Modeling three-dimensional dynamic stall

The dynamic stall process in three-dimensional (3D) cases on a rectangular wing undergoing a constant rate ramp-up motion is introduced to provide a qualitative analysis about the onset and development of the stall phenomenon. Based on the enhanced understanding of the mechanism of dynamic stalls, a 3D dynamic stall model is constructed with the emphasis of the onset, the growth, and the convection of the dynamic stall vortex on the 3D wing surface. The results show that this engineering dynamic stall model can simulate the 3D unsteady aerodynamic performance appropriately.     

含裂纹梁非线性静动力特性及简化动力学建模Static and dynamic behaviour of cracked beam and simplified dynamic model

主要研究裂纹对梁结构动力特性的影响规律,进而为含裂纹梁结构状态监测提供理论依据。首先,对裂纹影响区域进行分析,建立含裂纹梁二维接触非线性有限元模型,阐明含裂纹梁具有拉压不同刚度的静力特性;其次,通过对机理模型的分析,指出拉压不同刚度会引起轴向与弯曲的耦合振动;然后,通过非线性动力学分析方法研究其动力特性,观察到含裂纹梁在冲击荷载下会产生轴向与弯曲的耦合振动现象,并指出这种轴向与弯曲耦合振动的一个重要特征是轴向振动频谱图中含有弯曲振动基频的两倍频成分;最后,通过引入非线性弹簧建立一种新颖的含裂纹梁简化动力学模型,通过与精细有限元分析对比,验证了模型的合理性。该简化动力学模型将接触非线性问题转换为材料非线性问题,避免了费时的接触非线性动力学求解过程。     

动载荷的识别方法

大多数情况下, 作用在工程结构上的动载荷, 如高性能战斗机在大攻角机动飞行时作用在垂尾结构上的抖振载荷, 是无法直接测量的, 只能通过测试结构在动载荷作用下的动态响应来识别出结构的动载荷. 首先阐述了动载荷识别的基本原理, 然后根据结构模型的特点, 将动载荷识别方法分为确定性结构的动载荷识别方法和不确定性结构的动载荷识别两大类, 对近些年国内外学者在这两方面的研究进展进行述评, 最后针对目前动载荷识别方法研究中所存在的问题, 提出有待深入探讨的课题.      

Finite element method combined with dynamic photoelastic analysis to determine dynamic stress-intensity factors

The present paper is addressed to the finite element method combined with dynamic photoelastic analysis of propagating cracks, that is, on the basis of [1] by Chien Wei-zang, finite elements which incorporate the propagating crack-tip singularity intrinsic to two-dimensional elasticity are employed. The relation between crack opening length and time step obtained from dynamic photoelastic analysis is used as a definite condition for solving the dynamic equations and simulating the crack propagations as well. As an example, the impact response of dynamic-bending-test specimen is investigated and the dynamic stressintensity factor obtained from the mentioned finite element analysis and dynamic photoelasticity is in reasonable agreement with each other.Finally, my special thanks go to Prof. Chien Wei-zang for his kind guidance and I would like to express my thanks to Mr. Pang Jin-xiang for conducting the successful experiment.     

An introduction to dynamic photoelasticity

The general area of elastodynamics is divided into four more limited topics including, stress-wave propagation, vibration and impact, fracture propagation and quasi-static transients. The application of dynamic photoelasticity to each topic is discussed. Recording methods used in dynamic photoelasticity which are reviewed include the high-speed framing camera, the Cranz-Schardin system, Q-switched ruby lasers, and a stopaction strobe system. Advantages and disadvantages of each method of recording are covered. Analysis procedures used in interpreting the dynamic isochromatic-fringe patterns are described. Examples are illustrated where separation of the principal stresses is possible and a calibration method for determining the dynamic material-fringe valuef _σ ^* is reviewed. Finally, four applications of dynamic photoelasticity to problems arising in geophysics, fracture mechanics, flaw detection and mining are briefly reviewed to show the versatility of the dynamic photoelastic method are described.     

The dynamic response of an incompressible non-linearly elastic membrane tube subjected to a dynamic extension

The dynamic response of an isotropic hyperelastic membrane tube, subjected to a dynamic extension at its one end, is studied. In the first part of the paper, an asymptotic expansion technique is used to derive a non-linear membrane theory for finite axially symmetric dynamic deformations of incompressible non-linearly elastic circular cylindrical tubes by starting from the three-dimensional elasticity theory. The equations governing dynamic axially symmetric deformations of the membrane tube are obtained for an arbitrary form of the strain-energy function. In the second part of the paper, finite amplitude wave propagation in an incompressible hyperelastic membrane tube is considered when one end is fixed and the other is subjected to a suddenly applied dynamic extension. A Godunov-type finite volume method is used to solve numerically the corresponding problem. Numerical results are given for the Mooney-Rivlin incompressible material. The question how the present numerical results are related to those obtained in the literature is discussed.     

基于动态校准数据的压杆测试系统动态特性校正技术

针对应变式压杆测试系统在冲击波的超压测量中存在动态响应不足的问题,建立了激波管校准平台.采用系统辨识建模和动态补偿的方法,将时域分析和频域估计相结合,设计了一种动态补偿滤波器,改善了压杆测试系统的动态特性,使频带展宽,响应时间减少.     

切槽孔爆破动态力学特征的动焦散线实验

应用爆炸加载的透射式动焦散线测试系统,分析了有机玻璃切槽孔爆破模型的裂纹动态特征变化规律。比较了不同切槽角度、切槽深度的定向断裂裂纹尖端的扩展长度、扩展速度和动态应力强度因子。初步探讨了切槽爆破的动态效应,研究表明切槽孔爆破早期裂纹破坏模式为爆炸拉应力波作用下的I型快速扩展裂纹,裂纹尖端拉应力集中积聚的较大应变能维持了爆炸裂纹进一步扩展,裂纹尖端扩展后期表现为P波、S波共同作用下的复合型扩展特征。切槽角为60时获得的定向断裂效果最好,合理切槽深度为炮孔半径的1/4～1/2。     

On dynamic stability and quasi-bifurcation

In this paper, a dynamic stability concept and a quasi-bifurcation criterion introduced in a recent paper [1] are clarified. A simple dynamic system of four degrees of freedom is employed to illustrate the concept and the significance of the criterion.     

Lateral tyre dynamic characteristics

The ability to model the ride vibration of unsuspended vehicles is at present seriously limited by lack of knowledge about the lateral and longitudinal dynamic characteristics of rolling tyres and the interaction of these characteristics with the ground surface. In this paper, models for the lateral dynamic behaviour of a rolling tyre are reviewed. The beam model, string model and spring-damper models are described and compared. A new lateral tyre model is introduced and tested against experimental results. This model comprises a parallel spring and hysteretic damping unit, representing the tyre carcass, in series with a viscous damper representing the interaction between the tyre and ground. Model justification was made against the measurements of lateral tyre response. The results showed that the proposed model provided more accurate prediction of lateral dynamic tyre response than the other spring-damper models.     

动力荷载对围压卸载下岩石动态变形模量的影响

利用SHPB岩石动静组合加载实验系统,研究在不同轴压水平下围压以1 MPa/s速率卸载至预加值50%时矽卡岩受频繁冲击作用的动态变形模量变化规律。实验结果表明:高轴压促使岩石内部微裂纹萌发与扩展,降低了岩石抵抗外部冲击的能力。围压的侧向约束阻碍岩石内部裂纹的横向扩展,但在围压卸载时会加剧岩石内部的损伤,这是由于高轴压下,围压卸载导致岩石内部应力重新分布。轴压与围压卸载共同影响着冲击作用下的岩石动态变形模量,通过岩样在冲击荷载下的能量耗散分析岩石动态变形模量的变化规律,有助于了解深部岩体开挖的破坏机制。     

平面非协调动态有限元法

本文通过引入高阶非协调位移模式，从Ｈｅｌｉｎｇｅｒ－Ｒｅｉｓｓｎｅｒ泛函出发，给出了非协调动态有限元的一般列式，得到了相应的收敛准则。算例表明本文方法简单、有效     

Non-linear dynamic intertwining of rods with self-contact

Twisted marine cables on the sea floor can form highly contorted three-dimensional loops that resemble tangles. Such tangles or ‘hockles’ are topologically equivalent to the plectomenes that form in supercoiled DNA molecules. The dynamic evolution of these intertwined loops is studied herein using a computational rod model that explicitly accounts for dynamic self-contact. Numerical solutions are presented for an illustrative example of a long rod subjected to increasing twist at one end. The solutions reveal the dynamic evolution of the rod from an initially straight state, through a buckled state in the approximate form of a helix, through the dynamic collapse of this helix into a near-planar loop with one site of self-contact, and the subsequent intertwining of this loop with multiple sites of self-contact. This evolution is controlled by the dynamic conversion of torsional strain energy to bending strain energy or, alternatively, by the dynamic conversion of twist (Tw) to writhe (Wr).     

Dynamic photoelastic and dynamic finite-element analyses of dynamic-tear-test specimens

The transient response of the dynamic-tear-test specimen of a brittle material, Homalite-100, was investigated by dynamic photoelasticity and dynamic finite-element method. The dynamic stress-intensity factors obtained from dynamic photoelasticity and dynamic finite-element analyses were in reasonable agreement with each other. The dynamic finite-element analysis also showed that the dynamic-fracture-initiation toughness could be determined from the dynamicstrain response of a strain gage located near the crack tip in conjunction with a simple static analysis. Dynamic-fracture-toughness vs. crack-velocity relation was also obtained.     

Applications of the dynamic mode decomposition

The decomposition of experimental data into dynamic modes using a data-based algorithm is applied to Schlieren snapshots of a helium jet and to time-resolved PIV-measurements of an unforced and harmonically forced jet. The algorithm relies on the reconstruction of a low-dimensional inter-snapshot map from the available flow field data. The spectral decomposition of this map results in an eigenvalue and eigenvector representation (referred to as dynamic modes) of the underlying fluid behavior contained in the processed flow fields. This dynamic mode decomposition allows the breakdown of a fluid process into dynamically revelant and coherent structures and thus aids in the characterization and quantification of physical mechanisms in fluid flow.     

The onset of dynamic stall revisited

Dynamic stall on a helicopter rotor blade comprises a series of complex aerodynamic phenomena in response to the unsteady change of the blade’s angle of attack. It is accompanied by a lift overshoot and delayed massive flow separation with respect to static stall. The classical hallmark of the dynamic stall phenomenon is the dynamic stall vortex. The flow over an oscillating OA209 airfoil under dynamic stall conditions was investigated by means of unsteady surface pressure measurements and time-resolved particle image velocimetry. The characteristic features of the unsteady flow field were identified and analysed utilising different coherent structure identification methods. An Eulerian and a Lagrangian procedure were adopted to locate the axes of vortices and the edges of Lagrangian coherent structures, respectively; a proper orthogonal decomposition of the velocity field revealed the energetically dominant coherent flow patterns and their temporal evolution. Based on the complementary information obtained by these methods the dynamics and interaction of vortical structures were analysed within a single dynamic stall life cycle leading to a classification of the unsteady flow development into five successive stages: the attached flow stage; the stall development stage; stall onset; the stalled stage; and flow reattachment. The onset of dynamic stall was specified here based on a characteristic mode of the proper orthogonal decomposition of the velocity field. Variations in the flow field topology that accompany the stall onset were verified by the Lagrangian coherent structure analysis. The instantaneous effective unsteadiness was defined as a single representative parameter to describe the influence of the motion parameters. Dynamic stall onset was found to be promoted by increasing unsteadiness. The mechanism that results in the detachment of the dynamic stall vortex from the airfoil was identified as vortex-induced separation caused by strong viscous interactions. Finally, a revised criterion to discern between light and deep dynamic stall was formulated.     

兼顾静动荷载的结构拓扑优化方法

为避免考虑瞬态动力性能时拓扑优化的高计算成本,满足工程快速设计的需求,获得主要静动荷载作用下的合理结构形态,本文提出了一种低计算成本的兼顾静动荷载的结构拓扑优化方法。施加的动荷载是地震等效荷载,用振型分解法和抗震规范中的反应谱曲线确定;通过结构形态、动力特性和地震等效静载的相互反馈和作用实现了考虑结构动力特性的拓扑优化;此外,还提出方法的自动进化策略。算例表明,方法可有效实现兼顾静动性能的拓扑优化。     

How ‘mixed’ is dynamic mixed-mode crack propagation? — A dynamic photoelastic study

Dynamic photoelastic fringe recordings associated with rapid curved crack propagation, crack division and the interaction between moving cracks and elastic waves show global dynamic mixed-mode crack-tip fringe patterns. When analysed by means of K-determination procedures an ‘apparent’ dynamic mixed-mode ratio K₂/K₁ may be denned which turns out to be a function of the particular selection of measurement data points. This paper compares experimentally recorded fringe data with numerical results in an attempt to resolve the dynamic photoelastic mixed-mode crack problem.     

Static and dynamic behaviors of belt-drive dynamic systems with a one-way clutch

This paper focuses on the nontrivial equilibrium and the steady-state periodic response of belt-drive system with a one-way clutch and belt flexural rigidity. A nonlinear piecewise discrete–continuous dynamic model is established by modeling the motions of the translating belt spans as transverse vibrations of axially moving viscoelastic beams. The rotations of the pulleys and the accessory are also considered. Furthermore, the transverse vibrations and the rotation motions are coupled by nonlinear dynamic tension. The nontrivial equilibriums of the belt-drive system are obtained by an iterative scheme via the differential and integral quadrature methods (DQM and IQM). Moreover, the periodic fluctuation of the driving pulley is modeled as the excitation of the belt-drive system. The steady-state periodic responses of the dynamic system are, respectively, studied via the high-order Galerkin truncation as well as the DQM and IQM. The time histories of the system are numerically calculated based on the 4th Runge–Kutta time discretization method. Furthermore, the frequency–response curves are presented from the numerical solutions. Based on the steady-state periodic response, the resonance areas of the dynamic system are obtained by using the frequency sweep. Moreover, the influences of the truncation terms of the Galerkin method, such as 6-term, 8-term, 10-term, 12-term, and 16-term, are investigated by comparing with the DQM and IQM. Numerical results demonstrate that the one-way clutch reduces the resonance responses of the belt-drive system via the torque-transmitting directional function. Furthermore, the comparisons in numerical examples show that the investigation on steady-state responses of the belt-drive system with a one-way clutch and belt flexural rigidity needs 16-term truncation     

基于软件GPS接收机的高动态跟踪环路设计

介绍了三阶载波跟踪环路的设计方法,并在MATLAB软件上通过低动态实验和高动态仿真实验对设计的跟踪环路性能进行验证。低动态实验采用中频信号采样器采集的真实GPS信号,高动态实验采用GPS信号仿真器生成的高动态仿真信号。实验结果表明,基于软件GPS接收机设计的二阶载波跟踪环路可满足一般低动态要求,而三阶载波环路则可满足载体相对于卫星加速度为10个g的高动态要求。