Effects Analysis on the Mechanical Properties of Nanocrystalline Materials

Nanocrystalline materials are characterized by a typical grain size from 1 to 100nm. In order to study the nanocrystalline properties of nanocrystalline materials, we chose nanocrystalline coppers as the research object. The uniaxial tensile deformation of computer produced nanocrystalline coppers is simulated by using molecular dynamics with Finnis-Sinclair potential. The mean grain size of simulated nanocrystalline coppers is varied within the 5.38 to 1.79 nm range. The strength, Young's modulus and stress-strain are strongly depended on the grain size and nanocrystalline structure. The simulated nanocrystalline coppers show a reverse Hall-Petch effect.

Conjugate Model for Heat and Mass Transfer of Porous Wall in the High Temperature Gas Flow

Heat and mass transfer of a porous permeable wall in a high temperature gas dynamical flow is considered. Numerical simulation is conducted on the ground of the conjugate mathematical model which includes filtration and heat transfer equations in a porous body and boundary layer equations on its surface. Such an approach enables one to take into account complex interaction between heat and mass transfer in the gasdynamical flow and in the structure subjected to this flow. The main attention is given to the impact of the intraporous heat transfer intensity on the transpiration cooling efficiency.

COIL亚声速段横向喷流混合流场数值分析

COIL是一个气体动力学、化学反应动力学以及光学相互耦合的复杂过程.作为高总压COIL系统研究的第一步,利用三维CFD技术对传统的COIL亚声速段横向射流混合过程进行了数值分析,讨论了包括压力梯度驱动项的分子扩散机制,得到了横向射流的较精细的结构,如马蹄形射流界面、逆旋涡对以及射流剪切层.结果表明,压力梯度驱动项对重、轻组分的扩散作用相反,重组分沿压力梯度正向扩散,轻组分逆压力梯度方向扩散.在射流穿透不足的情况下,仍然在喷管出口得到了约为0.01 cm-1的小信号增益系数.

爆轰波在弯管内传播过程数值分析

应用基元反应模型和频散可控耗散格式(DCD)对氢氧爆轰波在弯管内的传播过程进行了数值模拟.计算中氢氧混合物化学反应采用了8种组分20个反应方程式.在处理化学反应引起的刚性问题时采用了时间算子分裂的方法.计算结果表明,在弯管小曲率半径壁面附近,由于膨胀稀疏作用,爆轰波强度减弱,在局部出现前导激波与放热反应区的解藕以及二次起爆现象;在弯管大曲率半径壁面上爆轰波在马赫反射和正规反射之间相互转变,使爆轰波加强.弯管内的爆轰现象与弯管曲率半径有关.

The Mach reflection of a detonation based on soot track measurements

This paper presents a series of soot tracks formed by gaseous detonation waves diffracting around wedges with different wedge angles. These cellular structure patterns describe the Mach-reflection processes of a detonation and reveal some unique characteristics. They can be used to analyze the relationship between the trajectory angle of the triple point, wedge angle, and initial pressure in Mach reflection. Compared to the Mach-reflected one-dimensional shock wave in nonreactive air, all these unique characteristics for a Mach-reflected detonation should be attributed to the transverse-wave structure of the detonation front; meanwhile, the precursor shock wave and transverse wave influence the Mach-reflected detonation, respectively. The experimental results support the recently published numerical simulation of this complex phenomenon.

侧加热腔体内重力波演化过程的数值模拟

利用二维数值模拟的方法研究了侧加热腔体内的自然对流．基于数值模拟结果，描述了水平热入侵流(intrusion)的整个演化过程，并对该过程的物理机制进行了讨论．结果表明：当热入侵流抵达腔体冷壁后，由于冷壁无法卷入所有的热入侵流，热入侵流在冷上角堆积并产生一个反向流动，在冷壁边界层附近形成一个顺时针涡，该涡在浮力效应驱动下可返回热壁，并在腔体的冷热壁之间形成了腔体尺度的流体振荡，即内重力波．

A coupling model for terrestrial processes in arid areas and its application

In this paper, the importance of investigation on terrestrical processes in arid areas for mankind's living environment protection and local economy development as well as its present state of the art are elucidated. A coupling model, which evaluates heat, mass, momentum and radiative fluxes in the SPAC system, is developed for simulating microclimate over plant and bare soil. Especially, it is focussed on the details of turbulence transfer. For illustration, numerical simulation of the water-heat exchange processes at Shapotou Observatory, GAS, Ninxia Province are conducted, and the computational results show that the laws of land-surface processes are rather typical in the arid areas.

Structure of Vortex Dislocations in a Wake-type Flow

Structure and dynamical processes of vortex dislocations in a kind of wake-type flow are described clearly by vortex lines, which are directly constructed from data of three-dimensional direct numerical simulations of the flow evolution.

Magnetic field design for floating zone crystal growth

The magnetic fields produced by electrical coils are designed for P-doped Si crystal growth in a floating full zone in microgravity environment. The fields are designed specially to reduce the how near the free surface and then in the melt zone by adjusting the coil positions near the melt zone. The effects of the designed magnetic fields on reducing the Row velocity and the non-uniformity of the concentration distribution in the melt zone are better than those of the case of a uniform longitudinal magnetic field, obtained by numerical simulation. It is expected to improve the radial macro-segregation and reduce the convection in the crystal growth at the same time by using the designed magnetic field.

Oscillatory instability of Rayleigh-Marangoni-Bénard convection in two-layer liquid systems

The oscillatory behaviour of the Rayleigh-Marangoni-Bénard convective instability (R-M-B instability) regarding two combinations of two-layer fluid systems has been investigated theoretically and numerically. For the two-layer system of Silicone oil (10cSt) over Fluorinert (FC70), both linear instability analysis and 2D numerical simulation show that the instability of the system depends strongly on the depth ratio Hr = H1/H2 of the two-layer liquid. The oscillatory regime at the onset of R-M-B convection enlarges with reducing Γ = Ra/Ma values. In the two-layer system of Silicone oil (2cSt) over water, it loses its stability and onsets to steady convection at first, then the steady convection bifurcates to oscillatory convection with increasing Rayleigh number Ra. This behaviour was found through numerical simulation above the onset of steady convection in the case of r = 2.9, ε=(Ra-Ruc)/Rac = 1.0, and Hr = 0.5. Our findings are different from the previous study of the Rayleigh-Benard instability and show the strong effects of the thermocapillary force at the interface on the time-dependent oscillations at or after the onset of convection. We propose a secondary oscillatory instability mechanism to explain the experimental observation of Degen et al. [Phys. Rev. E, 57 (1998), 6647-6659].

气相爆轰波基元反应模型数值模拟

应用频散可控格式和基元反应模型计算了氢气-氧气和氢气-空气混合物的爆轰波一维结构，并对爆轰波在二维轴对称突扩截面管内的传播过程进行了数值模拟。采用算子分裂的方法来处理化学反应的刚性问题。计算结果表明，频散可控格式可以精确捕捉爆轰波及其反射过程。爆轰波在突扩截面处绕射，由于稀疏波的作用而引起局部熄爆现象。数值结果还反应出，在一定条件下，熄爆区域可以二次起爆，而且二次起爆过程与可燃混合气体的敏感度有关，对于敏感度高的可燃气体，由于爆轰波具有较强的自持能力，其波阵面在绕射过程中所产生的局部熄爆区后会自动再次起爆

超声速气流中横向煤油射流的数值模拟

为了对超声速气流中液体横向射流进行数值模拟，采用Eulerian-Lagmngian方法描述气液两相流动现象，用随机轨道模型追踪液滴的运动历程，并考虑气流可压缩性、流场不均匀性及液滴变形对液滴运动的影响，开发了相应的计算程序。计算结果发现，液体射流与气流之间存在着强烈的相互作用，液滴在进入气流中不久就破碎成很小的子液滴，受煤油液雾的影响气流速度、温度急剧下降，同时在喷孔上游出现～道弓形激波。与液雾结构的纹影图像和液雾穿透的实验结果对比显示，数值计算结果基本合理。

Effects of the Spanwise Characteristic Length on the Transition Feature in the Wake of a Cylinder

The transition features of the wake behind a uniform circular cylinder at Re = 200, which is just beyond the critical Reynolds number of 3-D transition, are investigated in detail by direct numerical simulations of 3-D incompressible Navier-Stokes equations. The spanwise characteris-tic length determines the transition features and global properties of the wake.

Simulations of oil spread on a water surface

Numerical simulation of an oil slick spreading on still and wavy surfaces is described in this paper. The so-called sigma transformation is used to transform the time-varying physical domain into a fixed calculation domain for the water wave motions and, at the same time, the continuity equation is changed into an advection equation of wave elevation. This evolution equation is discretized by the forward time and central space scheme, and the momentum equations by the projection method. A damping zone is set up in front of the outlet boundary coupled with a Sommerfeld-Orlanski condition at that boundary to minimize the wave reflection. The equations for the oil slick are depth-averaged and coupled with the water motions when solving numerically. As examples, sinusoidal and solitary water waves, the oil spread on a smooth plane and on still and wavy water surfaces are calculated to examine the accuracy of simulating water waves by Navier-Stokes equations, the effect of damping zone on wave reflection and the precise structures of oil spread on waves.

Formation Mechanism of Cracks in Saturated Sand

The formation mechanism of “water film” (or crack) in saturated sand is analyzed theoretically and numerically. The theoretical analysis shows that there will be no stable “water film” in the saturated sand if the strength of the skeleton is zero and no positions are choked. It is shown by numerical simulation that stable water films initiate and grow if the choking state keeps unchanged once the fluid velocities decrease to zero in the liquefied sand column. The developments of “water film” based on the model presented in this paper are compared with experimental results.