Large Scale Numerical Simulations For Multi-Phase Fluid Dynamics With Moving Interfaces

This short communication presents our recent studies to implement numerical simulations for multi-phase flows on top-ranked supercomputer systems with distributed memory architecture. The numerical model is designed so as to make full use of the capacity of the hardware. Satisfactory scalability in terms of both the parallel speed-up rate and the size of the problem has been obtained on two high rank systems with massively parallel processors, the Earth Simulator (Earth simulator research center, Yokohama Kanagawa, Japan) and the TSUBAME (Tokyo Institute of Technology, Tokyo, Japan) supercomputers.

过渡领域高超声速圆柱绕流直接模拟

用直接模拟蒙特卡罗方法模拟了再入速度为7.5km/s,高度分别为80、85、90km,物壁是完全漫反射有限催化壁的圆柱绕流。在80km考察了壁面催化率的影响。计算结果表明: 稀薄程度对壁面物理量和流场结构有十分显著的影响,热力学和化学非平衡效应显著。随着高度降低,壁面催化率对壁面热流量等有明显影响。而流场沿物面方向变化比较缓慢,应用较粗的沿壁向网格也可以得到满意的宏观结果。作了球体绕流的数值模拟并与半球头柱头部热流量的实验结果作了比较,二者符合良好。

在位－流函数坐标系下求解非线性不定边界问题

自由边界问题是一类重要的非线性流体运动问题。这类问题在工程中有着广泛的应用，如过水坝或过水闸门的出流，给定流量的射流，以及翼型设计问题。由于这类流动求解区域不确定，导致求解问题的非线性特证，使数值计算十分困难。本文以过坝水流问题为例，提出一种求解不定边界问题的新方法。

激光双焦点测速中同一粒子的判别——一种研究气流湍流新方法的探讨

本文介绍了对激光双焦点技术的研究结果。从理论上分析了在该技术中,粒子依次穿越焦点的概率,提出了实际测量这种概率的逻辑判别方法。这样,解决了激光双焦点测速中同一粒子的判别问题而且对从拉格朗日观点研究湍流扩散提供了一种新的手段。文中给出了拉格朗日相关系数与同一粒子穿越概率的关系式和该技术对空气自由射流速度测量的结果。

自由湍流的后期运动

具有剪应力的自由湍流到下游相当远的地点由于能量的逐渐耗损将衰变成为类似均匀各向同性湍流的后期运动。如同处理后一种流动问题一样,我们现在根据湍流是由涡旋所组成的概念来求自由湍流的后期运动解。求涡旋运动解的动力学的基础是湍流速度涨落方程。在后期湍流场中湍流Reynolds数比较小,故方程中的非线性项可以略去。再考虑到组成湍流的涡旋尺寸比较小,在每个涡旋范围内平均湍流速度和它的坐标梯度可以近似地认为和坐标的改变无关。我们求线性化了之后的湍流速度涨落方程如下的近似解:涨落速度的一部分代表均匀各向同性湍流的后期运动:另一部分是和平均流速的坐标梯度成正比,后一部分要比前一部分为小。从这样的近似解得出的Reynolds剪应力是和平均流速的坐标梯度成正此。当作这一般解的特例我们求一个二元尾流的后期运动。在产生尾流的物体的后面还置放一个平面与尾流对称平面成垂直的栅格。这个栅格在它的下游可产生一个迭加在物体所产生的尾流场上的均匀各向同性的湍流(?)。我们的解是适用在离物体和栅格相当远处的后期运动,但此处的流场距栅格较近,所以栅格所产生的均匀各向同性湍流要比尾流的湍流度为高,因此一般解的近似条件是可以满足的。本论文给出尾流平均流速和速度涨落平方平均值的解。

Concentration distribution in crystallization from solution under microgravity

Concentration distribution in crystallization from solution under microgravity is numerically studied. A quasi-steady state growth and dissolution in a 2D rectangular enclosure filled with sodium chlorate (NaClO3) aqueous solution, in which one wall is the growth surface of the crystal and the opposite one is the dissolution surface, is considered. The solute transport process at the growth surface is described by the diffusion-reaction theory with finite interface kinetics coefficient. The results show that the concentration at the growth surface is supersaturated and the supersaturation distribution is of non-uniformity, i.e. the supersaturation in a region facing an incoming flow is high. On the other hand, the non-uniformity of supersaturation at the growth surface is closely related to the gravity level even under microgravity, it exponentially increases as the thermal Rayleigh number on behalf of the gravity level rises.

油气混输系统的实验研究与实验模拟准则

中国科学院九五重大项目!（KZ951－A1－405）

圆桩振荡绕流对淤泥床的冲蚀

本文实验研究不同圆柱直径和不同初始圆柱与床面距离时圆柱振荡绕流对其下方淤泥介质床面的冲蚀作用。实验得到了在产生冲蚀的临界条件下圆柱与床面初始无量纲距离随振荡流Ｋｃ数的变化规律。由于不同的无量纲间距下圆柱振荡绕流分离流动的旋涡运动类型的不同，形成了冲蚀床表面的不同类型的表面冲蚀图象。

微重力下环境压力和辐射再吸收对火焰沿薄燃料表面传播的影响

当环境气体不具备辐射特性时(例如环境气体为O2-N2),在较小的环境压力下,火焰向燃料表面的热传导是火焰传播的主要驱动力.但随着环境压力的增大,火焰传播速度越来越大,火焰对燃料表面的热辐射逐渐成为火焰传播的主要驱动力之一.当环境气体具有较强的辐射特性时(例如环境气体为O2-CO2),在较小的环境压力下,火焰前沿环境气体的辐射再吸收将强化火焰对燃料表面的传热、热辐射和热传导在火焰传播机理中均起重要作用.随着环境压力的增大,氧气向火焰的扩散加强,燃烧反应加剧,火焰传播加速.当环境压力大于一定值时,进一步增大环境压力,环境气体的辐射再吸收使火焰对燃料表面的热辐射减弱.因此,火焰传播速度反而随着环境压力的增大而减小,火焰向燃料表面的热传导逐渐成为火焰传播的主要驱动力.

关于吸气式高超声速推进技术研究的思考

回顾了吸气式高超声速推进技术的研究进展,分析了超燃冲压发动机研制面临的关键科学问题,并从不同角度探讨了增大超燃冲压发动机推力的可能方法.这些方法包括:能够降低总压损失的高超声速来流压缩方法、生成三维涡流的超声速混合增强技术、碳氢燃料的预热喷射、可以控制燃烧过程的燃烧室设计优化方法、通过减小发动机流道湿面积来降低摩擦阻力和催化复合解离的燃气降低高温气体效应.考虑到等压热力学循环的热效率,还建议研究在高超声速推进系统中应用热效率高的爆轰过程,并探讨了爆轰推进方法研究的进展与问题.吸气式高超声速推进技术是高超声速飞行器发展的关键技术,认真思考和探索其发展方向是非常必要的.

A Trabecular Bone Explant Model of Osteocyte–Osteoblast Co-Culture for Bone Mechanobiology

The osteocyte network is recognized as the major mechanical sensor in the bone remodeling process, and osteocyte-osteoblast communication acts as an important mediator in the coordination of bone formation and turnover. In this study, we developed a novel 3D trabecular bone explant co-culture model that allows live osteocytes situated in their native extracellular matrix environment to be interconnected with seeded osteoblasts on the bone surface. Using a low-level medium perfusion system, the viability of in situ osteocytes in bone explants was maintained for up to 4 weeks, and functional gap junction intercellular communication (GJIC) was successfully established between osteocytes and seeded primary osteoblasts. Using this novel co-culture model, the effects of dynamic deformational loading, GJIC, and prostaglandin E-2 (PGE(2)) release on functional bone adaptation were further investigated. The results showed that dynamical deformational loading can significantly increase the PGE(2) release by bone cells, bone formation, and the apparent elastic modulus of bone explants. However, the inhibition of gap junctions or the PGE(2) pathway dramatically attenuated the effects of mechanical loading. This 3D trabecular bone explant co-culture model has great potential to fill in the critical gap in knowledge regarding the role of osteocytes as a mechano-sensor and how osteocytes transmit signals to regulate osteoblasts function and skeletal integrity as reflected in its mechanical properties.

Red shift and broadening of backward harmonic radiation from electron oscillations driven by femtosecond laser pulse

The characteristics of backward harmonic radiation due to electron oscillations driven by a linearly polarized fs laser pulse are analysed considering a single electron model. The spectral distributions of the electron's backward harmonic radiation are investigated in detail for different parameters of the driver laser pulse. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width the broadening of the high harmonic radiations can be controlled.

The dispersive properties of an excited-doublet four-level atomic system

We have investigated the dispersive properties of excited-doublet four-level atoms interacting with a weak probe field and an intense coupling laser field. We have derived an analytical expression of the dispersion relation for a general excited-doublet four-level atomic system subject to a one-photon detuning. The numerical results demonstrate that for a typical rubidium D1 line configuration, due to the unequal dipole moments for the transitions of each ground state to double excited states, generally there exists no exact dark state in the system. Close to the two-photon resonance, the probe light can be absorbed orgained and propagate in the so-called superluminal form. This system may be used as an optical switch.

Ultranarrow absorptive spectral line induced by microwave field

This paper investigates the absorptive spectral lines of four-level atomic system driven by a coupling, probe and microwave fields. Due to the perturbation of the microwave field, the original electromagnetically induced transparency is changed to electromagnetically induced absorption and the absorptive spectral line can be very narrow. This ultranarrow spectral line has potential applications to the microwave atomic frequency standard and the measurement of very weak magnetic field.