Nodal Point Relation for the Distribution of Sediments at Channel Bifurcation

This technical note explores the applicability of the general nodal point relation to describe the distribution of sediments at channel bifurcation. Experiments were conducted in a physical model of channel bifurcation to estimate the coefficient and exponent of the nodal point relation for different nose angles and upstream discharges. It was found that the nose angle is the major variable for the distribution of sediments to the downstream branches. The value of the exponent k found from experimental results was compared with that of a theoretical analysis. For a stable equilibrium, the value of k is greater than 5/3 from theoretical consideration when the Engelund–Hansen sediment transport formula is used. This was confirmed from the experimental results. This suggests that the distribution of sediments at channel bifurcation can be expressed by the general nodal point relation.     

Nonlinear Coupled Seismic Sliding Analysis of Earth Structures

Earthquake-induced sliding displacements of earth structures are generally evaluated using simplified sliding block analyses that do not accurately model the seismic response of the sliding mass nor the seismic forces along the slide plane. The decoupled approximation introduced to capture each of these effects separately is generally believed to be conservative. However, recent studies using linear viscoelastic sliding mass models have revealed instances where the decoupled approximation is unconservative. In this paper, a coupled analytical model that captures simultaneously the fully nonlinear response of the sliding mass (necessary for intense motions) and the nonlinear stick-slip sliding response along the slide plane is presented. The proposed sliding model is validated against shaking table experiments of deformable soil columns sliding down an inclined plane. The effect of sliding on the response of earth structures is evaluated, and comparisons are made between sliding displacements calculated using coupled and decoupled analytical procedures with linear and nonlinear material properties. Nonlinearity resulting from stick-slip episodes is often the dominant source of nonlinearity in this problem. The decoupled approximation was unconservative primarily for intense ground motions for systems with low values of ky, larger values of ky∕kmax, and high period ratios (Ts∕Tm). Results indicate that a decoupled analysis is adequate for earth structures that are not expected to experience intense, near-fault motions. However, for projects undergoing intense, near-fault ground motions, a fully nonlinear, coupled stick-slip analysis is recommended.     

In-Plane Elastic Stability of Arches under a Central Concentrated Load

This paper is concerned with the in-plane elastic stability of arches with a symmetric cross section and subjected to a central concentrated load. The classical methods of predicting elastic buckling loads consider bifurcation from a prebuckling equilibrium path to an orthogonal buckling path. The prebuckling equilibrium path of an arch involves both axial and transverse deformations and so the arch is subjected to both axial compression and bending in the prebuckling stage. In addition, the prebuckling behavior of an arch may become nonlinear. The bending and nonlinearity are not considered in prebuckling analysis of classical methods. A virtual work formulation is used to establish both the nonlinear equilibrium conditions and the buckling equilibrium equations for shallow arches. Analytical solutions for antisymmetric bifurcation buckling and symmetric snap-through buckling loads of shallow arches subjected to this loading regime are obtained. Approximations for the symmetric buckling load of shallow arches and nonshallow fixed arches and for the antisymmetric buckling load of nonshallow pin-ended arches, and criteria that delineate shallow and nonshallow arches are proposed. Comparisons with finite element results demonstrate that the solutions and approximations are accurate. It is found that the existence of antisymmetric bifurcation buckling loads is not a sufficient condition for antisymmetric bifurcation buckling to take place.     

Buckling Mode Interaction in Fixed-End Column with Central Brace

The postbuckling behavior of an elastic fixed-end column with an elastic brace at the center is investigated. Attention is focused on those of brace stiffness near its threshold value at which, under axial load, the column becomes critical with respect to two buckling modes simultaneously. We show that, for the brace stiffness greater than the threshold value, there are precisely two secondary bifurcation points on each primary postbuckling path bifurcating from one of the least two classical buckling loads, and the corresponding secondary postbuckling paths connect all of these secondary bifurcation points in a loop. For the brace stiffness less than the threshold value, no secondary bifurcation occurs. The asymptotic expansions of the primary and secondary postbuckling paths are constructed. The stability analysis indicates that, when the brace stiffness goes beyond its threshold value, the primary postbuckling path with a node in the center becomes unstable from stable by means of the secondary bifurcation (i.e., secondary buckling occurs).     

The equilibrium shape of a misfitting precipitate

We examine the equilibrium morphologies of precipitates with either a tetragonal or purely dilatational misfit in an elastically anisotropic medium with cubic symmetry under conditions of plane strain. We find that particles with a dilatational misfit are nearly spherical at small sizes, take on four-fold symmetric shapes at intermediate sizes and then undergo a supercritical symmetry-breaking bifurcation to two-fold symmetric shapes aligned along the elastically soft directions of the crystal. A tetragonal misfit breaks this four-fold to two-fold supercritical bifurcation when the direction of the tetragonality is coincident with one of the elastically soft directions of the crystal. Such a tetragonal misfit can lead to two-fold equilibrium particle shapes which are local energy minima, or metastable, and in some cases have large negative interfacial curvatures. When the tetragonality is not in an elastically soft direction, the supercritical bifurcation is not broken and the particles can take on unusual diamond-like or S-shaped morphologies.     

An ultrasonic method for reconstructing the two-dimensional liquid-solid interface in solidifying bodies

A method is demonstrated for reconstructing the two-dimensional boundary of the liquid core in a solidifying strand of aluminum using ultrasonic time-of-flight data. Approximating the boundary by an ellipse leads to a model that can be used to calculate the time of flight along any path in the plane of the ellipse. The shape and location of the ellipse is specified by four free parameters. A least-squares algorithm is then used to find the set of four parameters that provides the best fit to the measured time-of-flight data on ten intersecting paths through the strand. For a strand with a 6-in. square cross section, the agreement between the actual interface and the computed interface was within about ±2 pct. Reference data for the temperature dependence of the longitudinal velocity for solid 1100 aluminum are included.     

基于时变局部模型的无人驾驶车辆路径跟踪

目前常用于无人驾驶车辆路径跟踪控制的有模型控制方法有两类,一类是基于全局模型的控制方法,另一类是基于局部模型的控制方法。基于全局模型的路径跟踪控制中无人驾驶车辆的纵向速度与全局坐标系中的横向、纵向位移误差之间存在随航向角变化的耦合关系,这种耦合关系使得控制器无法将纵向速度作为控制输入来提高路径跟踪控制的精确性。基于局部模型的路径跟踪控制器通常采用误差模型作为参考模型,这种模型使得控制器在参考路径曲率变化幅度较大时精确性较低。针对前述问题,基于非线性模型预测控制滚动优化的原理,提出一种基于时变局部模型的无人驾驶车辆路径跟踪控制方法,并在低速高附着路面、低速低附着路面和高速低附着路面等工况下进行仿真验证。在仿真结果中,相比于基于全局模型的路径跟踪控制器、基于局部模型的路径跟踪控制器以及Stanley路径跟踪控制器,基于时变局部模型的路径跟踪控制器精确性更高,其位移误差绝对值不超过0.3342 m,航向误差绝对值不超过0.0913 rad。      

液压溢流阀的失稳分析和实验研究

分析了液压回路中溢流阀的结构特点,考虑液压油压缩性、管道弹性和阀芯碰撞阀座时的能量损失,建立了溢流阀量纲一形式的数学模型,并进行了Lyapunov指数分析,目的是研究溢流阀的失稳机理和颤振行为.应用非光滑动态系统理论和MATLAB软件绘制单参数和双参数分岔图,理论解释了阀芯离开阀座时的擦边分岔.结果表明,溢流阀入口流量和预设压力直接决定着阀的振荡特性,并且存在着Hopf分岔、擦边分岔、周期和混沌等现象.搭建了测试平台,得到弹簧预压缩量x₀=5 mm情况下的阀芯位移分岔图,对数学模型进行了验证.      

The effect of the surface on grain boundary migration in austenitic stainless steel weld metal

The direction of grain boundary (GB) migration that occurs after solidification in the original surface of the weld of an austenitic stainless steel was compared to that in the interior (the depth 100 μm from the original surface). The results are as follows. In the interior, the direction of GB migration was decided by the moving direction of the triple junctions that bring about their equilibrium. This equilibrium results from the GB approaching a cross angle of 120 deg with another GB in the vicinity of the triple junction. Meanwhile, in the original surface, the direction of GB migration was decided by the direction that makes the GB plane, which is just at solidification, being perpendicular to the surface. That is, the GB plane migrates to be perpendicular to the original surface. This GB migration was often observed when the angle between the GB plane and the original surface had been less than 40 deg. Furthermore, the amount of the GB migration had rapidly increased with the decrease in the angle. The validity of these results is examined with a discussion of the reduction of the interfacial energy.     

Nonlinear Aeroelasticity: Continuum Theory, Flutter/Divergence Speed, and Plate Wing Model

This paper formulates flutter/divergence instability problems using continuum models for structure and air flow as coupled nonlinear partial differential equations. The structure model is a Kirchoff CFFF thin plate allowing for nonzero thickness and camber bending. The aerodynamics is modeled by the transonic small disturbance potential equation. The aeroelastic boundary conditions are derived for nonzero angle of attack. A central result is the time domain model as a nonlinear convolution/evolution equation in a Hilbert space. Flutter speed is characterized as a Hopf bifurcation point, completely determined by the linearized equations. The main tool in solving the linear equations is the Possio equation for nonzero angle of attack. Divergence speed is shown to be determined by an eigenvalue problem for linear operators. The corresponding stationary (steady state) solutions are more regular in the transonic range (as M goes to one) if the angle of attack is nonzero.     

Evaluation of oxygen uptake and delivery in critically ill patients: a statistical reappraisal

Computer simulation of pulsatile non-Newtonian blood flow has been carried out in different human carotid artery bifurcation models. In the first part of the investigation, two rigid walled models are analysed, differing in the bifurcation angle (wide angle and acute angle bifurcation) and in the shape of both the sinus (narrow and larger sinus width) and the bifurcation region (small and larger rounding of the flow divider), in order to contribute to the study of the geometric factor in atherosclerosis. The results show a significant difference in the wall shear stress and in the flow separation. Flow recirculation in the sinus is much more pronounced in the acute angle carotid. An important factor in flow separation is the sinus width. In the second part of the study, flow velocity and wall shear stress distribution have been analysed in a compliant carotid artery bifurcation model. In the mathematical model, the non-Newtonian flow field and the idealized elastic wall displacement are coupled and calculated iteratively at each time step. Maximum displacement of approximately 6% of the diastolic vessel diameter occurs at the side wall of the bifurcation region. The investigation demonstrates that the wall distensibility alters the flow field and the wall shear stress during the systolic phase. Comparison with corresponding rigid wall results shows that flow separation and wall shear stress are reduced in the distensible wall model.     

Visual input to the efferent control system of a fly's "gyroscope"

Dipterous insects (the true flies) have a sophisticated pair of equilibrium organs called halteres that evolved from hind wings. The halteres are sensitive to Coriolis forces that result from angular rotations of the body and mediate corrective reflexes during flight. Like the aerodynamically functional fore wings, the halteres beat during flight and are equipped with their own set of control muscles. It is shown that motoneurons innervating muscles of the haltere receive strong excitatory input from directionally sensitive visual interneurons. Visually guided flight maneuvers of flies may be mediated in part by efferent modulation of hard-wired equilibrium reflexes.     

Monte carlo-method simulation of the deformation-induced ferrite transformation in the Fe-C system

A Monte Carlo (MC) technique has been used to model deformation-induced ferrite transformation (DIFT) in an Fe-C binary system on a mesoscale. The effects of strain rate, strain, and recrystallization of the matrix on DIFT are investigated. Increasing the strain rate slightly retards the onset of DIFT. The volume fraction of ferrite increases gradually as the strain increases before the volume fraction of ferrite reaches its saturation value. After the volume fraction of ferrite becomes saturated, it oscillates around its saturation value. The recrystallization of austenite slightly retards the onset of the DIFT. Although the recrystallization of austenite reduces the equilibrium volume fraction of ferrite significantly, it cannot completely suppress DIFT. The stress concentration has been shown to induce the nucleation of ferrite near the grain boundaries and phase boundaries. The significance of the reverse transformation has been investigated. We found that there is a temporal oscillation of the volume fraction of ferrite and the stored energy after they arrive at their saturation values. We conclude that this oscillation and the effect of the strain rate on DIFT are both brought about by the reverse transformation from induced ferrite to undeformed austenite. The diffusion behavior of carbon atoms in the systems is different for different strain rates. The simulation shows that the dynamic recovery of austenite cannot occur in the system during deformation under the present conditions. The results of the simulation show that, other than the oscillation of the equilibrium volume fraction of ferrite and the unusual diffusion behavior of carbon atoms, the simulation agrees well with the corresponding experimental results. The temporal oscillation of the volume fraction of ferrite and stored energy and the unusual diffusion behavior are two new phenomena that have not been reported by other researchers.     

Buckling of Column with Base Attached to Elastic Half-Space

Buckling of a heavy elastic column loaded by a concentrated force at the top is analyzed. It is assumed that the base of the column is fixed to a rigid circular plate that is positioned on a homogeneous, isotropic, linearly elastic half-space. The plate has adhesive contact with the half-space. The constitutive equations for the column are assumed in the form that allows axial compressibility and takes into account the influence of shear stresses. It is shown that eigenvalues of the linearized equations determine the bifurcation points of the full nonlinear system of equilibrium equations. The type of bifurcation at the lowest eigenvalue is examined and is shown that it could be super- or subcritical. The postcritical shape of the column is determined by numerical integration of the equilibrium equations.     

Discontinuous Bifurcation Analysis of a Coupled Rate-Dependent Damage and Plasticity Model for Impact Responses

To identify the transition from continuous to discontinuous modes in the failure evolution of quasibrittle materials under impact, a coupled rate-dependent damage and plasticity model is developed within the thermodynamics framework. Due to the simplicity in model formulation, a continuum tangent stiffness tensor could be obtained for discontinuous bifurcation analysis, and the model parameters could be calibrated from split Hopkins pressure bar experimental data available. The coupled rate-dependent model could describe not only the pressure-dependent hardening/softening response but also the degradation of material stiffness under impact. A geometric criterion with a corresponding solution scheme is presented to explore the rate-dependent transition from continuous to discontinuous failure modes in the Mohr coordinates. The uniaxial compressive loading path is considered to illustrate the loading rate effect on the critical localization orientation and hardening parameters. It appears from the preliminary results that the coupled rate-dependent local continuum model might be combined with a decohesion model via discontinuous bifurcation analysis so that large-scale simulation of failure evolution could be performed without invoking higher-order spatial terms in the stress-strain space.     

机械实现的非正弦振动装置在方坯铸机上的应用

通过测试结晶器的振动位移曲线，分析了非正弦振动的波形失真率，该结构验证了振动装置设计的有效性，实验结果表明采用非正弦振动后，能够减少铸坯表面纵裂的发生率，降低铸坯表面的振痕深度，改善铸坯的表面质量。     

万向轴倾角对热连轧机振动的影响研究

四辊轧机在轧制过程中,辊系受力非常复杂,研究辊系的振动除考虑其它因素外还应考虑万向接轴附加弯矩的影响。建立工作辊耦合动力学模型,证明辊系振动存在水平和垂直方向的耦合关系,并进行数值仿真,得出万向轴倾角的存在使部分扭振转换成万向接轴弯振,进而影响辊系的动力学特性;仿真表明,万向轴倾角在一定角度下位移响应会出现分岔等现象。     

A numerical analysis of intergranular penny-shaped microcrack shrinkage controlled by coupled surface and interface diffusion

An axisymmetric finite-element method is developed and applied to simulate healing evolution of intergranular penny-shaped microcracks. In the finite-element method, grain-boundary diffusion and surface diffusion are coupled by the boundary conditions at the triple circle of the penny-shaped microcrack surface and the grain-boundary plane. Matter is transported to the triple circle by grain-boundary diffusion and is taken away from the microcrack tips and deposited onto the microcrack surfaces by surface diffusion, resulting in shrinkage of the intergranular microcracks. The numerical simulations show that the evolution processes of intergranular microcracks are controlled by equilibrium dihedral angle (defined by surface and grain-boundary tensions), microcrack spacing, ratio of grain-boundary diffusion to surface diffusion, and the applied pressure.     

Primary non-Hodgkin''s lymphoma of the larynx in an AIDS patient

BACKGROUND AND OBJECTIVES: Interscalene brachial plexus block is a useful technique to provide anesthesia and analgesia for the shoulder and proximal upper extremity. The initial needle direction at the interscalene groove has been described as being "perpendicular to the skin in every plane" (1). A cross-sectional (axial) approach may offer a more easily conceptualized directed needle placement. The purpose of this study is to define the cross-sectional anatomy and idealized needle angles important to interscalene brachial plexus block. METHODS: Following IRB approval, 50 patients were studied. Cross-sectional volume coil T1-weighted magnetic resonance images (MRI) were obtained from 50 patients undergoing cervical region imaging for other reasons. At the interscalene groove, a simulated needle path to contact the ventral rami or trunks of the brachial plexus was approximated at the level of C6 or C6-C7 interspace. The angle of this needle path intersecting the sagittal plane was recorded for each patient. RESULTS: The mean angle of the simulated needle path relative to sagittal plane was determined to be 61.1 +/- 6.1 degrees (range, 50-78 degrees). In 13 of 50 (26%) MRI scans, the cervical nerve roots were not visualized at the level of C6 and were measured at the C6-C7 level. CONCLUSIONS: These findings suggest initial needle placement at the interscalene groove should be angled less perpendicularly relative to the sagittal plane than is often observed. A cross-sectional approach enables more practical visualization of initial needle placement. A more accurate initial needle placement may minimize the number of needle passes necessary to contact the nerve roots, thereby more efficiently obtaining a successful block.     

Fluid Mechanism of Wing Rock for Configurations with Chine-Shaped Forebodies

The increasing demand to employ sharp-edged geometries in airframe design of advanced fighters to meet stealth requirements has nudged research to explore the new aerodynamic and dynamic characteristics of such configurations. In this paper, the wing rock oscillation is numerically simulated for a generic wing-body model consisting of cropped delta wing of 65°-sweep and chine-shaped forebody. The purpose is to develop a complete understanding of the complex flow interactions that drive the wing rock oscillation. The numerical simulation is based upon coupling the unsteady Euler fluid dynamic equations with the rigid-body dynamic equations in roll. A subiteration algorithm is employed to simultaneously solve the coupled equations. The numerical model exhibits a limit cycle oscillation in roll at an angle of attack of 35° with 16° peak-to-peak amplitude in roll angle. The complex interactions of the forebody-induced flow and the wing leading-edge vortices during wing rock are fully investigated and visualized. Analysis of the observed vortex breakdown dynamics during one entire cycle of the oscillation is also presented.