A Model of the Evolution of a Two-dimensional Defective Structure

A model of the anelastic evolution law of a two-dimensional defective solid crystal body is proposed. Assuming that the material body is made of triclinic crystals and that the evolution process does not alter the basic material symmetry group, we postulate that the evolution is driven by the present state of the density of the distribution of defects. We show that a linear relation between the inhomogeneity velocity gradient and the torsion tensor is rich enough to model such phenomena as relaxation of defects and dislocation pile-up.     

A mixed problem of plane elasticity for a domain with partially unknown boundary

The paper addresses a problem of plane elasticity for a doubly connected body with outer and inner boundaries in the form of regular polygons with a common center and parallel sides. The neighborhoods of the vertices of the inner boundary are unknown equal full-strength smooth arcs symmetric about the rays coming from the vertices to the center. It is assumed that this elastic body is inserted into a hole of a rigid body, with the hole boundary coinciding with the outer boundary of the elastic body. Absolutely smooth rigid punches with rectilinear bases are pressed into all the rectilinear sections of the inner polygonal boundary of the elastic body. There is no friction between the elastic and rigid bodies. The unknown arcs are free from external stresses. Complex variable theory is used to determine the unknown arcs and the stress state of the elastic body __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 3, pp. 110–118, March 2006.     

在质量块冲击作用下刚架大挠度响应的实验研究

本文报道刚架跨中受到以不同速度运动的质量块冲击时动态大挠度响应的实验研究工作,实验测取了冲击点位移、速度随时间变化的曲线及某些关键点的动态应变历史,由此描述了在响应过程中,质量块与刚架间动量动能的传递过程,找出了塑性区域形成和扩展的规律。研究表明:初始撞击接近于理想非弹性,动态荷载不宜简化为瞬时冲量,此时初始冲击动能与刚架所能吸收的最大弹性能之比即便很大,响应中弹性成分仍占有很大的比例,能量比率作为判断是否宜于采用刚塑性解答的条件已不再充分;刚架在初始撞击中吸收的能量和冲量与初始动能和动量之比,仅与刚架承受初始撞击的相当质量和冲击物质量之比有关,而与冲击速度无关,文中给出了二者的关系式。     

A new expression of force on a body in viscous vortex flow and asymptotic pressure field

Unsteady vortex flow around a fixed solid body in a viscous incompressible fluid is investigated for the case where the velocity field is assumed to vanish at infinity. Consideration of the asymptotic pressure field far from the body leads to a new formula for the force acting on the body, which is given by a volume integral whose integrand is linear with respect to the vorticity and does not include the velocity. This is facilitated by using a renormalized Green's function introduced by Howe. The formula offers an interesting interpretation for the force in the case of inviscid vortex rings moving near the body: that is, the force is proportional to the rate of change of volume flux through the rings of an imaginary potential flow around the body. The relation of the present subject to the excitation of acoustic waves by vortex motion moving near a compact body is considered.     

A stability analysis of an oscillating body located in fluid flow using automatic differentiation

This paper presents a stability analysis of an oscillating body subjected to fluid forces located in a transient incompressible viscous flow. If the body is supported by elastic springs, oscillation will begin. If the characteristic period of the body and the excited oscillating period due to fluid forces match each other, resonance can occur. Stability analysis is therefore needed to determine the nonlinear behavior of the body. This paper presents an analysis of the changing stability of bodies by the numerical computation. To implement the computation, the motion of fluid around a body is expressed by the Navier–Stokes equation described in the arbitrary Lagrangian–Eulerian form. The fluid influence on the body is discretized by the finite element method based on a mixed interpolation by the bubble function in space. The motion of the body is assumed to be expressed by the equations of motion. To evaluate stability, stability function is defined by the total energy of the oscillating body. The stability is judged according to a stability index, obtained by the use of the automatic differentiation (AD) of the stability function. AD is a derivative computation method that gives high accuracy. By the use of AD, the second‐order derivative matrix, which is needed to compute the stability index, can be obtained exactly. For the numerical studies, analyses of one degree of freedom and two degrees of freedom (2DOF) for a circular cylinder and 2DOF for a rectangular cylinder are carried out. A combination of a cylinder and supporting elastic spring can produce stable, neutral and unstable states. It is shown that the stability of the cylinder can be determined by the stability index. This paper shows new possibilities for stability analysis of bodies located in a fluid flow. Copyright © 2008 John Wiley & Sons, Ltd.     

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针对汽车冲撞行人抛出速度问题,利用人体直杆模型,运用有关力学原 理,得出汽车与行人第一次冲撞后人体的转动角速度和平动速度,进而讨论了不同冲撞车速 下人体倒向发动机罩的可能运动状态,结果比较符合实际情况.这对交通事故鉴定工作具有应 用价值.     

基于面元法回转体定长局部空泡的绕流计算

基于面元法 ,通过在回转体和空泡壁面放置源汇 ,对回转体定长局部空泡的绕流问题进行计算和分析 ,并讨论了空泡尾部速度过渡闭合模型对绕流计算的影响。计算结果表明 :本文的方法具有快速收敛的特征 ,第 1次叠代和最终收敛时空泡壁面切向速度的误差不超过 5 % ;随着回转体面元总数N的增加 ,局部空泡的空泡数趋于稳定 ;当回转体线型一定时 ,空泡数将随着局部空泡长度的增大而减小     

A Comparison of the Response of Isotropic Inhomogeneous Elastic Cylindrical and Spherical Shells and Their Homogenized Counterparts

All real bodies are inhomogeneous, though in many such bodies the inhomogeneity is “mild” in that the response of the bodies can be “approximated” well by the response of a homogeneous approximation. In this study we explore the status of such approximations when one is concerned with bodies whose response is nonlinear. We find that significant departures in response can occur between that of a “mildly” inhomogeneous body and its homogeneous approximation (if the approximate model is restricted to a certain class), both quantitatively and qualitatively. We illustrate this fact within the context of a specific boundary value problem, the inflation of an inhomogeneous spherical shell. We also discuss the inappropriateness of homogenization procedures that lead to a homogenized stored energy for the body when in fact what is required is a homogenized model that predicts the appropriate stresses as they invariably determine the failure or integrity of the body.     

A Generalized Formulation of Elastodynamics: Small on Rigid

The conventional theory of linearized elastodynamics addresses the case of motions that have small displacement gradients with respect to a reference configuration of the elastic body that is unstressed and at rest. Here, we develop a theory of much wider applicability in which the linearization is with respect to a reference configuration that is in rigid motion. More specifically, with an eye toward application of the theory to analysis of the motions of relatively inflexible machine parts, we view the motion as being composed of a rigid motion, which corresponds to the applied loads and initial conditions for the body under consideration, and an infinitesimal motion, in which the displacement from the rigid motion has a small gradient.     

FORCE PREDICTION BY PIV IMAGING: A MOMENTUM-BASED APPROACH

A momentum-based method for determining the instantaneous force on a body is formulated for application to quantitative imaging of vortex shedding from a cylinder. It involves a control volume that is fixed to the reference frame of an accelerating body and requires only the instantaneous velocity within the control volume and derivatives of velocity along the control surface.     

UNSTEADY LIFT FORCE ON A TOWED SPHERE

An experimental effort to characterize the broadband flow-induced lift forces on a spherical body that is towed underwater is described. The body itself is the transducer which is comprised of a small geophone encased in a near-neutrally-buoyant sphere, 7·62 cm in diameter. The research described in this paper quantifies the flow-induced unsteady lift force signal as a function of the sphere diameter Reynolds number (7620<Re<34 290) and the Strouhal number (1·5<St<30). It is found that the broadband flow-induced unsteady lift forces are proportional to the product of an area and the dynamic pressure of the flow, as expected. These data are compared to similar data measured previously on a finite-length, right-circular cylinder in cross flow. This comparison indicates that the cylindrical body creates more unsteady side force than does the spherical one, particularly at the lower end of the Strouhal number range.     

EXPERIMENTAL INVESTIGATION ON THE EVOLUTION OF INTERNAL SOLITARY WAVE OVER A RIDGE AND ITS ACTING FORCES ON A SUBMERGED SLENDER BODY

内孤立波沿山脊地形传播过程中的结构变化使得影响海洋结构物及水下航行器安全的不确定因素增加,用大型重力式分层流试验水槽系统研究内孤立波过山脊地形及其对细长潜体作用力特性,可有效提高对其危害性机理的认识.为此,利用电导率探头阵列,结合染色标识方法,测量内孤立波演变特性,同时利用三分量传感器测量水下细长体模型受力特性.研究结果表明：山脊地形显著改变下凹型内孤立波结构,表现为坡前波幅增大,坡顶背风波面抬升,坡后波长变长;内波结构变化直接影响细长体受力特性,表现为坡前向下作用力明显增强,坡顶向上作用力突显,坡后作用力持续时间变长;细长体受力特性变化影响其运动趋势,坡前下沉运动增强、朝山脊方向纵荡运动显著.     

A passive-biped model with multiple routes to chaos

This paper presents a new passive-biped model consisting of a simplest walking model beneath an upper body, with no kinematic constraint. The upper body is attached to the legs with a linear torsional spring. The model is a passive dynamic walker, so it walks down a slope without energy input. The governing equations of motion are derived and simulated for the parameter analysis purposes. Simulation results reveal some different routes to chaos that have not been observed in previous models.     

Determining Residual Stress from Boundary Measurements: A Linearized Approach

This article concerns the problem of recovering the residual stress in an elastic body from measurements taken on the boundary of the body. Since the Dirichlet problem for the equilibrium equation of elasticity has a unique solution, the Dirichlet to Neumann map is a well-defined map taking boundary displacement to boundary traction. The problem can thus be formulated by asking whether the mapping from possible residual stresses in a body to Dirichlet to Neumann maps is injective. As a first step in answering this question, we prove that the first order approximation of this mapping (in some cases) is injective. This revised version was published online in August 2006 with corrections to the Cover Date.     

A survey of strength calculations for ship hulls

Summary An elementary introduction is given to, and a survey is made of, practical methods employed for estimating the strength of ships. The evolution of techniques is described, showing their development from the statics of a rigid body to the dynamic analysis of an elastic body subject to random loading. The need for basic research on the side of structural dynamics is pointed out.     

A variational problem of supersonic gasdynamics

A large number of studies have been published which define bodies of minimal drag and nozzles of maximal thrust. A study has been made of the region of existence of the various solutions in planar flow, and it has been shown that the optimal profile in the presence of a length limitation may have a blunt end, which is a segment of a boundary extremum. The results of these studies and also the required bibiliography are contained in [1–3].In the following we present a joint solution of the problems of constructing the optimal afterbody and nozzle contour, which are the boundaries of a single planar body or of a ducted body. The investigation is limited to cases in which the form of the external contour (i.e., the afterbody contour) ensures the absence of shock waves in its influence region. The necessary conditions are obtained which determine the optimal configuration, and computational examples are given.     

高亚声速运输机后体减阻措施研究

为了降低高亚声速运输机的阻力,改善飞机的气动性能,本文在某一典型运输机机身外形的基础上,研究了后体参数和涡流发生器对机身后体流场及阻力的影响。通过数值模拟计算,得出了不同外形的后体流态分布及阻力特性数据。结果表明,小的上翘角和收缩比、大的扁平度和长细比能够显著地减小压差阻力,而涡流发生器能够有效地减小摩擦阻力。该结果可为后续的型号研制提供参考。     

A DIRECT NUMERICAL SIMULATION OF CHANNEL FLOW TRANSITION UNDER AN EXTERNAL BODY FORCE

This research seeks to increase our understanding on the laminar-turbulent transition under an external body force. Direct numerical simulation by the spectral method with a weak formulation is used to solve the transient 3-D Navier-Stokes equations. Initial disturbances consist of the finite-amplitude 2-D Tollmien-Schlichting wave and two 3-D oblique waves. Competitions among different modes were computed during transition for different Richardson numbers.

It is found that the body force can modify the transition mechanism of flows between two vertical plates. The body force was found to hasten the formation of three-dimensional flow. A non-laminar flow induced by the body force may present when the background flow is still laminar.     

A HYBRID VORTEX METHOD FOR FLOWS OVER A BLUFF BODY

A hybrid vortex method was developed to simulate the two-dimensional viscous incompressible flows over a bluff body numerically. It is based on a combination of the diffusion–vortex method and the vortex-in-cell method by dividing the flow field into two regions. In the region near the body surface the diffusion–vortex method is used to solve the Navier–Stokes equations, while the vortex-in-cell method is used in the exterior domain. Comparison with results obtained by the finite difference method, other vortex methods and experiments shows that the present method is well adapted to calculate two-dimensional external flows at high Reynolds number. It is capable of calculating not only the global characteristics of the separated flow but also the evolution of the fine structure of the flow field with time precisely. The influence of the grid system and region decomposition on the results will also be discussed. © by 1997 John Wiley & Sons, Ltd.