颗粒与高频感应热等离子体流的迭代计算

用完全二维自洽模型计算等离子体的速度场与温度场，用颗粒轨迹模型计算颗粒的运动轨迹与加热历程，但在等离子体的连续、动量和能量控制方程中，增加一项由于颗粒直径、运动速度及能焓的变化而引起的源项，计及颗粒的运动与加热对等离子体的速度场与温度场的影响；在颗粒的运动轨迹与加热历程计算中，考虑流体阻力和热泳力对颗粒运动与加热的影响；并在等离子体的速度场与温度场和颗粒的运动与加热之间进行迭代计算。通过对铝颗粒与     

颗粒沉降运动的虚拟区域法直接数值模拟

文采用基于四边形网格的分布式拉格朗日乘子／虚拟区域方法(DLM／FD method)对二维方槽内775个圆形颗粒在流体中的沉降过程进行了直接数值模拟。得到了颗粒流沉降过程中流体和颗粒速度和涡量分布、流场压力分布等流动细节,展示了颗粒在沉降过程中由于相间的相互作用以及颗粒间的作用,使得颗粒流在流场内形成大小不一的旋流区,颗粒回旋着沉降,同时颗粒的尾涡影响附近颗粒的运动．本文的结果说明分布式拉格朗日乘子／虚拟区域方法对模拟存在很多颗粒的悬浮体流动是可行的。     

温度体动网格模型中控制参数的研究

采用调整导热系数和源项分布的方式,对温度体动网格模型所生成的网格品质进行控制.温度体动网格模型将运动边界的位移虚拟为求解域的温度边界条件,以流体能量方程或固体导热方程作为控制方程,通过选取不同的导热系数或源项分布可以得到适合不同求解问题需要的温度分布,并将求解得到的网格节点的温度作为其动态位移量.采用温度体动网格模型计算平动、三维旋转运动、柔性体运动等流固耦合计算中可能涉及的动网格算例.结果表明,在计算效率和生成网格的品质方面,与已有的动态网格生成方法相比,具有较大的优势,可以有效解决流固耦合数值模拟中存在的动态网格生成难题.     

中国散裂中子源多功能反射谱仪屏蔽设计

本文介绍了利用蒙特卡罗粒子输运程序MCNPX2.5.0进行中国散裂中子源多功能反射谱仪屏蔽设计的屏蔽需求、辐射源项、计算方法和设计结果等内容.在计算中考虑慢化器泄漏源项、中子导管损失源项等不同辐射源项,使用分步计算和源项角度偏移、源项能量偏移、几何分裂等多种减方差方法,在保证计算结果精度的同时提高计算速度.在谱仪束线传输段、第二中子开关、散射室等的屏蔽计算中,通过比较了不同条件下的所需屏蔽确定最终屏蔽设计,确保谱仪屏蔽外人员可到达区域的剂量低于安全限值2.5μSv/h.     

虚拟流体方法的显隐算法

基于算子分裂,把欧拉方程分裂成对流项和非对流项两部分,建立一种基于原始变量的二阶显隐算法.由通常的虚拟流体方法显式地预估流场,用隐式的压力修正对预估解进行修正.计算结果表明,这样可以有效增大时间步长,提高计算效率.     

界面不稳定性数值模拟中的虚拟流动方法

研究了流体界面不稳定性的一类数值模拟方法——虚拟流动方法(Ghost Fluid Method).在算法中直接针对多维问题设定虚拟区域的流动参数,在流体力学方程的计算中采用了非分裂型的高分辨SCB格式,最后利用该方法完成了R-M和R-T不稳定性问题的数值计算,得到了满意的计算结果.     

滴形颗粒在通道中沉降的格子Boltzmann-虚拟区域方法模拟

采用格子Boltzmann-虚拟区域方法对滴形颗粒在垂直通道中的沉降过程进行直接数值模拟.通过格子Boltzmann方法求解N-S方程,流体与固体之间的相互作用通过虚拟区域方法描述.研究雷诺数在10^-2到100范围内颗粒形状因子对其摩擦系数和阻力系数的影响.为便于比较,给出了圆形颗粒沉降的结果.结果发现,当雷诺数小于1的时候,颗粒的摩擦系数始终保持常数,而当雷诺数大于1时摩擦系数随雷诺数的增大而增大.此外,当雷诺数小于约30时圆形颗粒的摩擦系数和阻力系数均小于滴形颗粒,而当雷诺数大于30时情况正好相反.颗粒周围的压力分布证明了这一结论.     

颗粒与顺列管束磨损的数值模拟

采用直接数值模拟方法模拟了在管道中直径80μm的颗粒对顺列10×10管束的碰撞和磨损.圆管与流场流体之间的相互作用采用内嵌边界方法进行计算并得到流场结构.颗粒的运动采用了拉格朗日跟踪方法计算并与流场之间进行了双向耦合.计算分析了沿流向以及垂直于流向各排管束受到颗粒的碰撞产生的磨损量.计算结果表明沿流向第三排管束以后的各排管束应采取防磨措旌.     

颗粒相干发射源的2π和3π玻色-爱因斯坦关联

在假设由同一颗粒发射的π介子为相干发射的条件下, 给出了颗粒发射源模型的2π及3π关联函数, 发射源的混沌性随颗粒数目的增加而增大. 此外, 对颗粒相干发射源及部分相干高斯源的归一化纯3π关联进行比较, 发现当2π关联的混沌性参量小于0.8时, 两种发射源的归一化纯3π关联值在小Q₃区域有明显差别.     

纳米流体多相流动的多尺度模拟方法

针对纳米流体多相流动的微观特征,提出一种基于格子Boltzmann的多尺度耦合方法,在速度和悬浮纳米粒子分布变化比较剧烈的区域采用细网格多相模型,在其它区域视纳米流体为均匀混合的单相流体,使用粗网格单相模型.为保证不同尺度区域之间的物理信息(参数)的准确传递,运用质量和动量守恒原理,建立跨区域的边界耦合模型.几个算例表明,该方法既可以反映纳米流体流动的微观特征,又能提高计算效率,与单纯使用多相模型相比,节省大量时间.     

Conservative discretization of Coriolis force in a finite volume framework

In this article we are interested in the problem of numerical simulations for a shallow fluid flow in a rotating system. This problem is closely related to climate or meteorological simulations. Our purpose is to introduce a new finite volume technique which allows us to guarantee conservation of linear momentum in an inertial frame of reference. Furthermore, we show that this method introduces a new discrete Coriolis term which is based on the interface mass fluxes instead of on straightforward cell-centered evaluation of the source term. Some numerical tests exhibit that this approach significantly reduces the numerical diffusion and is particularly interesting when considering nonisotropic meshes or long time simulations.     

Transient coupled radiation-conduction in infinite semitransparent cylinders

A method is developed to analyze the transient coupled radiation-conduction in infinite semitransparent cylinders surrounded by isothermal black walls. The radiative heat source term is calculated by the radiative transfer coefficients and the transient energy equation is solved by an implicit finite difference method. The radiative transfer coefficients are deduced by use of the ray tracing method in combination with the Hottel and Sarofim zonal method. The effects of the related parameters on the transient radiative heat source and temperature distribution are analyzed. It is found that the peak of the dimensionless radial radiative heat source can be located at the interior shell of the cylinder with small optical thickness when heated by the surrounding irradiation. Treating the volume radiation as a surface radiation will result in large errors of transient temperature distribution for the cylinder with small optical thickness.     

A multi-moment finite volume formulation for shallow water equations on unstructured mesh

A novel and accurate finite volume method has been presented to solve the shallow water equations on unstructured grid in plane geometry. In addition to the volume integrated average (VIA moment) for each mesh cell, the point values (PV moment) defined on cell boundary are also treated as the model variables. The volume integrated average is updated via a finite volume formulation, and thus is numerically conserved, while the point value is computed by a point-wise Riemann solver. The cell-wise local interpolation reconstruction is built based on both the VIA and the PV moments, which results in a scheme of almost third order accuracy. Efforts have also been made to formulate the source term of the bottom topography in a way to balance the numerical flux function to satisfy the so-called C-property. The proposed numerical model is validated by numerical tests in comparison with other methods reported in the literature.     

Numerical solution of under-resolved detonations

A new fractional-step method is proposed for the numerical solution of high speed reacting flows, where the chemical time scales are often much smaller than the fluid dynamical time scales. When the problem is stiff, because of insufficient spatial/temporal resolution, a well-known spurious numerical phenomenon occurs in standard finite volume schemes: the incorrect calculation of the speed of propagation of discontinuities. The new method is first illustrated considering a one-dimensional scalar hyperbolic advection/reaction equation with stiff source term, which may be considered as a model problem to under-resolved detonations. During the reaction step, the proposed scheme replaces the cell average representation with a two-value reconstruction, which allows us to locate the discontinuity position inside the cell during the computation of the source term. This results in the correct propagation of discontinuities even in the stiff case. The method is proved to be second-order accurate for smooth solutions of scalar equations and is applied successfully to the solution of the one-dimensional reactive Euler equations for Chapman–Jouguet detonations.     

龙格库塔间断有限元方法在计算爆轰问题中的应用

构造求解带源项守恒律方程组的龙格库塔间断有限元(RKDG)方法,并分别结合源项的Strang分裂法和无分裂法数值求解模型守恒律方程和反应欧拉方程.为了和有限体积型WENO方法进行比较,设计计算源项的WENO重构格式.对一维带源项守恒律的计算表明,对于非刚性问题,RKDG方法比有限体积型WENO方法的误差更小;对于刚性问题,RKDG方法对于间断面位置的捕捉更为精确.对于一二维爆轰波问题的计算结果表明,RKDG方法对爆轰波结构的分辨和爆轰波位置的捕捉能力更强.     

II. Recirculation flow induced by a bubble stream rising in a viscous liquid

The recirculation flow induced by the rising motion of a bubble stream in a viscous fluid within an open-top rectangular enclosure is studied. The three-dimensional volume averaged conservation equations are solved by a control-volume method using a hybrid finite differencing scheme to describe the liquid phase hydrodynamics. The momentum exhange between the bubbles and the liquid phase is modeled with a source term equals to the volumetric buoyancy force acting on the gas in the bubble stream. The volumetric buoyancy force accounts for in line interactions between bubbles through the average gas volume fraction in the gas liquid column which depends on the size and the rising velocity of bubbles. The fluid flow within an open-top rectangular enclosure is further investigated by particle image velocimetry for a bubble stream rising in a water-glycerol solution. The measured fluid velocities in a vertical plane are compared with the predictions of the numerical model over a wide range of fluid viscosity (43 mPa s-800 mPa s) and gas flow rates. Finally, the recirculation flows resulting from the interaction of two neighbouring vertical bubble streams are studied. Received: 23 July 1997 / Revised: 19 December 1997 / Accepted: 11 May 1998     

三维浅海下弹性结构声辐射预报的有限元-抛物方程法

利用多物理场耦合有限元法对结构和流体适应性强、抛物方程声场计算高效准确的特点,提出了三维浅海波导下弹性结构声振特性研究的有限元-抛物方程法.该方法采用多物理场耦合有限元理论建立浅海下结构近场声辐射模型,计算局域波导下结构声振信息,并提取深度方向上复声压值作为抛物方程初始值;然后采用隐式差分法求解抛物方程以步进计算结构辐射声场.重点介绍了该方法对浅海下结构声辐射计算的准确性、高效性以及快速收敛性后,对Pekeris波导中有限长弹性圆柱壳的声振特性进行了分析.研究得出,当圆柱壳靠近海面(海底)时,其耦合频率比自由场下的要高(低),当潜深达到一定范围时,与自由场耦合频率基本趋于一致;在低频远场,结构辐射场与同强度点源声场具有一定的等效性,且等效距离随着频率增加而增加;由于辐射声场受结构振动模态、几何尺寸和简正波模式影响,结构辐射场传播的衰减规律按近场声影响区、球面波衰减区、介于球面波和柱面波衰减区、柱面波衰减区四个扩展区依次进行.     

A novel coupled level set and volume of fluid method for sharp interface capturing on 3D tetrahedral grids

We present a new three-dimensional hybrid level set (LS) and volume of fluid (VOF) method for free surface flow simulations on tetrahedral grids. At each time step, we evolve both the level set function and the volume fraction. The level set function is evolved by solving the level set advection equation using a second-order characteristic based finite volume method. The volume fraction advection is performed using a bounded compressive normalized variable diagram (NVD) based scheme. The interface is reconstructed based on both the level set and the volume fraction information. The novelty of the method lies in that we use an analytic method for finding the intercepts on tetrahedral grids, which makes interface reconstruction efficient and conserves volume of fluid exactly. Furthermore, the advection of volume fraction makes use of the NVD concept and switches between different high resolution differencing schemes to yield a bounded scalar field, and to preserve both smoothness and sharp definition of the interface. The method is coupled to a well validated finite volume based Navier–Stokes incompressible flow solver. The code validation shows that our method can be employed to resolve complex interface changes efficiently and accurately. In addition, the centroid and intercept data available as a by-product of the proposed interface reconstruction scheme can be used directly in near-interface sub-grid models in large eddy simulation.     

非结构网格上的B-B单方程模型湍流计算

研究如何在非结构网格上进行Navier Stokes(N-S)方程湍流计算.采用格心有限体积方法离散N-S方程.为了适应非结构网格,计算所用的湍流模型特别选用Baldwin Barth(B-B)单方程模型.此模型由一个单一的具有源项的对流扩散方程组成.为了能在非结构网格上求解B B单方程模型,提出一显式有限体积格式,并直接对带源项的格式进行稳定性分析,得到了相应的时间步长限制条件.最后以平板、RAE 2822翼型、多段翼型绕流等数值算例验证了计算方法的有效性.