学习支持服务在MOOCs平台中的需求研究——以强动机学习者为例

随着MOOCs快速发展，一些问题也渐渐地浮现。当前大众对MOOCs关注的问题之一就是MOOCs的高辍学率。动机和学习支持服务是网络时代成功学习的两大要素，影响着学习者能否成功完成学习。本研究以强动机学习者为例，探究强动机学习者对MOOCs平台中学习支持服务中最迫切的需求，以期促使强动机学习者能顺利完成课程。研究结果表明：强动机学习者在人员服务中的需求最大，希望助教及时答疑以及组织线上线下活动，同时也希望提供有效分组功能支持协作学习。在学习管理工具层面需求相对较小，也其是笔记功能。

强预冷发动机新型热力循环布局及性能分析

为兼顾高超声速强预冷发动机的比冲及系统复杂度,提出了一种耦合闭式氦循环的强预冷发动机新型热力循环布局。该热力循环采用空气适度预冷,闭式氦循环采用较为简单的二支路分流冷却压缩方式。在支路1中,使用较少的液氢即可保证氦被冷却至低温以便于压缩,可提高闭式循环增压比进而增加闭式循环输出功率;在支路2中,利用温度相对较低的氢气对预冷器出口分流的部分高温氦冷却,保证支路1和支路2掺混后的氦温度满足预冷器要求。对该新型强预冷发动机的设计点性能进行了参数化影响分析及优化设计,马赫数5工作点比冲可达3287 s。通过旁路冲压当量比和喷管喉道面积的调节,可保证进气道、预冷涡轮通道和旁路冲压通道的匹配工作,且能实现冷却与燃烧所需燃料量的平衡;通过对闭式氦循环基准压力的调节可实现发动机推力的有效调节,且各主要部件均能匹配稳定工作。沿典型飞行轨迹的发动机总体性能计算表明,在马赫数0~5工作范围内,该新型强预冷发动机具有较高的推力和比冲性能;且闭式氦循环较为简单,利于工程实现,该新型热力循环布局可为高超声速强预冷发动机设计提新的思路。

高超声速强预冷航空发动机技术研究进展

高超声速强预冷航空发动机技术是1项具有巨大潜在技术优势的革命性动力技术,已成为高超声速动力的研究热点。调研了国外具有代表性的强预冷发动机技术发展脉络及现状,并对不同发动机方案的典型技术特征进行了分析与总结, 详细介绍了中国在强预冷航空发动机热力循环设计分析、紧凑强预冷器设计制造和试验、超临界氦叶轮机设计、宽域进排气系统优化设计及高效燃烧等技术方面的最新研究进展。国内外已有研究表明:高超声速强预冷航空发动机原理先进、综合性能优异,多项核心技术已取得重大突破,无"卡脖子"难题, 中国可进一步开展强预冷航空发动机核心系统和整机集成验证,提升强预冷航空发动机技术成熟度,为水平起降重复使用高超声速飞行器研制提供有力支撑。

拟陷阱同步自动机的最短同步字的长度

既非陷阱也非强连通的同步自动机称为拟陷阱同步自动机。对于任意的拟陷阱同步自动机A,利用其强连通子自动机的状态数给出了A的最短同步字的长度的一个上界,进而获得了A满足C∨ern猜想的一个充分条件。

高超声速强预冷发动机热力循环研究进展

先进动力系统是水平起降、可重复使用高超声速飞行器的核心支撑,其中,高超声速强预冷发动机是一种极具潜力的动力方案,近年来受到广泛关注。深入研究强预冷发动机的热力循环,掌握发动机热力循环工作特性对发动机的方案设计至关重要。本文对近年来国内外在高超声速强预冷发动机热力循环方面的研究进展进行了综述,主要包括发动机热力循环建模分析方法、性能分析手段、典型强预冷发动机热力循环方案研究等。其中,根据热力循环方案的显著差异,分别介绍了开式直接预冷循环及中间介质闭式预冷热力循环。已有研究表明,对于开式直接预冷循环,燃料类型是决定其性能的根本,提升燃料的热沉是提升发动机性能的重要途径。对于中间介质闭式预冷热力循环,发动机的比冲、单位推力等性能与闭式循环系统的复杂性存在一定的矛盾。整体来看,需继续开展高超声速强预冷发动机核心部件的研究,提炼更加准确的部件性能模型,完善部件尺寸、重量等估算模型,实现对于发动机比冲、单位推力、推重比等整机性能参数的准确评估,支撑高可行性的高超声速强预冷发动机热力方案设计。

Application of the Sub-Model Method in the Engine Strength Analysis

On the basis of introducing the fundamental theory and the basic analysis steps of the sub model method, the strength of the new engine complex assembly structure was analyzed according to the properties of the engine structures, some of the key parts of the engine were analyzed with refined mesh by sub model method and the error of the FEM solution was estimated by the extrapolation method. The example showed that the sub model can not only analyze the comlex structures without the restriction of the software and hardware of the computers, but get the more precise analysis result also. This method is more suitable for the strength analysis of the complex assembly structure.

Aging behavior and mechanical properties of 6013 aluminum alloy processed by severe plastic deformation

Structural features, aging behavior, precipitation kinetics and mechanical properties of a 6013 Al–Mg–Si aluminum alloy subjected to equal channel angular pressing （ECAP） at different temperatures were comparatively investigated with that in conventional static aging by quantitative X-ray diffraction （XRD） measurements, differential scanning calorimetry （DSC） and tensile tests. Average grain sizes measured by XRD are in the range of 66-112 nm while the average dislocation density is in the range of 1.20×10^14-1.70×10^14 m^-2 in the deformed alloy. The DSC analysis reveals that the precipitation kinetics in the deformed alloy is much faster as compared with the peak-aged sample due to the smaller grains and higher dislocation density developed after ECAP. Both the yield strength （YS） and ultimate tensile strength （UTS） are dramatically increased in all the ECAP samples as compared with the undeformed counterparts. The maximum strength appears in the samples ECAP treated at room temperature and the maximum YS is about 1.6 times that of the statically peak-aged sample. The very high strength in the ECAP alloy is suggested to be related to the grain size strengthening and dislocation strengthening, as well as the precipitation strengthening contributing from the dynamic precipitation during ECAP.

Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al Zn Mg Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al Zn Mg Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al3(Sc,Zr) and Al3Zr dispersoids. The volume of Al3Zr particles is less than that of Al3(Sc,Zr) particles, but the distribution of Al3(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.

Experimental Investigation on Mixing Enhancement Mechanism of Turbulent Jet Flow with Tabbed Nozzle

Flow visualization and hot-wire measurement techniques were combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation mechanism. Streamwise vortices generated by the tabs of different sizes and numbers were observed from the flow visualization images. Combined with flow visualization, hot-wire measurement gave a quantitative insight of the effect of various tabbed jet flows. Instantaneous two-component velocity signals (longitudinal and transverse velocity components) at different cross sections along radius direction and streamwise direction with different tabbed jet nozzles were measured using hot-wire anemometer. Average flow field parameters of tabbed jet flow such as mean velocity, tur-bulence intensity, vorticity were analyzed and the effects of tabs with different sizes and numbers were compared with that of circular no-tab jet flow. It is revealed that the generation of a series of counter-rotating quasi-streamwise vortices, azimuthal vortices and double-row azi-muthal vortex are the reasons for mixing enhancement of tabbed turbulent jet flow.

Improving impact wear resistance of Ti-6Al-4V alloy treated by laser shock peening

The effects of laser shock peening (LSP) on the impact wear behavior of Ti-6Al-4V alloys were investigated by a homemade impact wear test rig. The microstructure and mechanical properties of the peened samples were studied. During the impact wear test, the energy absorption, impact force, wear contact time and wear mechanism of all the test samples were investigated in terms of the influence of the impact kinetic energy. The results showed that microhardness, elastic modulus and residual compressive stress of the treated samples were markedly improved. The wear resistances of both treated samples were highly improved after LSP, and a higher pulse energy corresponded to a more obvious effect. Besides, the wear in all test samples involved a combination of abrasive and oxidation wear and fatigue spalling.

Response of train-bridge system under intensive seismic excitation by random vibration method

Earthquake is a kind of sudden and destructive random excitation in nature.It is significant to determine the probability distribution characteristics of the corresponding dynamic indicators to ensure the safety and the stability of structures when the intensive seismic excitation,the intensity of which is larger than 7,acts in train-bridge system.Firstly,the motion equations of a two-dimensional train-bridge system under the vertical random excitation of track irregularity and the vertical seismic acceleration are established,where the train subsystem is composed of 8 mutually independent vehicle elements with 48 degrees of freedom,while the single-span simple supported bridge subsystem is composed of 102D beam elements with 20 degrees of freedom on beam and 2 large mass degrees of freedom at the support.Secondly,Monte Carlo method and pseudo excitation method are adopted to analyze the statistical parameters of the system.The power spectrum density of random excitation is used to define a series of non-stationary pseudo excitation in pseudo excitation method and the trigonometric series of random vibration history samples in Monte Carlo method,respectively solved by precise integral method and Newmark-βmethod through the inter-system iterative procedure.Finally,the results are compared with the case under the weak seismic excitation,and show that the samples of vertical acceleration response of bridge and the offload factor of train obeys the normal distribution.In a high probability,the intensive earthquakes pose a greater threat to the safety and stability of bridges and trains than the weak ones.

Numerical Simulation of Coseismic Response and Its Mechanism of Well Water Temperature to Farfield Strong Earthquakes1

Heat flow is inevitably accompanied by temperature change,thus,the water temperature coseismic response during earthquake activity should also obey the laws of thermodynamics.Taking the M S8.0 Wenchuan,Sichuan,China earthquake and the M9.0Tohoku,Japan earthquake as an example,and based on the data of water temperature coseismic responses observed in well ZK26 in Haikou,Hainan Province,China,we investigate the relationship between well water temperature change and heat transfer in the coseismic response process and the relevant thermodynamic mechanism by using the numerical simulation method for thermodynamic equations.Then,through forward modeling,we obtain several simulation curves of water temperature change in response to earthquakes along the well depth at different times.The simulated curves of water temperature change approximately fit the observed curves.Consequently,based on the variation of temperature,we find that the modes of well water temperature coseismic response( ascending,descending or stable) are related to factors such as the location of sensors,distribution and location of heat sources,the span between sensors and heat sources.

Numerical Investigation on the Initiation Mechanism of Debris-Flow under Rainfall

Rainfall is an important factor to trigger the debris flow.Numerical simulation on the responses of slopes and the initiation of debris flow under rainfall was processed by using the software FLAC2D based on the soil parameters in Weijia Gully,Beichuan County,Sichuan Province,China.The effects of the slope angle,rainfall intensity,soil parameters on the developments of the stress and pore pressure and deformation of the slope were studied.It indicates that large displacements of the slope are mainly located near the slope toe.With the increase of the rainfall intensity the stability of the slope decreases and so the debris-flow is easy to occur.

Simulation of dynamic recrystallization in 23Co13Ni11Cr3Mo steel using a modified cellular automaton

A modified cellular automaton(CA) program was developed to simulate the process of dynamic recrystallization(DRX) for 23Co13Ni11Cr3Mo ultrahigh strength steel.In this model,influences of deformation parameters on hardening rate and solute drag effect were considered.Moreover,an inverse analysis method was proposed for parameters identification of dislocation model and solute drag effect based on the results of isothermal compression tests on Gleeble-1500.Then,simulated microstructures under different deformation conditions were compared with those of experiments.A good agreement is achieved.Furthermore,influences of deformation parameters on microstructure evolution for 23Co13Ni11Cr3Mo steel were investigated in details.High strain is an effective measure to refine grain and improve homogeneity.Meanwhile,the desired deformation parameters are temperature of 1000-1050 °C and strain rate of 0.008-0.01 s-1 for obtaining grains smaller than 22.5 μm.

Forced transverse vibration of rolls for four-high rolling mill

In order to investigate the forced transverse vibration of rolls under distributed draught pressure and moment of bending roll force, the forced transverse vibration model of rolls for four-high rolling mill was established. The work roll and backup roll were considered as elastic continuous bodies that were joined by a Winkler elastic layer. According to Euler-Bemoulli beam theory, the forced transverse vibration of rolls was analyzed based on modal superposition method. The forced vibration equations were established when the draught pressure and moment of bending roll force were imposed on the rolls respectively. Numerical modeling was made on 2 030 mm cold tandem rolling mill of Baoshan Iron and Steel Company. Simulation results show that when the work roll is only subjected to different forms of draught pressures, the vibration curves of work roll and backup roll are quadratic curves with amplitudes of 0.3 mm and 45 μm, respectively. When only the moments of bending roll force are imposed on the work roll and backup roll, the vibration curves of work roll and backup roll are quadratic curves, and the amplitudes are 5.0 and 1.6 μm, respectively. The influence of moment of bending roll force on the vibration of work roll is related with the bending roll force.

Seismic Sequence Characteristics of the March 21,2008,M_S 7. 4 Yutian,Xinjiang Earthquake and Seismological Anomalies before the Earthquake

The parameters, stress field background, geological tectonics and seismic sequence of the March 21, 2008, Ms7.4 Yutian, Xinjiang earthquake are discussed in this paper. The characteristics of seismic activity in the epicenter and its adjacent region before the earthquake are analyzed; and a comparison is made between the Ms7.4 Yutian earthquake sequence, the Wenchuan Ms8.0 earthquake sequence and the Wuqia Ms6.9 earthquake sequence. The results show the Ms7.4 Yutian earthquake occurred in the junction between the Aityn Tagh fault and the western Kunlun fault, resulting perhaps from the tensile fracture of the branch fault located southwest of the Ashikule basin due to left-lateral dislocation of the Aityn Tagh fault; the seismic sequence is of main shock-aftershock type, the strongest aftershock being Ms5.8. The aftershocks attenuated quickly, and occurred in groups; the focal mechanism solutions and epicenter distribution have revealed a unilateral rupture source of this earthquake. Seismic activity shows that there was medium and medium-short term abnormity before the earthquake, but there was not short-imminent abnormity. Seismic activity of this earthquake sequence enhanced before the Ms8. 0 Wenchuan and the Ms6. 9 Wuqia earthquakes, showing the window effect to some extent.

Study on Dynamic Response of Fiber-Reinforced Plastic Float for Light Seaplane

The marine area of Japan, including territorial waters and the exclusive economic zone, is the sixth largest in the world at about 4,470,000 km2. Therefore, it is becoming necessary to establish appropriate means of transportation other than ships in order to utilize the area efficiently. In this respect, ultra-light seaplanes are attracting attention from the viewpoint of protecting the natural environment. Accordingly, JRPS （Japan Reinforced Plastics Society） is currently developing FRP （fiber-reinforced plastic） floats for such planes. In this study, we conducted simulations of seaplane behavior during alighting by using the smoothed particle hydrodynamics method, which is one of the functions in the PAM-CRASH solver, and we present the observed trend in the vertical acceleration of the floats as a first step toward deriving the impact force from analytical data.

A scalable and efficient IPv4 address sharing approach in IPv6 transition scenarios

IPv6 has been an inevitable trend with the depletion of the global IPv4 address space. However, new IPv6 users still need public IPv4 addresses to access global IPv4 users/resources, making it important for providers to share scarce global IPv4 addresses effectively. There are two categories of solutions to the problem, carrier-grade NAT （CGN） and ＇A＋P＇ （each customer shaving the same IPv4 address is assigned an excluded port range）. However, both of them have limitations. Specifically, CGN solutions are not scalable and can bring much complexity in managing customers in large-scale deployments, while A＋P solutions are not flexible enough to meet dynamic port requirements. In this paper, we propose a hybrid mechanism to improve current solutions and have deployed it in the Tsinghua University Campus Network. The real traffic data shows that our mechanism can utilize limited IPv4 addresses efficiently without degrading the performance of applications on end hosts. Based on the enhanced mechanism, we propose a method to help service providers make address plans based on their own traffic patterns and actual requirements.

Approaches to Stable Operation of Shaftless Centrifugal Pump

The shaft-less impeller of the centrifugal pump, which is driven by the magnetic induction, was developed. Theimpeller rotates under the floating condition without any control device. The floating condition of the impeller isrealized by utilizing the pressure in pump casing. The pump performance and the rotational behavior of the impellerwere investigated experimentally. It has found that the floating position of the impeller become quite unstablein a partial flow rate operation. And the pressure distribution at the casing wall affects the rotational posture ofthe impeller. This paper presents the effect of the casing treatment on the rotational posture of the impeller and thepressure distribution. The reasonable casing treatment is available for the stable rotational posture of the impeller.

Numerical Computations of Confined Swirl Flows in Augmentor of Jet Engines and Comparisons with Experiments

This paper describes the design of four types of swirlers which produce different velocity profiles.The data of confined turbulent swirling flowfields are obtained from experiments and numerical calculations.The actual flow profiles at the swirler exit basically agree with the desired profiles. Two kinds of turbulence models,k-ε model and DDC model,are used to predict the flowfields produced by swirl augmentors.The results show that the DDC model,based on interaction between dissipation and dispersion,can overcome the defect of k-ε model and predict the swirling flowfields more accurately than k-ε model.