Numerical investigation on the shock wave transition in a three-dimensional scramjet isolator

The scramjet isolator, which is used to prevent the hypersonic inlet from disturbances that arise from the pressure rise in the scramjet combustor due to the intense turbulent combustion, is one of the most critical components in hypersonic airbreathing propulsion systems. Any engineering error that is possible in the design and manufacturing procedure of the experimental model, and the intense heat release in the scramjet combustor, may cause the performance of the isolator to decrease, leading to its lack of capability in supporting the back pressure. The coupled implicit Reynolds Averaged Navier–Stokes (RANS) equations and the two-equation standard k?ε turbulent model have been employed to numerically simulate the flow fields in a three-dimensional scramjet isolator. The effects of the divergent angle and the back pressure on the shock wave transition and the location of the leading edge of the shock wave train have been estimated and discussed. The obtained results show that the present numerical results are in very good agreement with the available experimental shadow-pictures, and the numerical method is more suitable for capturing the shock wave train and predicting the location of the leading edge of the shock wave train in the scramjet isolator than the present two-dimensional numerical methods. This is due to the small width-to-height ratio of the isolator and the intense three-dimensional flow structures. On increasing the divergent angle of the scramjet isolator, the static pressure along the central symmetrical line of the isolator decreases sharply. This is due to the strong expansion wave generated at the entrance of the isolator, and when the divergent angle of the isolator is sufficiently large, namely 1.5°, a zone of negative pressure is formed just ahead of the leading edge of the shock wave train. At the same time, the shock wave train varies from being oblique to being normal, and then back to oblique. With an increase in the prescribed back pressure at the exit of the scramjet isolator, the leading edge of the shock wave train moves forward towards the entrance of the isolator, and when the back pressure is sufficiently large, unstart conditions in the hypersonic inlet can take place if the shock train reaches the inlet.     

The effects of simulated microgravity on cultured chicken embryonic chondrocytes.

Using the cultured chicken embryonic chondrocytes as a model, the effects of simulated microgravity on the microtubular system of the cellular skeleton, extracellular matrix, alkaline phosphatase activity, intracellular free calcium concentration and mitochondrial ATP synthase activity with its oligomycin inhibition rate were studied with a clinostat. The microtubular content was measured by a flow cytometer. The decrease of microtubular content showed the impairment of the cellular skeleton system. Observation on the extracellular matrix by the scanning electron microscopy showed that it decreased significantly after rotating, and the fibers in the extracellular matrix were more tiny and disorderly than that of the control group. It can be concluded that the simulated microgravity can affect the secreting and assembly of the extracellular matrix. In contrast to the control, there was a time course decrease in alkaline phosphatase activity of chondrocytes, a marker of matrix mineralization. Meanwhile a significant drop in the intracellular calcium concentration happened at the beginning of rotation. These results indicate that simulated microgravity can suppress matrix calcification of cultured chondrocytes, and intracellular free calcium may be involved in the regulation of matrix calcification as the second signal transmitter. No significant changes happened in the mitochondrial ATP synthase activity and its oligomycin inhibition rate. Perhaps the energy metabolism wasn't affected by the simulated microgravity. The possible mechanisms about them were discussed.     

集成制造车间生产控制和管理技术研究

在介绍了MRPⅡ驱动下的生产管理系统 (PMS)和典型机加车间的基本结构、特点和不足的基础上 ,提出了一种适合国情的“集成制造车间”生产控制和管理技术 (IMS -PCMS) ,增强了车间级生产计划控制和管理的能力 ,并将MRPⅡ按计划集中控制和管理生产作业运行模式转变成可视化“计划主导型”并基于车间实际情况实现滚动生产作业调度控制和管理的模式。     

一种用于交会对接全过程的测量敏感器——复合式激光雷达

本文介绍激光雷达系统的组成和测量原理;提出了一种作为交会对接全过程测量敏感器的复合式激光雷达。所谓复合式就是把激光雷达与光学成象敏感器有机结合起来,远、中距离由激光雷达承担测量任务,近距离测量由CCD光学成像系统来完成。     

工具与方法:中国现代惟科学主义文化哲学的兴起

近现代史上,中国的惟科学主义者试图建构一种新的文化,他们肯定科学的工具价值,认为科学是文化的物质承担者,文明的进步即物质的进步,亦即科学的进步。他们继而高扬科学的认识论价值,确认科学建立知识的方式和原则具有普遍性,科学精神是文化进步的重要尺度。由此,中国现代惟科学主义的建设理路已经比较清晰,等待它是历史的选择与检验。     

Implementation of a single-phase AC/AC converter based on neutral-point-clamped topology

A single-phase ac/ac converter based on neutral-point-clamped scheme is proposed to perform unity input power factor and to provide a stable ac voltage to the critical loads. The ac/dc rectifier part is controlled to generate a unipolar pulsewidth modulation (PWM) waveform on the ac terminal by using four power switches with voltage stress of half the dc-link voltage. The carrier-based current control scheme is employed in the inner control loop to track the line current command. To regulate the dc bus voltage, a proportional-integral (PI) control is adopted in the outer control loop. The dc/ac inverter part of the system with four power switches is employed to generate a stable and clean sinusoidal output voltage to the critical load. The instantaneous current control scheme is used to track the output voltage command. To verify the effectiveness of the proposed control algorithm, the simulation and experimental results based on a laboratory prototype were implemented and discussed.     

典型R-C系统主光学装置的热光学分析

文章采用热光学集成法对典型R-C成像系统某主光学装置进行热光学分析。     

平均绝对差图象匹配系统的定位精度

本文证明了两个基本定理。在假定图象信号和附加噪声是相互独立的零均值齐次高斯随机场、且二次可微的条件下,解决了平均绝对差图象匹配系统的定位精度的分析和计算问题。用实例说明了精度分析和计算的方法,分析了影响定位精度的因素和参数。为匹配性能的定量分析提供了理论依据和方法。     

Observations of inverted-V electron precipitation

The characteristics of inverted-V electron precipitation fluxes deduced predominantly from observations by the Atmosphere Explorer satellites are reviewed. The energy and pitch angle distributions are presented and shown to be generally in agreement with acceleration by a parallel electrostatic potential. Characteristics of secondary electrons are examined, and effects of beam plasma instabilities on these electrons are discussed. The properties of the monoenergetic component are compared with theoretical models of creating parallel DC electric fields, and found to favor the anomalous resistivity model. The article also discusses relations of inverted-V events with other auroral phenomena including auroras, electrostatic shocks, convective electric field reversals, field-aligned currents and wave emissions. The principal conclusions are: (1) plasma sheet electrons are continuously accelerated to form inverted-V structures in the pre-midnight hemisphere independent of substorm phase, (2) the acceleration processes are probably related to large scale electrostatic wave turbulence observed at altitudes of a few thousand kilometers, (3) narrow bursts of intense electron precipitation fluxes are found to be imbedded within some inverted-V's. It is argued that the narrow bursts of intense electron precipitation have the proper characteristics to cause discrete auroral arcs in the atmosphere. We suggest that these narrow bursts are accelerated by an electrostatic shock at higher altitude and capable of producing discrete auroral arcs below the observing satellite.     

稀薄效应对可倾瓦动压气体轴承性能的影响

以微型三可倾瓦动压气体轴承为研究对象,采用连续模型、一阶滑移模型和WU新滑移模型速度边界条件,建立考虑稀薄效应的滑移修正雷诺方程。结合牛顿迭代法和有限差分法求解修正雷诺方程,计算考虑稀薄效应的三可倾瓦动压气体轴承各瓦块压力分布和承载力。计算结果表明:三可倾瓦动压气体轴承偏心率越大,转速越大,压缩系数越大,轴承承载力越大;考虑稀薄效应后,计算出的可倾瓦动压气体轴承承载力明显下降,且随着克努森数的增大,WU新滑移模型计算得到的承载力明显低于一阶滑移模型。