A surface thermal model and exospheric ballistic transport code of planet Mercury

The atmosphere of Mercury is of an exospheric nature. Its formation is due to several physical mechanisms including meteoroid impact, surface sputtering by solar wind ions and photon sputtering by solar UV radiation. The molecules and atoms emitted from the surface materials of Mercury include H, He, O, Ar, and S, etc. It is important to study their spatial distributions across the planetary surface via ballistic random walk. We have developed a surface thermal model coupled with Hodges-type Monte Carlo calculations to simulate the exosphere of Mercury, which will be a major scientific target of the BepiColombo mission of ESA and JAXA.     

引脚式表面贴装元件的数值热分析

建立了四边引线塑料扁平封装(PQFP, Plastic Quad Flat Package)数值热模拟的详细模型和简化模型,实验验证了这两种模型的模拟精度.对PQFP在机载恶劣环境下的稳态热性能进行了研究,分析了影响元件内、外热阻的各种因素.结果表明,内部采用多层结构设计是改善PQFP元件热性能的最佳方案,而在采用强迫空气冷却时,空气速度不应大于5m/s.对承受脉冲形式热载荷和环境温度随时间变化两种情况下的PQFP元件进行了瞬态热特性研究,获得了芯片结点温度随时间变化的曲线,可用于研究元件因过热引起的热应变、热损坏和电信号失真,为改进和优化元件热设计提供科学依据.      

某型飞机水上迫降数值化模型

近海/跨海使用的飞机必须考核水上迫降性能,而目前飞机抗坠毁标准、设计方法和分析方法通常只考虑硬着陆情况,很少考虑水面迫降情况,这是由于水上迫降涉及多场耦合,问题十分复杂.为了简化问题,采用了解耦的方法,将仿真与试验相结合,利用MSC.PATRAN/DYTRAN软件平台,建立全机有限元模型,包括全尺寸机身、机翼和尾翼结构,并导入具体参数,对13种工况条件进行了动力学数值仿真计算.预测了飞机撞击水面过程中结构的瞬态应力分布,对飞机机身下部蒙皮进行了强度分析,表明飞机在着水过程中蒙皮不会发生损坏.      

移动电子设备相变热控单元热性能的数值仿真

采用相变材料(PCM,Phase Change Material)的热控制单元(TCU,Thermal Control Unit)可以很好地实现对可移动电子设备的热控制,避免设备过热而引起的热故障.建立了分析TCU热性能的二维数学模型,并进行了数值计算.结果表明:加肋和填充泡沫-PCM复合材料的设计方案可以显著提高TCU热性能,能很好地满足电子元件的工作要求.此外,还对加肋和填充泡沫-PCM提高TCU性能的效果进行了比较,结果表明:填充泡沫-PCM复合材料的方案更具优势.所得结论对移动电子设备TCU的设计和优化有一定的指导作用.     

旋转状态下气膜冷却模型的数值模拟

通过对带有90°倾角圆柱形交错孔排的涡轮叶片模型进行数值模拟,得到了不同主流雷诺数、旋转数和吹风比情况下前缘面与后缘面侧的气膜冷却流动与换热特性及各气膜孔流量系数的分配规律.结果表明,冷气受到离心力与哥氏力的共同作用向高半径处发生偏转,导致壁面冷却效率降低;雷诺数的增大会削弱气膜冷却效果,高吹风比则不利于气膜孔下游区域的冷却.各气膜孔的流量系数随吹风比的增大而增大,随旋转数的提高而减小.在后缘面侧,相同工况下各气膜孔的流量系数明显高于前缘面侧对应气膜孔的值.      

The measurement of water vapour in thermal emission on studies of nimbus-7 stratoprobe flights for correlative studies of nimbus-7

A scanning infrared radiometer used to measure nitric acid was flown on the STRATOPROBE Flight of November 8, 1978. Using the observed thermal emission from 6.5 to 7.4 microns during the balloon ascent, a water vapour profile has been derived using a band model of the water vapour lines in this spectral region. The resulting profile has a minimum of 3.3 ppmv at the tropopause and then rises to a value of 5 ppmv by 30 km. The profile is comparable to the profile from another water vapour instrument from the National Physical Laboratory which was flown on the LIP balloon payload on the same day. A coincident profile from the LIMS experiment on NIMBUS 7 was also obtained since the STRATOPROBE experiment was flown as a correlative measurement for the LIMS experiment.     

复合材料热膨胀性能的细观分析模型与预报

基于细观力学有限元方法对T300/5208,P75/934,P75/CE339,C6000/PMR 15,HMS/Borosilicate,P100/2024 Al六种复合材料的纵向和横向热膨胀系数进行了预报,并将预报结果与国内外常用的几种理论方法的预报结果以及试验值进行了对比,分析了各分析方法的预报精度.各分析方法对于复合材料的纵向热膨胀系数的预报结果与试验值均能很好吻合;对于横向热膨胀系数,各分析方法的预报结果相差较大.Rosen and Hashin(RH)方法与所建立的细观力学有限元分析模型的预报结果与试验值的吻合程度比其他理论方法要高,进而验证了细观力学有限元方法的可靠性.此外,基于细观力学有限元方法建立了各种材料体系热膨胀系数对纤维体积分数的响应关系,得出复合材料纤维/基体性能比对其热膨胀系数的影响,为航空航天结构先进复合材料的应用提供了技术基础支撑.     

Steady and oscillatory convection in vertical cylinders heated from below. Numerical simulation of asymmetric flow regimes

Three-dimensional buoyancy driven flows in vertical cylinders are simulated with a finite difference technique. Asymmetric convective flows are studied for three values of the (length-to-radius) aspect ratio A. One result concerns the occurrence of asymmetric convection in a flat A=0.485 cylinder for Pr=6.7, near the threshold and up to 3Ra_c (where Ra_c is the critical Rayleigh number and Pr is the Prandtl number). Complex (steady and time-dependent) supercritical regimes have been simulated in cylinders of aspect ratios A=2 and A=4 for Pr=0.02 and in ranges of Ra up to 8Ra_c and 6Ra_c, respectively. The flow patterns are analysed graphically and discussed with respect to a weakly nonlinear analysis and to experiments. In a particular case, A=4 and Pr=0.02, an oscillatory motion has been obtained and some features of the flow pattern are shown during a period.     

Plasma convection model and generation of longitudinal currents in the earth's magnetosphere

The aim of this paper is to investigate processes in the magnetosphere and in particular the problems of the interaction of the solar wind with the Earth's magnetic field to produce large-scale convection, electric fields and longitudinal currents in the magnetosphere. The investigation is carried out in the frame of magnetic hydrodynamics. The reason for such an approach can be found in /1/. When calculating the transfer coefficients, the Böhm approximation is used, i.e. it is considered that the plasma in the near-equatorial part of the magnetosphere (quasiplanar geometry is used in the problem for simplification) is sufficiently turbulent that the condition ωτ ≈ 1 is valid (ω is the Larmor frequency of electrons, τ is effective time between two Quasi-collisions). The main subjects of investigation in this paper are the input near the equatorial boundary layer and the plasma layer of the magnetosphere tail.     

Mini-magnetosphere: Laboratory experiment,physical model and Hall MHD simulation

Magnetosphere with a size comparable to the ion kinetic scales is investigated by means of laboratory experiment, analytical analysis and Hall MHD simulation. In experiment a specific magnetic field was observed which is non-coplanar to dipole field, does not change sign at dipole moment inversion and could be generated only via the quadratic Hall term. Magnetopause position and plasma stand off distance were found to be profoundly different between the experimental regimes with small and large ion inertia length. In the previous studies of a mini-magnetosphere by kinetic codes such novel features were observed as absence of the bow shock and plasma stopping at the Stoermer particle limit instead of the pressure balance distance. Proposed analytical model explains these features by Hall currents which tend to cancel magnetic field convection by ions. Performed numerical simulation shows a good agreement with experiment and analytical model. It gives detailed spatial structure of the Hall field and reveals that while ions penetrate deep inside mini-magnetosphere electrons overflow around it along magnetopause boundary.     

Nusselt number for convection driven by tidal and radiogenic heating

The ‘traditional’ formulas giving the Nusselt number Nu as a function of Rayleigh number Ra cannot be used for low and moderate values of Ra. Moreover, the recent progress in 3D numerical modeling makes possible to determine the Nusselt number Nu as a function of Rayleigh number Ra for convection driven by radiogenic and tidal heating. We found that for low and moderate Ra: Nu(Ra) = ε(Ra+ξ)^λ where λ depends on rheology and boundary conditions, ε depends only on the mode of heating, and ξ = ε^−1/λ · Nu(Ra) makes possible to develop a parameterized theory of convection in medium size icy satellites. We also indicate some differences between convection driven by tidal and radiogenic heating and convection driven exclusively by radiogenic heating.     

等离子涂层典型界面损伤与破坏的数值模拟

基于界面损伤力学思想,提出了三结点界面单元的概念,和传统界面单元相比,具有能够表征任意形状界面法线方向的优点,将改进的单元通过ABAQUS的用户子程序UEL与有限元软件进行了结合.考虑等离子涂层界面粗糙度的影响,模拟了余弦形界面在热循环载荷下的损伤以及在拉伸载荷下的破坏过程.计算结果表明,界面单元结点间位移出现不连续现象,体现了裂纹的逐渐张开过程,界面损伤随热循环次数的增加而增加,其中第一个循环造成的损伤最大,波峰处是界面断裂的危险位置,法向分离起主导作用;等离子涂层粗糙界面承受法向拉伸载荷的能力较平直界面显著增加.采用改进的界面单元模拟异质材料复杂形状界面的损伤与破坏是可行的,结果是合理的.      

热沉结构管接头的应力计算与实验验证

由于大型空间环境铝热沉设备KM6承受周期性的应力,使空间实验室结构存在安全问题,因此选择T型、L型两类典型热沉构件,运用大型计算软件PATRAN,NASTRAN对构件在压力谱与温度谱共同作用情况下进行应力分析,得到关键部位的应力.采用实验方法确定了热沉各连接部位的应力,并把有限元计算结果和实验结果进行比较.结果表明:有限元计算的结果与实验测量结果基本一致,从而可以采用计算方法确定空间实验室的工作应力,进行其结构的安全性预报.     

卫星热平衡试验的温度预测与仿真模型

为了解决卫星热平衡试验中涉及的温度预测与变工况仿真试验问题,建立起星载仪器温度变化的双层集总参数模型,这一动态特性模型采用简单的嵌套结构,模型参数可以方便地运用系统辨识算法从卫星热平衡试验数据中获取,应用这一模型可以对不同星载仪器的温度变化进行分别的预测与仿真计算,从而大大减少了温度预测与仿真工作的计算量.对比研究表明:温度预测与仿真计算的结果与实际热平衡试验的结果一致.     

Development of a micro-balance system for dust and water vapour detection in the Mars atmosphere

Quartz crystal piezoelectric sensors are suitable for deposition analyses that need very high sensitivity. Due to the wide working ranges and high performances, micro-balances can measure the mass settling in average Mars conditions during a period of months before saturation is reached. This ensures a proper use for short and long term water and dust deposition monitoring. Micro-balances have been studied, calibrated and used for the GIADA (grain impact analyser and dust accumulator) experiment for the ESA-Rosetta space mission. Experience on micro-balance performance study by dust deposition has been acquired and water vapour deposition studies are in progress in a Martian atmosphere simulation chamber. Preliminary results show that micro-balances are capable to detect up to partial pressure values corresponding to parts per billion of the typical Martian atmosphere.     

Electron density near the plasmapause,modified experimental parameters for the volland convection electric field and field-aligned current distribution deduced from the global field model

This paper presents the results of a statistical analysis made on the electron density data obtained over one year by the Relaxation Sounder (RS) onboard the GEOS-2 ESA satellite. The annually overaged daily profile is given. A seasonal variation of the plasma density in the equatorial plane is clearly apparent, i.e. the electron density is larger in summer than in winter. By combining the equatorial electron density data and data obtained in the auroral ionospheric region, we can deduce the parameters of the Volland convection electric field model and calculate the associated field-aligned current distributions, which roughly agree with the observed ones.     

基于Transition SST模型的高雷诺数圆柱绕流数值研究

为了研究高雷诺数下圆柱绕流边界层的转捩现象和圆柱尾迹近壁区的流动特征,首先通过在典型雷诺数下采用Transition SST四方程转捩模型模拟圆柱绕流得到的结果与实验结果及采用SST k-ω两方程湍流模型模拟结果的对比分析,验证了Transition SST模型在模拟高雷诺数下圆柱绕流的优越性,并较为准确地预测出了圆柱绕流边界层的转捩现象及尾迹近壁区的流动特征。然后分别对亚临界区、临界区、超临界区和过临界区的圆柱绕流问题进行了数值模拟,分析了不同雷诺数下圆柱绕流的流场结构及圆柱表面压力系数、摩擦力系数的变化规律,研究了圆柱绕流近壁区的流动特征、边界层转捩的流动机理、转捩位置及其随雷诺数的变化规律。结果表明,亚临界区二维圆柱绕流边界层发生层流分离,无分离泡和转捩现象;临界区和超临界区二维圆柱绕流边界层先产生了分离泡现象,之后流动发生了转捩并在转捩后发生湍流分离;过临界区二维圆柱绕流边界层流动在转捩之后发生湍流分离,无分离泡现象;在临界区、超临界区和过临界区,二维圆柱绕流边界层转捩位置随雷诺数增大向前驻点移动。     

Numerical investigation of the influence of large turbulence scales on the parallel and perpendicular transport of cosmic rays

In recent analytical investigations it has been demonstrated that the turbulence behavior at large scales has a very strong influence on the perpendicular diffusion coefficient of charged particles. In the present paper we use computer simulations to investigate numerically cross field transport and particle propagation along the mean magnetic field for different turbulence models at large scales. Our results are compared with quasilinear theory and nonlinear diffusion theories. We show that for different forms of the turbulence spectrum at large scales, the perpendicular mean free paths obtained numerically are in agreement with recent predictions made by analytical theory. It is also shown that the parallel diffusion coefficient contains always a strong nonlinear contribution which is, however, independent of the assumed spectrum at large scales.     

Numerical techniques for generating and refining solar sail trajectories

Like all applications in trajectory design, the design of solar sail trajectories requires a transition from analytical models to numerically generated realizations of an orbit. In astrodynamics, three numerical strategies are often employed. Differential correctors (also known as shooting methods) are perhaps the most common techniques. Finite-difference methods and collocation schemes are also employed and are successful in generating trajectories with pseudo-continuous control histories. These three numerical techniques are employed here to generate periodic trajectories displaced below the Moon in a circular restricted three-body system. All these approaches reveal trajectory options within the design space for solar sail applications.