粒子输运离散纵标方程基于界面修正的并行计算方法

为了改造粒子输运方程求解的隐式格式,研究设计适应大型并行计算机的并行计算方法,介绍一类求解粒子输运方程离散纵标方程组的基于界面修正的源迭代并行计算方法.应用空间区域分解,在子区域内界面处首先采用迎风显式差分格式进行预估,构造子区域的入射边界条件,然后,在各个子区域内部进行源迭代求解隐式离散纵标方程组.在源迭代过程中,在内界面入射边界处采用隐式格式进行界面修正.数值算例表明该并行计算方法在精度、并行度、简单性诸方面均具有良好的性质.     

扩散方程的守恒型并行计算格式

辐射流体力学实际问题计算中扩散方程的计算量极大,必须采用并行计算.研究易于在并行机上实施的高效的并行计算方法,通过采用预估修正等多种方式,构造和发展既保持隐式格式的守恒性、同时能保持所需精度与无条件稳定性的并行计算格式,以满足大规模数值求解辐射流体力学问题的需求.     

算子分裂法在对流占优三维对流—弥散问题中的应用

本文提出了一种求解对流占优的三维对流－弥散问题的新方法。首先用算子分裂技术把对流－弥散方程在时间上分裂为三步，然后根据各步中方程的特点，分别用常数变易法、匹配伪弥散系数法和交替方向隐式差分法求解。对流－弥散方程中还考虑了吸附作用、衰变作用和化学反应项。最后把该法应用于一个理想的污染物三维运移问题的求解。通过与解析解及其它数值解对比，表明本法的模拟结果是令人满意的。     

二维柱几何非定常中子输运方程基于格式的界面预估校正并行算法

针对二维柱几何非定常中子输运方程的Sn-间断有限元方法,提出基于格式的界面预估校正并行算法.数值算例表明,该并行算法在精度与并行度等诸方面均具有良好的性质,与已有的基于隐式格式的并行扫描算法相比,对于二维中子输运大规模计算问题,并行计算效率较高,并行加速比可增加-倍以上,且可保持原隐式格式的计算精度.     

弹头激波诱导燃烧特性的数值模拟

为了研究弹头激波诱导燃烧，基于有限体积的考虑化学反应的Navier-Stokes（N-S）方程，对预混氢气-空气化学恰当比时的燃烧流场进行了数值模拟.时间项基于2阶隐式LU-SGS格式，对流项基于Steger-Warming进行离散，化学反应源项采用对角化隐式处理.首先，研究了网格对燃烧爆轰流场结构的影响，并利用Lehr实验结果验证了计算方法的可靠性；其次，研究了弹头的飞行Mach数（Ma=4.18，5.11，6.46）、弹头直径（D=5，10，15 mm）对燃烧流场稳定性的影响.研究表明:计算网格对氢气-空气爆轰流场结构影响很大；弹头直径一定时，氢气-空气燃烧流场稳定性随着飞行Mach数的增大而增强；弹头飞行Mach数一定时，氢气-空气燃烧流场稳定性随着弹头直径减小而增强.      

自然对流换热的高精度非结构网格有限体积法

用非结构网格有限体积法求解自然对流换热时,传统的对流项离散格式难以兼顾数值精度与计算效率,我们发展了一种耦合高精度格式的延迟修正方法,用于对流项的离散.高Re数下方腔驱动流数值计算验证了该方法具有较高的计算精度和较好的稳定性.Boussinesq流体的自然对流换热数值模拟,表明该方法能有效克服高Ra数时数值计算发散,可准确捕捉自然对流换热问题中不同偏心率下的等温线和流线分布特征.     

一种基于特征理论模拟凝聚炸药爆轰的单元中心型Lagrange方法

提出一种数值模拟凝聚炸药爆轰问题的单元中心型Lagrange方法.利用有限体积离散爆轰反应流动方程组,基于双曲型偏微分方程组的特征理论获得离散网格节点的速度与压力,获得的网格节点速度与压力用于更新网格节点位置以及计算网格单元边的数值通量.以这种方式获得的网格节点解是一种"真正多维"的理论解,是一维Godunov格式在二维Riemann问题的推广.有限体积离散得到的爆轰反应流动的半离散系统使用一种显-隐Runge-Kutta格式来离散求解：显式格式处理对流项,隐式格式处理化学反应刚性源项.算例表明,提出的单元中心型Lagrange方法能够较好地模拟凝聚炸药的爆轰反应流动.     

激光辐照高超声速舵面热力响应研究

建立了基于流固耦合的高超声速飞行器舵面结构在激光辐照下静气动弹性模型,流体控制方程为三维雷诺平均N-S方程,分别采用了中心格式和AUSM+up格式对粘性项和对流项进行空间离散,时间推进采用了高斯-塞德尔隐式推进方法,湍流粘性系数求解使用Menter SST模型。利用冯·卡门研究所高超风洞实验结果对模型进行了校核,预估了激光辐照对高超舵面的热力影响。结果表明,气动力/热计算模型与实验数据符合较好,能够准确模拟高超飞行器的热力参数,根据模型外推结果,激光在较低功率下加热高超飞行器舵面可能导致舵面材料弹性模量大大降低,继而发生弯曲发散而折断,高超飞行器可能因此发生气动失稳而坠毁。     

强爆炸火球数值模拟中的算子分裂方法

采用算子分裂方法将辐射流体力学方程组分裂为对流项和刚性源项,设计一种高效求解刚性源项方程组的数值方法.数值实验表明:该方法对时间步长不敏感,计算精度能够满足工程计算要求.在不对方程组做任何近似的情况下,数值给出了较长时间内火球冲击波阵面压力峰值及阵面位置的变化,结果与实际强爆炸中的经验公式吻合较好.     

一种基于AUSM思想的通量分裂方法

根据对流迎风分裂(AUSM)思想提出一种通量分裂方法,称为K-CUSP格式.它与传统H-CUSP和E-CUSP格式的最大差异在于总能量的分裂:K-CUSP格式将无粘守恒通量中所有的运动学量分裂到对流项,所有的热力学量分裂到压力项,即总能量被分裂成动能和静焓.对于压力项的数值通量,采用一种新的界面构造方法.数值测试表明:①K-CUSP格式继承了FVS格式的简单性和稳健性.在激波后不易出现压力过冲,在膨胀区域没有振荡,优于AUSM和WPS格式;②K-CUSP格式继承了FDS格式的分辨率.激波间断的分辨率和H-CUSP、E-CUSP格式基本相同,接触间断的分辨率高于FVS格式,低于Roe、AUSM和WPS格式.AUSM和WPS格式在计算运动接触间断时,速度存在很大振荡,而新格式不存在振荡.     

湍流热对流的高效并行直接求解方法

结合泊松方程直接求解的PDD算法,创建二维湍流热对流DNS模拟的并行直接求解方法（PDM-DNS）.在"天河二号"超级计算机上完成高Ra数湍流热对流大规模DNS计算,计算效果令人惊喜,实现了一年前难以达到的计算规模.高分辨率的湍流热对流计算结果显示,不同高Ra数的湍流热对流流动状态完全不同.为高Ra数湍流热对流大规模并行计算和数值模拟研究提供了有价值的计算方法.     

Temperature and velocity measurement of a 3-D thermal flow field using thermo-sensitive liquid crystals

It is an important challenge to analyze a three-dimensional thermal flow field in engineering, science, and agriculture. For such an analysis, it is essential to measure physical quantities such as temperature and velocity over the entire thermal flow field. This paper presents a measurement system based on color image processing for temperature and velocity vector distributions in a three-dimensional thermal flow field. Flow visualization is accomplished by the use of thermo-sensitive liquid crystal tracers. An algorithm for the color-to-temperature transformation using a multi-layer feed-forward neural network is applied to three-dimensional natural convection in a rotating cylindrical cell. Two-dimensional temperature distributions in a slit plane are obtained by using the algorithm. A three-dimensional temperature distribution is consequently constructed by interpolating the two-dimensional distributions using the B-spline function. In addition, the Spatio-Temporal correlation method is applied to the natural convection to obtain a three-dimensional velocity vector distribution.     

Two-phase viscoelastic jetting

A coupled finite difference algorithm on rectangular grids is developed for viscoelastic ink ejection simulations. The ink is modeled by the Oldroyd-B viscoelastic fluid model. The coupled algorithm seamlessly incorporates several things: (1) a coupled level set-projection method for incompressible immiscible two-phase fluid flows; (2) a higher-order Godunov type algorithm for the convection terms in the momentum and level set equations; (3) a simple first-order upwind algorithm for the convection term in the viscoelastic stress equations; (4) central difference approximations for viscosity, surface tension, and upper-convected derivative terms; and (5) an equivalent circuit model to calculate the inflow pressure (or flow rate) from dynamic voltage.     

Numerical simulation on dendritic growth of Al-Cu alloy under convection based on the cellular automaton lattice Boltzmann method

A numerical model is developed by coupling the cellular automaton (CA) method and the lattice Boltzmann method (LBM) to simulate the dendritic growth of Al-Cu alloy in both two and three dimensions. An improved decentered square algorithm is proposed to overcome the artificial anisotropy induced by the CA cells and to realize simulation of dendritic growth with arbitrary orientations. Based on the established CA-LBM model, effects of forced convection and gravity-driven natural convection on dendritic growth are studied. The simulation results show that the blocking effect of dendrites on melt flow is advanced with a larger number of seeds. The competitive growth of the converging columnar dendrites is determined by the interaction between heat flow and forced convection. Gravity-driven natural convection leads to highly asymmetric growth of equiaxed dendrites. With sinking downwards of the heavy solute, chimney-like or mushroom-like solute plumes are formed in the melt in front of the columnar dendrites when they grow along the gravitational direction. More details on dendritic growth of Al-Cu alloy under convection are revealed by 3D simulations.     

Effect of gas convection induced by cross-field discharge in a copper vapor laser

Magnetic field has been applied to a copper vapor laser with an intention of improving the laser characteristics by inner gas convection. The convection induced by the J×B cross-field discharge (the Lorentz force) is used for partially replacing or mixing the laser medium in a highly heated discharged region with relatively cold gas surrounding the region. It is observed that the output power increases with the increase of the magnetic field. A 23% increase in the output power is obtained with a magnetic field of 150 gauss. The cross-field discharge causes an increase in the peak discharge voltage and a reduction in the initial part of discharge current (Phantom current). Based on the results observed, the possible mechanisms for increasing the power are discussed.     

Time Step Validation Method Research for Low-Prandtl Number Fluid Numerical Simulation

The stability condition of Courant number and diffusion number is proved for an SGSD (stability guaranteed second-order difference) scheme by von Neumann method in implicit and explicit discretization of the one-dimensional convection and diffusion terms. Then, a series of numerical simulations of fluid flow and heat transfer based on two-dimensional unsteady state model is used to study the combined natural and MHD (magnetohydrodynamics) convection in a Joule-heated cavity using the finite volume methods, for the fluid of Pr = 0.01, also we use an SGSD scheme and IDEAL (inner doubly iterative efficient algorithm for linked equations) algorithm. It is found that periodic oscillation flow evolves.We propose a new convergence concept for the simulation oscillation results; the results of the numerical experiments are presented and they confirm our theoretical conclusions. The convergence result is checked in another way. It is found that the two approaches have the same results and can judge the validity of the time step. The proposed method is helpful to get reliable results efficiently.     

粘性耗散及变物性对多孔介质中对流换热的影响研究

本文通过理论分析、数值模拟与实验研究的综合,研究了粘性耗散及变物性对多孔介质中对流换热的影响。发现：用理想的等热流边界条件可以较好地数值模拟等热流边界条件下多孔介质中的对流换热;在实际可实现的参数条件下,粘性耗散对多孔介质中对流换热的影响不大;油的变物性对换热的影响很大—热流密度越高,对流换热越强。     

管内对流换热的场协同分析及换热强化

导出管内湍流换热Nu与局域时均参数的关系式,将对流换热的场协同理论扩展至湍流换热。分析了管内对流换热的特点,并根据场协同理论提出强化湍流换热的方法,发展了一种新型强化换热管一交叉椭圆管,既适合于层流换热强化也适合于湍流换热强化,其强化传热效果显著而流阻较小。     

微重力条件下熔-固相变对流与浮区对流的耦合

本文以空间材料生长为背景,用有限元法研究微重力条件下的浮区单晶生长过程,考虑了表面张力对流与浮区对流的耦合作用,计算了凝固界面形状,结果表明,晶体的生长速度是个重要的影响因素,生长时,单晶-熔体界面比多晶-熔体界面更倾斜;当生长速度超过临界值时,多晶-熔体界面附近的Marangoni涡会分裂出二次涡。 关键词：     

竖壁传热对方腔内Benard对流的影响

本文采用具有QUICK差分格式的SIMPLE算法对边界竖壁传热的方腔内空气Benard对流进行了数值计算,根据计算结果探讨了竖壁传热对Benard对流的影响.计算表明,在所考虑的几何和物理条件下,所有竖壁绝热时,腔内流体形成平行与短轴的多个涡卷;竖壁存在传热时,腔内流体形成平行于长轴的两个涡卷,并且平行于长轴竖壁的传热热流方向相反时,涡卷的旋转方向也相反;垂直于长轴蛏壁的传热对近壁附近的流动有一定影响.