Flutter analysis of an articulated high aspect ratio wing in subsonic airflow

We present a methodology for calculating flutter speeds of a high aspect ratio flying wing articulated with point masses in inviscid air flow. This highly flexible wing configuration typically models a HALE (High Altitude Long Endurance) UAV (Unmanned Aerial Vehicle) type aircraft. To demonstrate the procedure, we perform flutter analysis on an actual articulated wing model and we investigate the dependence of the flutter speed on the number of loads mounted onto the structure and the number of panels comprising the flying wing for both varying and constant span. The results show that the flutter speed decreases as more panels and point masses are incorporated into the flying wing. On the other hand, the number of point masses mounted onto the structure has a small effect on the flutter speed if the wing span is kept constant.     

Bending-torsional flutter of a cantilevered wing containing a tip mass and subjected to a transverse follower force

This paper is a study of the bending-torsional flutter of a cantilevered wing subjected to a follower force, and containing a lumped mass, at the free end. In addition, a distributed aerodynamic loading is introduced along the wing. This results in a set of nonself-adjoint differential equations with variable, complex coefficients whose solutions are obtainable only in series form. Using the Frobenius method, a direct procedure is employed which retains the exact expression of the Theodorsen function and the unknown coefficients are evaluated on a computer which numerically converge to any prescribed accuracy. It is found that, as a result of the interaction of the two sources of non-conservative loadings, the follower force reduces considerably the critical speed of flows in all cases studied. An increase in the tip mass, however, has a stabilizing influence. The effect of structural damping is also examined and it is shown that internal damping forces may have pronounced influences on the flutter speed of the system.     

Robust active suppression for body-freedom flutter of a flying-wing unmanned aerial vehicle

Flying-wing unmanned aerial vehicles have received extensive attention over the past decade because of their excellent aerodynamic and stealth performance. However, the aeroelastic interaction problems among unsteady aerodynamics, flight dynamics, and structural dynamics, such as the body-freedom flutter, are still open. This paper presents the study of a robust control scheme for active body-freedom flutter suppression of a flexible flying-wing unmanned aerial vehicle. The control objective is to expand the boundary of body-freedom flutter and to enhance the control robustness to external unknown disturbance simultaneously. The paper begins with the modeling procedure of a parameter-varying aeroservoelastic plant for the design of control law. Then, it presents how to synthesize a robust controller so as to suppress the flutter instability for a wide flight range of dynamic pressures. Afterwards, the paper shows how to analyze the flutter stability of the closed-loop system and the robustness of the controller, respectively. The numerical results demonstrate that the proposed robust controller can not only expand the flutter boundary of the unmanned aerial vehicle by 30%, but also exhibit the strong robustness to external disturbance.     

模型翼拍动中动态柔性变形效应的数值研究

本文利用数值求解二维不可压缩Navier-Stokes方程，研究了在两种拍动基元运动中，模型翼动态柔性变形对其气动性能的影响。由柔性翼和刚性翼的结果比较发现，附加惯性效应是动态变形影响气动性能的主要原因，变形加速率决定了瞬时气动性能改变量的大小，相比之下，动态变形对气动性能时均值的影响则弱得多。     

Asymptotics of aeroelastic modes and basis property of mode shapes for aircraft wing model

In this paper, we announce our recent results on the asymptotic and spectral analysis of the model of an aircraft wing in a subsonic air flow. This model has been developed in the Flight Systems Research Center of UCLA and is presented in the works by Balakrishnan. The model is governed by a system of two coupled integro-differential equations and a two-parameter family of boundary conditions modeling the action of the self-straining actuators. The differential parts of the above equations form a coupled linear hyperbolic system; the integral parts are of the convolution type. We provide the spectral asymptotics for the eigenfrequencies of the system (or aeroelastic modes) and the asymptotical approximations for the corresponding eigenfunctions (or the mode shapes). Based on the asymptotical results, we (a) state that the set of the mode shapes is complete in the energy space; (b) construct a system which is biorthogonal to the set of the mode shapes in the case when there might be multiple aeroelastic modes; and (c) show that the mode shapes form a Riesz basis in the energy space.     

价值性差异、知识域结构与知识计量研究——基于知识的一般性经济原理

 知识分类的混沌现象,不能较好解释知识的动态转化与经济之间的关系。因此,根据知识拥有的主体不同,从异质动态多样性、层次性、结构性并反映知识价值差异性特征角度,对具有整合、交易价值属性的不同层次、不同形态的同质知识和异质知识,按照时间序列和空间结构关系,从静态到动态螺旋上升整合过程,提出了可用于规范经济学理论阐述的静态知识、静态点知识、个体动态点知识、个体结构知识、个体边际结构知识、组织结构知识、组织边际结构知识等知识域纳入到经济学原理框架下分析、评价和计量,并揭示了知识租金在相对溢出和绝对溢出效应影响下,知识经济战略选择是专业化技术跳跃的创新产品知识速度经济→持续改进与学习的知识固本规模经济→研发专业化更新产品的速度经济交替循环战略发展组合过程,是知识经济规模总量与速度经济净增加量交替转换竞争战略关系.     

后扰流板角度对汽车尾部受力影响的分析

以捷达为例通过分析在不同速度下,汽车后扰流板起升不同角度时,汽车所受到的气动升力和气动阻力的关系,以及由扰流板本身带来的形状阻力对汽车受力的综合影响,得出在满足最低的气动力系数条件下的最优角度。     

The transonic dip in aeroelastic divergence speed—An explicit formula

Using continuum models we develop a closed form analytical formula for the divergence speed as a function of the Mach number and in particular its dependence on wing camber. There is a transonic dip even when the angle of attack is zero and it depends on the wing thickness ratio. For nonzero camber the angle of attack in fact plays a lesser role. The main analytical tool is the Possio integral equation which is shown to have an explicit solution. Neither CFD nor FEM is employed.     

CFM56-3发动机N2转速管理系统原理

CFM56-3系列发动机是由美国通用电气公司(GE)和法国国营航空发动机研制公司(SNECMA)组成的国际公司(CFMI)研制生产的为波音B737-300/400/500型航空器翼吊发动机;其N2转速管理系统为飞机提供精确的燃油控制功能以防止发动机超速及失速情况的发生。N2转速管理系统主要通过接受发动机的核心转子转速(N2)和动力角(PLA)来控制燃油流量使发动机实际转速与目标转速相符。     

On an approximate effective speed of propagation

An algorithm is suggested for the determination of an effective speed of propagation for a random medium whose mean speed of propagation is perturbed by a small random term of the white-noise type. We study the propagation of disturbances in the limiting geometrical optics case, and pursue the analysis within the framework of stochastic optimal control. The statement of an appropriate Fermat's principle for the random medium and invoking the principle of dynamic programming lead to a non-linear elliptic equation—the classical Eikonal equation—perturbed by terms associated with the stochastics. This equation and estimates for the departure of the stochastic trajectories from the free-space geodesic path of propagation enable us to calculate the approximate speed.     

圆翼管换热器传热分析

圆翼管换热气是采暖通风工程中常用的换热设备。本文通过对圆翼管传热性能进行分析，提出在暖通工程中合理采用圆翼管换热气器的设想。     

牛津高中英语“读听说写”单元板块教学新探

本文在常见的整合教学思想的基础上对现行牛津高中英语教材进一步发掘,探索出＂读、听、说、写＂单元板块教学结构,试图通过这样的探索,遵循输入——吸收——输出的语言习得过程,力求符合学习规律,增强语言学习的效率,提高学生综合运用语言的能力。     

蜻蜓拍翼前飞中模型翼动态柔性变形的气动效应研究

根据蜻蜓拍翼前飞激发的流场具有St>>1/Re的特点，在理论模化框架下，本文采用势流假设，用二维变形体前、后缘脱涡的面元法研究了前飞模型蜻蜓翼在周期性拍动中动态柔性变形引起的气动力响应。通过比较柔性模型翼和刚性模型翼气动力随时间的变化，发现在下拍行程中动态柔性变形显著地提高了升力和推力；值得注意的是，蜻蜓前飞时，翼的实际柔性变形幅值恰处于一个最佳范围内，它大幅地提高了升力，同时使消耗的气动功率趋近于最低，有利于蜻蜓维持前飞状态。     

中国省域尺度低碳经济发展综合水平评价

低碳经济方兴未艾,是未来世界的发展趋势。本文运用层次分析和k均值聚类方法,构建了一套针对中国省域尺度的低碳经济发展水平评价指标体系和指数。论文对中国大陆30个省(市、自治区)的低碳经济发展水平现状进行了评价,并对其进行类型划分,得出了4种类型的低碳经济发展水平包括低碳区、相对低碳区、相对高碳区和高碳区,并分析了影响省域低碳经济综合发展水平的相对优势指标和相对劣势指标。评估结果表明,我国一多半的省份属于相对高碳和高碳区,由此说明我国目前经济发展处于相对高碳阶段。论文最后指出,推进低碳经济的发展,重要的是"开源节流":所谓"开源",即指开发新能源和增加林地植被覆被,以降低温室气体的排放和增大对温室气体的吸收;"节流"指通过产业结构的调整和能源利用效率的提高,来减少碳排放。     

Unraveling the links between dimensions of innovation and organizational performance

Economists typically define innovation as a process or practice that is new to an industry; hence they emphasize a firm's speed of innovation relative to other firms in the industry. Organizational theorists, on the other hand, usually focus on the number of products or processes that are new to the firm; hence, they emphasize innovation magnitude. This study builds a bridge between these two approaches by exploring the link between two dimensions of innovation—speed and magnitude—and two measures of a firm's performance—objective financial reports and executive ratings of perceived effectiveness. We propose that each dimension of innovation will be associated with a different measure of firm performance. Using data from the commercial banking industry, we find interesting results that partially support our predictions based on the theory that different dimensions are indeed linked to different measures of performance. Implications for future research and practice are discussed.     

上市公司自由现金流与资本结构动态调整研究

本文采用我国沪深两市A股非金融上市公司2004–2012年的财务数据,首先通过动态面板系统GMM模型,估算出我国上市公司的最优资本结构,并计算出资本结构调整速度为0.186;然后建立了反映自由现金流盈亏程度和资本结构偏离程度如何影响资本结构调整的固定效应模型,分析得出与单独调整资本结构的情况相比,企业同时调整资本结构和自由现金流时,调整成本降低,调整速度加快;当资本结构调整至(或接近)最优状态后,企业会保持资本结构在最优水平,不受自由现金流调整的影响。上述结论对企业是否存在融资约束具有稳健性,但受融资约束的企业调整成本相对较高,调整速度相对较慢。     

考虑渗流作用的基坑围护结构稳定性分析

推导了一维稳定渗流条件下基坑内外被动区和主动区的水头计算公式,并分析了渗流对作用在围护结构上的水压力、土压力及侧压力的影响,结合算例讨论了渗流作用对围护结构稳定性的影响．研究表明,渗流作用使得被动区和主动区作用在围护结构上的侧压力均减小,在地下水位较高的地区开挖基坑时应当考虑渗流作用的影响。     

某超临界机翼风洞模型转捩方式研究

本文介绍了风洞试验转捩技术的常规方法和应用,通过对某超临界机翼在风洞试验中自由转捩和固定转捩对阻力的影响结果,结合该机翼的压力分布云图进行了分析,提出了超临界机翼在风洞试验中模拟转捩时的方法。     

To have or to be? The interplay between knowledge structure and market identity in knowledge-based alliance formation

We explore how a firm’s knowledge structure characteristics and market identity—as reflected in its position in the industry value chain and market status—influence knowledge-based alliance (KBA) formation. We propose that a firm’s propensity to form KBAs is affected by the congruence (or misalignment) between its knowledge structure characteristics (i.e., knowledge depth and scope) and the role it plays in the industry, as demanded by its position in the value chain. We further argue that while a firm’s market status can amplify the positive effect of the congruence, thereby enhancing a firm’s proclivity to form KBAs, being prominent can also hurt a firm when it faces a misalignment between its knowledge structure characteristics and externally induced expectations related to its value chain position. We validate our arguments using panel data on 1051 firm-year observations for 197 firms patenting in the global fuel cell industry for the period 1999–2009. By developing a richer understanding of the relationship between a firm’s knowledge-based resources and KBA formation, we answer the call for a better understanding of how and when firms can leverage their internal knowledge configurations to improve their proclivity to form alliances.