一种基于感知的大规模场景粒子系统绘制方法

在分析已有的大规模场景下使用粒子系统对自然现象等模拟方法的基础上,提出了一种绘制大规模粒子系统的方法.在大规模粒子系统的绘制过程中引入基于视截体模型的感知算法对粒子进行细化并引入双重缓冲技术以及粒子动态纹理.将粒子按所属不同区域进行分层次绘制,提高了真实性和灵活性,最终解决了传统绘制方法不适用大规模粒子系统的问题.实验表明,系统的模拟效果真实,满足系统对实时性的要求,适合大规模粒子系统的绘制.     

云的建模与渲染技术

云的建模和渲染技术对仿真系统的研究有着重要的理论和现实意义.该文利用粒子系统,结合了云的运动规律和光照规律对其进行建模.并对其运动模型、光照模型以及相函数都进行了简化.渲染时考虑了风速、光源等因素,提出两阶段算法来渲染云.试验结果表明,该文采用的运动模型,光照模型和渲染方法有较好的真实感和良好的实时性.     

超声磨料对TC4钛合金电火花加工表面质量的影响

为改善TC4钛合金电火花加工表面质量,减少表面微裂纹、重熔层等表面缺陷,提出了基于电极振动的超声磨料电火花复合加工方法。在煤油工作液中混入12g/L的SiC磨料粉,进行了有无超声磨料作用的窄脉冲(脉宽小于1μs)电火花加工的表面粗糙度对比实验研究。实验结果表明:SiC磨料的超声振动作用使零件表面粗糙度Ra由0.5μm降到0.2μm左右;重熔层厚度、表面裂纹的扫描电子显微镜(SEM)照片显示,超声磨料作用使重熔层厚度减薄20～30μm,表面微裂纹得到有效控制;机理分析认为工作液高频振动及磨料对工件的冲击作用,是改善TC4钛合金电火花加工表面质量的主要原因。研究表明磨料的超声振动作用可显著改善TC4钛合金电火花加工的表面质量。     

有限姿控能力的低RCS微小卫星姿态实时规划

 为提高在轨微小卫星使用效能及生存能力,提出一种受姿控能源消耗及驱动能力约束的低雷达散射截面（RCS）微小卫星姿态实时规划算法。算法应用曲面像素法、时域有限差分法及假设检验,在三维空间内对微小卫星的RCS及雷达探测水平进行建模,并结合雷达分布模型构建了相应的威胁评估函数及规划代价评估函数。同时,为提高算法的实时性能,采用了粒子群优化(PSO)算法以降低计算复杂度。仿真结果表明,在有限计算量的基础上,算法能够以较小规划代价有效降低卫星威胁方向的RCS及雷达探测概率,满足对微小卫星飞行姿态实时规划的需要。     

Physics of ion acceleration in the solar flare on 2005 September 7 determines γ-ray and neutron production

Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 min with high statistical significance. Intense emissions of γ-rays were also registered by INTEGRAL, and during the decay phase by RHESSI. We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky, B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140, 563–579, 2002], and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch-angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the γ-ray line emission and that ions were continuously accelerated at the emission site.     

Solar wind low-energy energetic ion enhancements: A tool to forecast large geomagnetic storms

Predicting the occurrence of large geomagnetic storms more than an hour in advance is an important, yet difficult task. Energetic ion data show enhancements in flux that herald the approach of interplanetary shocks, usually for many hours before the shock arrival. We present a technique for predicting large geomagnetic storms (Kp  7) following the arrival of interplanetary shocks at 1 AU, using low-energy energetic ions (47–65 keV) and solar wind data measured at the L1 libration point. It is based on a study of the relationship between energetic ion enhancements (EIEs) and large geomagnetic storms by Smith et al. [Smith, Z., Murtagh, W., Smithtro, C. Relationship between solar wind low-energy energetic ion enhancements and large geomagnetic storms. J. Geophys. Res. 109, A01110, 2004. doi:10.1029/ 2003JA010044] using data in the rise and maximum of solar cycle 23 (February 1998–December 2000). An excellent correlation was found between storms with Kp  7 and the peak flux of large energetic ion enhancements that almost always (93% of time in our time period) accompany the arrival of interplanetary shocks at L1. However, as there are many more large EIEs than large geomagnetic storms, other characteristics were investigated to help determine which EIEs are likely to be followed by large storms. An additional parameter, the magnitude of the post-shock total magnetic field at the L1 Lagrangian point, is introduced here. This improves the identification of the EIEs that are likely to be followed by large storms. A forecasting technique is developed and tested on the time period of the original study (the training data set). The lead times, defined as the times from the arrival of the shock to the start of the 3-h interval of maximum Kp, are also presented. They range from minutes to more than a day; the average for large storms is 7 h. These times do not include the extra warning time given when the EI flux cross the high thresholds ahead of the shock. Because the data-stream used in the original study is no longer available, we extended the original study (1998–2000) to 2001, in order to: (a) investigate EIEs in 2001; (b) present a validation of the technique on an independent data set; (c) compare the results based on the original (P1) energy channel to those of the replacement (P1′) and (d), determine new EIE thresholds for forecasting geomagnetic storms using P1′ data. The verification of this P1′ training data set is also presented, together with lead times.     

无网格SPH法中守恒光滑法的修正

守恒光滑法CSA(Conservative Smoothing Approach)是解决传统SPH(Smoothed Particle Hydrodynamics)法稳定性问题的一种很有效的方法.研究发现在SPH法中运用所谓的守恒光滑法并不能保证系统的总物理量(质量、动量或者能量)在每个时间步均守恒,而是随着时间在理论值附近波动,对计算结果的精度有一定的影响.对此提出了守恒光滑法的修正公式,对SPH控制方程得到的各个粒子的物理量进行CSA光滑,然后运用文中提出的CSA的修正公式把对该粒子物理量的改变量加权平均给邻域内各个粒子,从而确保了SPH算法中系统的总物理量(质量、动量以及能量等)在每个时间步均守恒,而且由于减少了CSA对SPH得到的各个粒子物理量的过分光滑,从而提高了计算精度.物理意义、理论推导以及文中的算例均证明了这种修正的有效性.     

固体发动机燃烧室离散相颗粒相变对喉衬烧蚀的影响

针对喉衬烧蚀试验中高燃温推进荆对喉衬烧蚀较普通推进剂偏小的现象,采用Euler-Lagrangian方法,对燃烧室内离散相颗粒的传热和相变过程进行数值分析,对烧蚀偏小现象给出了理论解释.分析结果表明,在发动机燃烧室内的高温环境(4000 K)下,约有离散项总质量40%左右的颗粒发生相变成为气体,大部分离散相颗粒从喷管迟逸;离散项颗粒的相变,一方面会减弱对喉衬的烧蚀,另一方面会造成燃烧室内温度等参数分布的不均匀.     

物理规划方法及其在飞机方案设计中的应用

 物理规划方法是一种高效的设计优化方法,它与设计者的工程经验紧密联系,能以较低的计算代价获得真实反映设计者偏好的折衷解,对解决多目标优化设计问题具有重要意义。将物理规划设计方法与稳定、高效的粒子群优化算法相结合,有效地求解多目标优化问题中的折衷非劣解。通过求解数值算例和民用飞机方案设计问题,验证了该方法的有效性。     

大型运输机全寿命成本模型分析

大型运输机的发展,对于国民经济及国防事业发展具有重要的意义。由于研制大型运输机发展人力、财力投入大,使用周期长,因而,早期研制阶段进行全寿命成本模型分析对于研制方案的优选、提高竞争水平具有重要影响。本文从大型运输机概念设计中对经济性分析的需求出发,结合我国国情,探讨了大型运输机全寿命成本模型的建立方法。