科创中国

根据陕西省17个村庄的农地流转情况的调查资料,采取随机抽样法和问卷调查法,利用Logistic模型对陕西农村农地流转的影响因素进行分析,结果显示农户的个体因素是影响农地流转的主要因素,而一般认为的工资性收入和政策因素对其影响并不显著,由此应调整促进农地流转的长短期政策.文中的"农地流转"是指土地承包经营权的流转,不涉及农村建设用地的流转.

流行病学研究发现,人群中心力衰竭的发生率达到2.0%,而年龄大于60岁的老年人群发生率高达10%〔1〕.因此心力衰竭已经成为老年人群常见的致死和致残疾病之一.由于老年人抵抗力下降等原因,心力衰竭患者并发肺部感染的风险亦增加〔2〕.本研究探讨心力衰竭患者并发肺部感染的相关危险因

在种群的增长率满足广义logistic方程的情况下,建立了在污染环境中一类新的单种群模型,给出了该模型中种群一致持续生存和灭绝的充分条件.这里建立的模型是He和Wang[Appl.Math.Modell.31(2007)2227-2238]中模型的改进.

Several typical flexible pneumatic actuators(FPA) and different mechanical models describing their behaviors have been proposed, however, it is difficult to balance compliance and load capacity in conventional designs, and these models still have limitations in predicting behavior of FPAs. A new flexible pneumatic bending joint(FPBJ) with special anisotropic rigidity structure is proposed. The FPBJ is developed as an improvement with regard to existing types of FPA, and its principal characteristic is derived from the special anisotropic rigidity structure. With this structure, the load capacity in the direction perpendicular to bending plane is strengthened. The structure of the new FPBJ is explained and a mathematical model is derived based on Euler-Bernoulli beam model and Hook's law. To obtain optimum design and usage, some key structure parameters and input-output characteristics are simulated. The simulation results reveal that the relationship between the structure parameters and FPBJ's bending angle is nonlinear. At last, according to the simulation results, the FPBJ is manufactured with optional parameters and tested. The experimental results show that the joint's statics characteristics are reflected by the mathematical model accurately when the FPBJ is deflated. The maximum relative error between simulation and experimental results is less than 6%. However, the model still has limitations. When the joint is inflated, the maximum relative error reaches 20%. This paper proposes a new flexible pneumatic bending joint which has sufficient load capacity and compliance, and the mathematical model provides theoretical guidance for the FPBJ's structure design.

我国作为发展中国家,城市化总体水平不高,并符合Logis-tic曲线的初期发展规律.但自2004年开始,我国上海市的非农人口比重竟呈现出逐年下降的趋势,同时,长三角地区其他城市非农人口比重以缓慢的速度在上升.这是否意味着我国长江三角洲地区已然步入逆城市化阶段?本文将对长三角地区城市化拐点进行预测分析,研究所谓的逆城市化现象的原因.对如何推进长江三角洲地区城市化进程提出对策建议.

地下水数值模拟是目前定量研究地下水的重要手段.分析了数值法求解地下水流数学模型的基本步骤,对于采用试估一校正法进行模型识别率定和验证过程中,在遵循模型的识别率定和验证的基本要求和四个基本原则基础上,对于如何评价模型率定和验证的效果、定量分析模型精度、以及达到模型可实际应用的要求,总结众多地下水数值模型调试的经验,提出了地下水数值模型识别率定和验证的定量评价标准.

Most researches on transient fuel control of port fuel injection S.I. engine are carried out from the perspective of advanced mathematical theories. When it comes to practical control, there exist many limitations although they are more intelligent. In order to overcome the fuel wetting effect of PFI engine, the application-oriented transient fuel control is studied by analyzing the key parameters which are closely related with the engine transient characteristics. Both validity and simplicity are taken into consideration. Based on the fuel wall-wetting theory and popular fuel compensation strategy, short-term transient fuel(STF) and long-term transient fuel(LTF), as well as their individual decay approaches, are introduced. STF is to compensate the drastic fuel film loss caused by sudden throttle change, while the function of LTF is to compensate the fuel film loss by manifold air pressure(p) fluctuation. Each of them has their respective pros and cons. The engine fuel mass and air mass are also calculated for air-fuel ratio(AFR) according to ideal gas state equation and empirical equations. The vehicle acceleration test is designed for model validation. The engine experiences several mild and heavy accelerations corresponding to the gear change during vehicle acceleration. STF and LTF control are triggered reliably. The engine transient fuel control simulation adopts the same inputs as the test to ensure consistency. The logged test data are used to check the model output. The results show that the maximum fuel pulse width(FPW) error reaches 2 ms, and it only occurs under engine heavy acceleration condition. The average FPW error is 0.57 ms. The results of simulation and test are close overall, which indicates the accuracy of steady and transient fuel. The proposed research provides an efficient approach not only suitable for practical engineering application, but also for AFR prediction, fuel consumption calculation, and further studies on emission control.

目的:探讨脑出血并发应激性溃疡(SU)的相关危险因素。方法回顾性分析2006年3月至2014年3月南京军区南京总医院急救医学科抢救室收治1185例脑出血患者的临床资料,以是否合并SU将患者分为两组。采集两组患者入院8h内的数据,包括性别、年龄、出血量、出血部位(基底节区、丘脑、脑干、脑叶、脑室、蛛网膜下腔、小脑)、意识障碍、急性生理学与慢性健康状况评分系统Ⅱ(APACHEⅡ)评分、收缩压(SBP)、高血压病史、脑出血病史9项指标进行单因素分析,将有统计学意义的危险因素进行多因素logistic回归分析,筛选独立危险因素,并绘制受试者工作特征曲线(ROC曲线),评价其检验效能。结果1185例脑出血患者入选本研究,293例发生SU,占24.7%;892例未发生SU,占75.3%。单因素分析显示,脑出血并发SU的危险因素包括年龄、出血量、出血部位、意识障碍、APACHEⅡ评分、SBP。在出血部位中,脑室、丘脑、脑干出血并发SU的比例较高,分别为45.3%(43/95)、39.1%(63/161)、36.9%(48/130),显著高于脑叶〔26.2%(33/126)〕、小脑〔18.8%(15/80)〕、基底节区〔16.1%(78/485)〕、蛛网膜下腔〔12.0%(13/108)〕。多因素logistic回归分析显示,出血量〔优势比(OR)=3.305,P=0.001,95%可信区间(95%CI)为2.213~48.634〕、出血部位(OR=1.762,P=0.008,95%CI为0.123~2.743)、SBP(OR=1.223,P=0.034,95%CI为0.245~2.812)是脑出血并发SU的独立危险因素。出血量、SBP的ROC曲线下面积(AUC)分别为0.846、0.597,提示出血量有中等的诊断价值,SBP的诊断价值较低。结论出血量大,出血部位在脑室、丘脑或脑干,SBP高的脑出血患者并发SU的风险较大,应尽早降低血压并及时给予SU的预防性治疗。

Parallel kinematic machines have drawn considerable attention and have been widely used in some special fields.However,high precision is still one of the challenges when they are used for advanced machine tools.One of the main reasons is that the kinematic chains of parallel kinematic machines are composed of elongated links that can easily suffer deformations,especially at high speeds and under heavy loads.A 3-RRR parallel kinematic machine is taken as a study object for investigating its accuracy with the consideration of the deformations of its links during the motion process.Based on the dynamic model constructed by the Newton-Euler method,all the inertia loads and constraint forces of the links are computed and their deformations are derived.Then the kinematic errors of the machine are derived with the consideration of the deformations of the links.Through further derivation,the accuracy of the machine is given in a simple explicit expression,which will be helpful to increase the calculating speed.The accuracy of this machine when following a selected circle path is simulated.The influences of magnitude of the maximum acceleration and external loads on the running accuracy of the machine are investigated.The results show that the external loads will deteriorate the accuracy of the machine tremendously when their direction coincides with the direction of the worst stiffness of the machine.The proposed method provides a solution for predicting the running accuracy of the parallel kinematic machines and can also be used in their design optimization as well as selection of suitable running parameters.

为了研究中国区域间跨阶段的创业性别差异,论文使用了随机系数二元Logistic回归模型,将那些超越特定创业阶段的个体与那些恰恰处于该阶段的个体进行动态比较.通过使用区域环境变量和控制个体层面变量,在基于中国15个城市的5083份问卷的调查,五个阶段的创业性别差异得到解释.研究认为,中国跨阶段的创业性别差异主要表现为行业选择的性别差异.从动因来看,区域环境约束差异不明显,但个体偏好的作用不容忽视.具体而言,跨阶段创业性别差异的部分原因是女性过多地聚集于特定行业;家庭资本和行政管理在形成创业意愿方面存在显著的性别差异.不过,数据没有发现在获得金融支持方面存在性别歧视.