含化学反应膛口流场的无网格数值模拟

基于无网格方法,对包含大位移运动边界和非平衡化学反应的膛口流场进行了数值模拟。所发展算法是基于线性基函数最小二乘显式无网格方法,忽略黏性及湍流的影响,对流场采用ALE(arbitrary Lagrangian-Eulerian)形式的Euler方程描述,对流通量和化学反应源项采用多组分HLLC(Harten-Lax-van Leer-Contact)格式和有限速率反应模型计算,对于运动边界造成的点云畸形采用局部点云重构方法处理,重构过程中采用虚拟边阵面推进。对圆柱绕流和激波诱导燃烧流场进行了数值模拟,验证了重构方法和化学反应计算的有效性。最后对12.7 mm口径机枪膛口流场进行了模拟,结果同实验照片、非结构网格方法结果吻合较好,数值结果清晰地再现了膛口初始冲击波、膛口冲击波、欠膨胀射流波系结构的动力学发展过程,以及膛口焰的时间、空间分布特征。     

基于ALE方程的动网格膛口流场数值研究

基于ALE(Arbitrary Lagrangian-Eulerian Equation)方程的有限体积法,采用高精度Roe方法及结构化动网格,利用嵌入网格技术及动边界条件,对弹丸由膛内高压气体推动射出到完全飞高初始流场的整个过程进行了数值模拟.根据数值结果绘制了膛口流场密度和压力分布的时序图,形象地再现了膛口流场中初始激波、弓形激波及膛口冲击波,以及接触闻断、漩涡和剪切层等的动力学发展过程.结果基本反映了膛口流场的典型变化特征,为进一步研究真实膛口流场(如带化学反应、湍流等)奠定了基础.     

膛口反应流并行数值模拟

采用轴对称多组分N-S方程对含有高速运动弹丸的膛口反应流进行了数值模拟.控制方程采用时间分裂方法并在大型计算机上采用MPI方法进行多核并行求解,其中对流项采用二阶AUSM+格式和MUSCL插值方法进行处理,燃气采用氢气-空气混合气,反应机理为9组分19步基元反应.对于弹丸引起的网格运动,采用嵌套网格法处理.并行验证算例与串行计算结果一致,采用20个CPU计算时效率为64％.根据数值结果详细讨论了发射过程中的气体动力学和化学动力学过程,并且通过对两种条件下的计算结果比较分析了化学反应对膛口流场发展的影响.结果表明,上述算法能够较为正确地模拟弹丸和化学反应对膛口流场的影响,并大大提高了计算速度.     

基于非结构化同位网格的SIMPLE算法

通过基于非结构化网格的有限体积法对二维稳态Navier—Stokes方程进行了数值求解。其中对流项采用延迟修正的二阶格式进行离散；扩散项的离散采用二阶中心差分格式；对于压力-速度耦合利用SIMPLE算法进行处理；计算节点的布置采用同位网格技术，界面流速通过动量插值确定。本文对方腔驱动流、倾斜腔驱动流和圆柱外部绕流问题进行了计算，讨论了非结构化同位网格有限体积法在实现SIMPLE算法时，迭代次数与欠松弛系数的关系、不同网格情况的收敛性、同结构化网格的对比以及流场尾迹结构。通过和以往结果比较可知，本文的方法是准确和可信的。     

用高阶位移模式分析复合材料层合板的自由振动

建立一种新的高阶位移模式：分层假设复合材料层合板的位移场，利用各层间应力及位移的连续性条件，导出了作为整体的层合板控制微分方程。最后，采用这一高阶剪切理论来分析复合材料矩形层合板的自由振动。数值计算采用有限差分法，并编制了计算程序，计算得出了六种不同边界条件、不同铺层、不同宽厚比工况下的矩形层合板的自振频率。同经典理论解进行比较，可以看出本文方法简便易行，且精度较高，可以在微机上实用。     

膛口流场的数值模拟

本文使用严格二维的FCT算法数值模拟美国M16 5.36毫米自动步枪的非定常的轴对称膛口流场。用此方法不仅可得到膛口冲击波的发展过程,而且可得到不少数值方法不易得到的马赫盘的发展过程。本文计算结果与Schmidt的实验结果作了比较,二者吻合较好。这一结果为进一步开展更复杂(考虑弹丸、膛口制退器等)的膛口流场的数值模拟奠定了基础。     

基于动态面搭接技术的直升机旋翼流场分析

为了进一步提高直升机旋翼流场计算的精度与效率,本文根据直升机旋翼的运动特点,发展了一种基于结构化网格动态面搭接技术及低速预处理技术的直升机旋翼流场的非定常数值分析方法。采用该方法分别对Caradonna-Tung、UH-60A旋翼模型的悬停流场及NASA某旋翼无升力前飞流场进行了数值分析;将计算所得的旋翼表面压力分布与风洞试验结果进行了对比,对旋翼的尾涡结构进行了相关分析。分析结果表明:本文所发展的方法具有较高的精度且计算速度快,并能够比较准确地捕捉直升机旋翼的尾涡结构,可以用于直升机旋翼流场的分析研究,从而为开展考虑挥舞及变距运动的直升机旋翼前飞状态非定常气动特性的研究奠定了基础。     

三维裂纹应力强度因子计算综述——Ⅱ、数值计算法

对于三维裂纹应力强度因子的计算,在第一部分中曾介绍了一些分析解法,但这类方法由于数学上的困难还存在较大的局限性。为了解决上述矛盾,近年来,近似的数值计算方法有了相当大的发展。本文将对近十年来国际上在三维裂纹问题方面所发展的各种数值解法作一简要的综述。在三维裂纹问题的计算中,目前比较行之有效的数值解法有边界积分方程法[3—7]、有限单元法[8—13]、有限差分法[14—16]、直线分析法[17—19]以及边界积分方程和有限单元的混合应用法[20—21]等。总的来说这些方法,尽管离散化      

不同边界条件正交各向异性中厚板的振动与稳定分析

本文采用高阶剪切变形理论对正交各向异性中厚矩形板进行振动与稳定分析,数值计算采用样条有限点法,得出了六种不同边界条件矩形板的自振频率和屈曲载荷,并与相应的经典板理论的结果进行比较.结果说明横向剪切变形对复合材料层合板的影响与板的各向异性程度、板的宽厚比(b/h)、层合板的层数和板的支承条件有关,它随着层合板各向异性程度的增加而增加,随着层合板宽厚比的增加而逐渐消失.     

剪切流作用下层合梁非线性振动特性研究

针对剪切流中层合梁的大变形非线性振动问题, 采用高阶剪切变形锯齿理论和冯·卡门应变描述层合梁的变形模式和几何非线性效应, 构建了大变形层合梁非线性振动有限元数值模型; 采用基于任意拉格朗日?欧拉方法的有限体积法求解不可压缩黏性流体纳维-斯托克斯方程, 结合层合梁和流体的耦合界面条件建立了剪切流作用下层合梁流固耦合非线性动力学数值模型, 采用分区并行强耦合方法对层合梁的流致非线性振动响应进行了迭代计算. 研究了不同速度分布的剪切流作用下单层梁和多层复合材料梁的振动响应特性, 并验证了本文数值建模方法的有效性. 结果表明: 剪切流作用下单层梁的振动特性与均匀流作用下的情况不同, 梁的运动轨迹受剪切流影响向下偏斜, 随着速度分布系数增加, 尾部流场中的涡结构发生改变; 刚度比对剪切流作用下层合梁的振动特性有显著影响, 随着刚度比的增加, 层合梁振动的振幅增大, 主导频率下降, 运动轨迹由‘8’字形逐渐变得不对称; 发现了不同厚度比和铺层角度情况下, 层合梁存在定点稳定模式、周期极限环振动模式和非周期振动模式三种不同的振动模式, 改变层合梁铺层角度可实现层合梁周期极限环振动模式向非周期振动模式转变.      

Numerical solution of the turbulent flow of an incompressible fluid in curved planar and axially symmetrical channels

The governing equations for axially symmetric flow, where the Reynolds stresses are expressed by scalar turbulent viscosity, are the Reynolds equations. The turbulence model k, ? is used in the well-known form for fully developed turbulent flow.The numerical method, a continuation of the MAC system¹, is adapted so that even for high Reynolds cell numbers precision (δx²) can be achieved for the steady flow. Irregular cells join the rectangular network on the curved surface. Von Neumann's stability condition of the linearised numerical system is investigated. Special problems concerning the numerical solution of the turbulence model equations are stated and a special procedure is worked out to ensure that the fields k, ? do not converge to physically meaningless values. The program for the computer is universal in that the boundary problems can be assigned by input data.As an example, an axially symmetrical diffuser with an area ratio of widening 1.40 is computed. Fields of velocity and pressure at the wall as well as fields v_T and k are assessed. The results are compared with an experiment. The conclusion is that this method is suitable for the problems mentioned in this study as well as for unsteady flow.     

Downstream evolution of junction flow three-component velocity fluctuations through the lens of the diagnostic plot

Experimental measurements of the streamwise and transverse velocity components have been acquired in three spanwise/wall-normal planes in the wakes of both a streamlined ‘wing’ and a bluff ‘wing’ junction. The ‘extended’ diagnostic plot introduced by Alfredsson et al. (2011) (see figure 3 therein) is used as a benchmark to locally evaluate the departure of turbulent wing-body junction flow wakes from ‘equilibrium’ boundary layers. Both obstacles produce a secondary flow of Prandtl’s first kind, which disrupts the equilibrium implied by the universality of the extended diagnostic plot. The plane wake of the obstacle itself (away from the junction) also disrupts this equilibrium. It is found that with downstream development the boundary layer eventually recovers to the base zero-pressure-gradient ‘equilibrium’, and that this recovery process emanates from the near-wall region. The transverse velocity components are also examined in “extended diagnostic” form, revealing that the wall-normal fluctuations recover to the zero-pressure-gradient case near the wall more rapidly than the wall-parallel components.     

Improving the accuracy of the capacitance method for void fraction measurement

Void fraction measurements were made using capacitance method. Five capacitor configurations were manufactured and tested; parallel, strip-type plates, ring-type plates, unidirectional, and double-helix. The void fraction was simulated by nonflow air-paraffin wax, air-glass, air-wood, and air-Freon 113 systems. The relative statistical error in void fraction measurement was minimized by taking into account the spacing between the ends of the two electrodes.     

Purely irrotational theories for the viscous effects on the oscillations of drops and bubbles

In this paper, we apply two purely irrotational theories of the motion of a viscous fluid, namely, viscous potential flow (VPF) and the dissipation method to the problem of the decay of waves on the surface of a sphere. We treat the problem of the decay of small disturbances on a viscous drop surrounded by gas of negligible density and viscosity and a bubble immersed in a viscous liquid. The instantaneous velocity field in the viscous liquid is assumed to be irrotational. In VPF, viscosity enters the problem through the viscous normal stress at the free surface. In the dissipation method, viscosity appears in the dissipation integral included in the mechanical energy equation. Comparisons of the eigenvalues from VPF and the dissipation approximation with those from the exact solution of the linearized governing equations are presented. The results show that the viscous irrotational theories exhibit most of the features of the wave dynamics described by the exact solution. In particular, VPF and DM give rise to a viscous correction for the frequency that determines the crossover from oscillatory to monotonically decaying waves. Good to reasonable quantitative agreement with the exact solution is also shown for certain ranges of modes and dimensionless viscosity: For large viscosity and short waves, VPF is a very good approximation to the exact solution. For ‘small’ viscosity and long waves, the dissipation method furnishes the best approximation.     

Sidewall flow stability in the MHD channel flows

The velocity distribution between two sidewalls is M-shaped for the MHD channel, flows with rectangular cross section and thin conducting walls in a strong transverse magnetic field. Assume that the dimensionless numbersR _m ?1,M, N? 1, and σ* and that the distance between two perpendicular walls is very long in comparison with the distance between two sidewalls. First, the equation for steady flow is established, and the solution of M-shaped velocity distribution is given. Then, an equation for stability of small disturbances is derived based on the velocity distribution obtained. Finally, it is proved that the stability equation for sidewall flow can be transformed into the famous Orr-Sommerfeld equation, in addition, the following theorems are also proved, namely, the analogy theorem, the generalized Rayleigh's theorem, the generalized Fjørtoft's theorem and the generalized Joseph's theorems.     

Study of the Effect of the Depth of the Cavity of a Braking Device on the Aerodynamic Drag of a Model in Transonic and Supersonic Flow

The flowfield and the aerodynamic drag of a model consisting of a pair of bodies (leading body a cylinder and trailing body a hollow cylinder) connected by a cylindrical bar along the axis of symmetry is experimentally investigated at Mach numbers ranging from 0.6 to 1.7. In the course of the experiments, the trailing body cavity depth and the connecting bar length were varied.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, 2005, pp. 186–192.Original Russian Text Copyright © 2005 by Pilyugin and Khlebnikov.     

On the self-induced motion of vortex sheets

An accurate numerical scheme has been devised to study the self-induced motion of an infinitely thin, free vortex sheet of finite span in an unbounded, inviscid, incompressible fluid. The new numerical scheme has been tested against two vortex sheet problems for which exact solutions have also been obtained. The agreement between the numerical and exact solutions is excellent. The scheme has been further tested against two more examples for which analytical solutions for small times were available. Here too the agreement is excellent.     

Numerical predictions of the bifurcation of confined swirling flows

The bifurcation of confined swirling flows was numerically investigated by employing both the k-? and algebraic stress turbulence models. Depending upon the branch solution examined, dual flow patterns were predicted at certain swirl levels. In the lower-branch solution which is obtained by gradually increasing the swirl level from a low-swirl flow, the flow changes with increasing swirl number from the low-swirl flow pattern to a high-swirl flow pattern. In the upper-branch solution which is acquired by gradually decreasing the swirl level from a high-swirl flow, on the other hand, the flow can maintain itself in the high-swirl flow pattern at the swirl levels where it exhibits the low-swirl flow pattern in the lower branch. The bifurcation of confined swirling flows was predicted with either the k-? model or the algebraic stress model being employed. Both the k-? and algebraic stress models result in comparable and sufficiently good predictions for confined swirling flows if high-order numerical schemes are used. The reported poor performance of the k-? model was clarified to be mainly attributable to the occurrence of the bifurcation and the use of low-order numerical schemes.     

空间模式下可压缩气固两相混合层流动特性研究

采用Euler-Lagrange颗粒-轨道双向耦合模型对空间模式下含有固粒的二维可压缩混合层流场进行了研究。气相流场采用具有空间三阶精度的WNND格式进行数值模拟,固相方程采用单边三点差分离散。在考虑流场对固粒作用的同时,也计及了固粒对流场的反作用。在对流马赫数为0.5时,研究了颗粒相对密度、颗粒尺寸、Stokes数等因素对粒子运动和流场结构的影响。研究结果表明:在可压缩空间模式混合层中,固粒的Stokes数仍然是主要影响参数之一;相同Stokes数下不同密度的固粒对流场的干扰不同,轻固粒对流场的干扰明显要小。