Optimization of structural parameters for elliptical cross-section spiral equal-channel extrusion dies based on grey theory

The elliptical cross-section spiral equal-channel extrusion (ECSEE) process is simulated by using Deform-3D finite element software. The ratio m of major-axis to minor-axis length for ellipse-cross-section, the torsion angle u, the round-ellipse cross-section transitional channel L1, the elliptical rotation cross-section transitional channel L2 and the ellipse-round cross-section transitional channel L3 are destined for the extrusion process parameters. The average effective strain eave on cross-section of blank, the deformation uniformity coefficient a and the value of maximum damage dmax are chosen to be the optimize indexes, and the virtual orthogonal experiment of L16 (45) is designed. The correlation degree of the process factors affecting eave, a and dmax is analyzed by the numerical simulation results using the weights and grey association model. The process parameters are optimized by introducing the grey situation decision theory and the ECSEE optimal combination of process parameters is obtained: u of 120 , m of 1.55, L1 of 7 mm, L2 of 10 mm, and L3 of 10 mm. Simulation and experimental results show that the material can be refined with the optimized structural parameters of die. Therefore, the optimization results are satisfactory.     

战场复杂电磁环境下的无人机作战应用探讨

通过对战场复杂电磁环境的研究,分析总结无人机系统在侦察作战全过程中可能受到各种电磁干扰,结合无人机系统抗干扰特性,提出应对战场复杂电磁环境的无人机系统作战运用方法,有效提高了无人机在战场中的作战效能和生存能力。     

Detached-eddy simulation of supersonic flow past a spike-tipped blunt nose

Space vehicle in atmosphere travels mostly at supersonic speed and generates a very strong bow shockwave around its blunt nose. Oblique shock and conical separated flow zone generated by a forward disk-tip spike significantly reduce the drag by reducing the high pressure area on the blunt nose. This study employs improved delayed detached eddy simulation to investigate the characteristic flow structures around a spike-tipped blunt nose at Mach number of 3 and Reynolds number(based on the blunt-body diameter) of 2.72x10~8. The calculated time-averaged quantities agree well with experimental data. Characteristic frequencies in different flow regions are extracted using fast Fourier transform. It is found that two distinct instability modes exist: oscillation mode and pulsation mode. The former is related to the foreshock/turbulence interaction with nondimensional frequency at around 0.004. The latter corresponds to the interaction between turbulence and shock structures around the blunt nose, with a typical coherent structure shedding frequency at 0.092.     

重力和重力梯度基准图构建的多参量联合网格化

高精度、高分辨率的重力及重力梯度基准图是决定潜艇水下辅助导航定位精度的关键因素。我国海洋重力测量目前的主要比例尺为1:1000000，在局部区域可达1:500000，测线间隔分布相对稀疏。传统的网格化插值技术在远离测点位置时容易产生虚假异常，或异常特征发生偏移。构建了全张量重力梯度数据的6个梯度分量的联合网格化方法,利用各个梯度分量与引力位在波数域中的关系重构了引力位，实现了重力和重力梯度数据的再计算，从而实现了网格处理。通过模型数据和实测数据，验证了该方法在插值异常分辨率和位置准确性上具有优势。     

3J53弹簧表面裂纹缺陷原因分析

3J53钢丝在车间缠绕成弹簧并经热处理后发现其表面存在线性缺陷。用扫描电镜对钢丝线性缺陷的形貌进行了分析,对线性缺陷区域进行了能谱分析,并利用金相显微镜观察了线性缺陷的中心部位及其周围区域的金相组织。结果表明:钢丝表面的线性缺陷呈锯齿状,沿钢丝轴向方向,为裂纹型折叠缺陷。折叠缺陷是钢丝在生产过程中形成,在绕簧和热处理过程中,裂纹型折叠缺陷发生扩展和延伸,最终形成线性缺陷。     

A novel predictive algorithm for double difference observations of obstructed BeiDou geostationary earth orbit (GEO) satellites

Transmission link disturbances and device failure cause global navigation satellite system (GNSS) receivers to miss observations, leading to poor accuracy in real-time kinematic (RTK) positioning. Previously described solutions for this problem are influenced by the length of the prediction period, or are unable to account for changes in receiver state because they use information from previous epochs to make predictions. We propose an algorithm for predicting double difference (DD) observations of obstructed BeiDou navigation system (BDS) GEO satellites. Our approach adopts the first-degree polynomial function for predicting missing observations. We introduce a Douglas-Peucker algorithm to judge the state of the rover receiver to reduce the impact of predictive biases. Static and kinematic experiments were carried out on BDS observations to evaluate the proposed algorithm. The results of our navigation experiment demonstrate that RTK positioning accuracy is improved from meter to decimeter level with fixed ambiguity (horizontal?<?2?cm, vertical?<?18?cm). Horizontal accuracy is improved by over 50%, and the vertical accuracies of the results of the static and kinematic experiments are increased by 47% and 27% respectively, compared with the results produced by the classical approach. Though as the baseline becomes longer, the accuracy is weakened, our predictive algorithm is an improvement over existing approaches to overcome the issue of missing data.     

Ionospheric correction using GPS Klobuchar coefficients with an empirical night-time delay model

We proposed an ionospheric correction approach called NKlob to mitigate the ionospheric delay errors. NKlob is a modification of the original GPS Ionospheric Correction Algorithm (ICA), which uses an empirical night-time model depending on the time, geomagnetic location and periodicities of the ionospheric behavior to replace the night-time constant delay in GPS ICA. Performance of NKlob was evaluated by the independent total electron contents (TECs) derived from Global Ionospheric Maps (GIMs) of the International GNSS Services (IGS) and Jason-2 altimetry satellite during 2013–2017. Compared to GIM TECs, NKlob corrects 51.5% of the ionospheric delay errors, which outperforms GPS ICA by 6.3%. Compared to Jason-2 TECs, NKlob mitigates the ionospheric errors by 58.1%, which is approximately 3.7% better than that of GPS ICA. NKlob shows significant improvement in low-latitude and equatorial regions with respect to GPS ICA, meanwhile exhibiting comparable performance at middle and high latitudes. Since NKlob only requires slight technical changes at the processing level of GPS receivers, we suppose that it can be easily implemented for better ionospheric delay corrections of real-time GPS single-frequency applications.     

CAASSE Signed a MOU with HASSE

正On May 27-June 2,China Aerospace Academy of Systems Science and Engineering(CAASSE)visited the Houston Association for Space and Science Education(HASSE)and singed a MOU after met with HASSE CEO James Liu and HASSE special advisor,astronaut Dr.Leroy Chiao.