure is further investigated by particle image velocimetry for a bubble stream rising in a water-glycerol solution. The measured fluid velocities in a vertical plane are compared with the predictions of the numerical model over a wide range of fluid viscosity (43 mPa s-800 mPa s) and gas flow rates. Finally, the recirculation flows resulting from the interaction of two neighbouring vertical bubble streams are studied. Received: 23 July 1997 / Revised: 19 December 1997 / Accepted: 11 May 1998     

The Green's matrix and the boundary integral equations for analysis of time-harmonic dynamics of elastic helical springs

Helical springs serve as vibration isolators in virtually any suspension system. Various exact and approximate methods may be employed to determine the eigenfrequencies of vibrations of these structural elements and their dynamic transfer functions. The method of boundary integral equations is a meaningful alternative to obtain exact solutions of problems of the time-harmonic dynamics of elastic springs in the framework of Bernoulli-Euler beam theory. In this paper, the derivations of the Green's matrix, of the Somigliana's identities, and of the boundary integral equations are presented. The vibrational power transmission in an infinitely long spring is analyzed by means of the Green's matrix. The eigenfrequencies and the dynamic transfer functions are found by solving the boundary integral equations. In the course of analysis, the essential features and advantages of the method of boundary integral equations are highlighted. The reported analytical results may be used to study the time-harmonic motion in any wave guide governed by a system of linear differential equations in a single spatial coordinate along its axis.