三维高超声速稀薄气体流场通量分裂式DSMC-IP方法研究

结合非结构网格对复杂外形精确描述的能力和IP（Information Preservation）方法降低统计耗散的特点,该文在非结构网格的基础上构建了改进型通量分裂式DSMC（Direct Simulation of Monte Carlo）-IP数值方法,对三维高超声速稀薄气体流场进行了数值模拟。采用渐变尺度的非结构网格划分流场区域,在满足DSMC方法对网格尺度需求的同时,对流场的结构特点进行精确描述;另外根据Van Leer格式的思想改进IP方法中的通量计算格式,解决了激波处的数值间断问题;并采用单元信息保存法对IP方法进行补充,提高了计算方法的稳定性。通过对三维圆球绕流和航天飞机头部绕流流场的数值模拟,验证了该文方法的有效性和可行性,并对模型中的气动特性和参数分布规律进行了分析。     

超弹性柔性结构与流体耦合运动的浸入边界法研究

浸入边界法是模拟大变形柔性弹性结构和粘性流体相互作用的重要数值方法之一。该文有效结合传统的反馈力方法和混合有限元浸入边界方法,对圆柱和方柱绕流后柔性悬臂梁流固耦合振动问题进行数值模拟。其中,固体采用超弹性材料,利用有限单元法求解,流体为不可压缩牛顿流体,使用笛卡尔自适应加密网格,利用有限差分法进行求解。通过数值计算,得到柔性超弹性结构的耦合振动特性和流场动态分布特性,并将计算结果同其他文献计算结果进行比较,验证了该耦合计算方法的可靠性。     

三维实体网格划分算法的研究

根据三维实体的几何特征和物理特征来确定网格划分的加密规则,设计基于Delaunay剖分的动态节点单元一体化三维网格自适应生成算法。通过对对象的加密区域、区域布点等前期处理,对选取的节点集进行Delaunay三角剖分,并对特征集中区域进行局部加密,与传统网格划分方法相比具有更精确性和高效性。将此算法应用于水泥熟料流动冷却工程模拟分析中,实验结果表明能准确描述出熟料流动过程的几何形体的特征和物理特征,实现网格的疏密分布。     

流固耦合问题的网格更新与信息传递新方法

研究流固耦合问题中的网格技术。针对流体域的网格移动,提出基于映射结构化网格的插值更新的新方法,采用映射插值函数计算流体网格节点位移并与初始网格坐标值叠加,以获取流体新的节点坐标。对二维正方形、梭形及三维立方体流场网格更新开展数值计算。计算表明,该方法可保持原网格的拓扑关系,且更新速度快,更新质量好。使用约束反力分配法和投影插值法分别传递流体域到结构域、结构域到流体域的信息,运用基于该方法的自编程序对典型形体的结构流固耦合界面进行信息传递计算模拟。通过对比传递前后结构与流体的作用,验证了基于该方法的数值传递效果理想。     

等螺距螺杆真空泵内气体流动的数值模拟研究

应用计算流体力学（CFD）方法对螺杆干式真空泵进行数值模拟计算,建立了针对螺杆干式真空泵流场的分析方法。对泵腔中流动区域进行三维建模,采用SCORG和ANSYS-ICEM软件对转动域和固定域分别进行了网格划分,利用瞬态模拟方法得到了泵内压力场、速度场和温度场的分布情况,并计算出抽气速率等参数。网格无关性分析结果与理论数据对比表明数值模拟结果可靠,符合预期。     

微型三角楔超声速绕流特性的研究

基于大涡模拟(LES)方法,结合WENO格式与自适应网格加密(AMR)技术及沉浸边界法(IBM),对来流马赫数为Ma =2.5条件下的平板上微型三角楔绕流流场进行了数值模拟。数值模拟表明微型三角楔涡流发生器可以显著改变超声速流体边界层结构。计算结果清晰地显示了三角楔上游分离区的流场结构和下游各涡的流态,同时计算表明,微楔对边界层控制过程中,其下游的流向涡对与涡环结构都起了重要作用,并对其作用过程进行了讨论。数值计算与相关实验结果相符,且提供了流场的重要细节,揭示了微楔的控制机理,可为超声速边界层控制研究提供重要支持。     

大规模四面体网格并行生成方法

针对大规模网格串行生成的时间和内存瓶颈问题,阐述了一种非结构化四面体网格并行生成方法,首先对几何体进行初始网格划分,并通过相对体积比及最优分区控制初始网格数量;然后采用图论图划分方法对初始网格进行区域分解;最后采用分裂法进行并行网格生成。提出的基于共享单元的边界判定方法,有效地解决了分区边界节点的匹配问题。通过算例证明了该并行网格生成方法具有良好的并行效率,同时可以产生高质量的四面体网格。     

基于映射理论的网格生成新方法及应用研究

基于映射函数理论提出二维/三维流场的网格生成新方法。针对实际应用中对场函数变密度的要求,提出可控制网格疏密度的插值公式,并以此对典型物体周围流场进行网格剖分,验证其有效性。针对圆形域(柱形域)流场网格生成质量的缺陷,提出在目标域中建立内置块的新策略,统计分析网格最小内角分布,并与原方法对比分析,以验证改进效果。运用自编程序,对具有复杂外形的空间膜结构进行流场网格剖分,并将该网格导入Ansys-CFX软件,进而数值模拟结构表面风压分布。经验证,该方法可有效地运用于流场网格剖分。     

弹簧近似法在二维非结构动网格生成技术中的应用

通过控制网格质量改进弹簧近似法,并将其结果与传统方法进行比较。数值模拟结果显示：基于弹簧近似法的非结构动网格随着模型边界旋转角度的增加网格质量逐渐下降;在发生相同旋转角度时,改进弹簧近似法生成网格的质量较传统方法有了较大的改善;传统弹簧近似法模型边界最大旋转角度为13°左右,改进方法可达到27°,较大地提高了网格的变形能力。最后,运用改进的弹簧近似法对二维翼型进行数值模拟,并与传统方法及实验数据进行比较,得到了较满意的结果,该方法能够较好地运用于实际数值模拟中。     

内埋武器发射参数对下落轨迹的影响

采用改进的延迟分离涡模拟方法结合动网格技术,对某一简化内埋弹舱-舱门-弹体模型的三维流场进行非定常计算,得到了不同来流马赫数、弹体发射速度和初始角速度等参数对弹体下落轨迹的影响规律。结果表明:亚声速条件下来流马赫数对弹体偏航角度影响较小,跨声速及超声速条件下受流场波系的影响,弹体均向右偏航。超声速条件下,随马赫数增加弹体偏航角和俯仰角的变化幅度减小。提升弹体初始发射速度可以使弹体快速达到安全分离距离,并减小姿态角变化。增加弹体初始低头角速度可以在不增加弹射力的情况下,减小弹体俯仰角度,提高弹体分离速度,有利于机弹安全分离。     

A structured/unstructured grid generation method and its application

Summary. A hybrid structured/unstructured grid is generated by using both the advanced layer method and the advanced front method. A domain decomposition method in this hybrid grid to simulate 2-D viscous flow is discussed. The Euler equations are solved on the unstructured grid in the flow field region far from the wall, and the RNS equations are solved on the structured grid in the flow field region near the wall. Two solutions are matched on the interface of these two regions. Based on this viscous flow solver the mono-objective/bi-objective optimization and the inverse design calculation are given. Numerical results show that calculated pressure distributions of given airfoils agree well with the corresponding experimental data and performances of optimal airfoils are much better than the initial airfoils. The agreement between designed and target pressure is fine in the inverse design calculation.     

An implicit finite-element method for high-speed flows

A fast algorithm is presented for constructing continuous lines, made up of element sides, which pass once through each node of a general unstructured triangular mesh and which are generally aligned in prescribed directions. The lines are used as the basis of an adaptive fully implicit unstructured grid procedure for the solution of two-dimensional problems of steady compressible inviscid and laminar viscous high-speed flows, where the equation system is solved by line relaxation using a block tridiagonal equation solver. For three-dimensional laminar viscous simulations it is proposed to utilize an implicit/explicit finite-element formulation. In the vicinity of solid walls a grid exhibiting structure in the normal direction is employed while, away from this region, the grid will be totally unstructured. In the structured region, lines in the normal direction to the wall are readily identified, while lines in the surfaces parallel to the solid wall are constructed using the proposed two-dimensional procedure. The implicit algorithm is then used in the structured region and the equation solution is achieved via line relaxation. An explicit form of the solution algorithm is used elsewhere. To illustrate the performance of the proposed method, solutions are obtained for both transonic inviscid and transonic and hypersonic laminar viscous problems in two dimensions. The application of the proposed procedure to the solution of three-dimensional hypersonic laminar viscous flow over a double ellipsoid configuration is also described.     

Application of the method of splitting by physical processes for the computation of a hypersonic flow over an aircraft model of complex configuration

The capabilities of the computing complex developed at the Institute for Problems in Mechanics of the Russian Academy of Sciences are studied by the example of an external hypersonic flow over a model of an unmanned experimental aircraft X-43 of complex configuration. The program complex includes a generator of the unstructured surface and volumetric computational meshes and a series of computer codes that implement the integration of the closed system of Euler and Navier-Stokes equations using the method of splitting by physical processes on an unstructured mesh. The results of application of the developed numerical method on the generated unstructured computational meshes for three-dimensional modeling of the aerothermodynamics of the complex hypersonic aircraft X-43 are presented. The flow field and heat transfer processes in the entire region of the flow from the bow wave to the far wake are investigated. The configuration of the shock waves generated in the flow over a hypersonic flying vehicle (HFV) is studied. The effect of the angle of attack and free stream on the flow flown and the aerodynamic and thermal characteristics of the HFV surface is studied. The integral aerodynamic characteristics of X-43, lift coefficients C _L , and drag coefficients C _D are calculated for each regime of the flow. The HFV lift-drag ratio K dependences on the Mach number and angle of attack are found. The data of the X-43 flight and wind-tunnel tests are compared with the results of numerical modeling.     

An adaptive mesh refinement technique for two-dimensional shear band problems

We have developed an adaptive mesh refinement technique that rezones the given domain for a fixed number of quadrilateral elements such that fine elements are generated within the severely deformed region and coarse elements elsewhere. Loosely speaking, the area of an element is inversely proportional to the value of the deformation measure at its centroid. Here we use the temperature rise at a material point to gauge its deformations which is reasonable for the shear band problem since the material within the shear band is deformed intensely and is heated up significantly. It is shown that the proposed mesh refinement technique is independent of the initial starting mesh, and that the use of an adaptively refined mesh gives thinner shear bands, and shaper temperature rise and the growth of the second invariant of the plastic strain-rate within the band as compared to that for a fixed mesh having the same number of nodes. The method works well even when the deformation localizes into more than one narrow region.     

气流分级机流场的数值模拟与优化

运用计算流体力学数值模拟的方法,研究气流分级机的内部气流流场情况,并对于分级机的工作参数进行优化。通过采用滑移网格技术模拟分级轮在气流分级机内的旋转,用结构网格与非结构网格结合的方法对气流分级机进行网格划分,主要研究了分级轮不同叶片数(4、6、12片),不同转速(500、1000、2000、3000r/min)下分级机内部流场情况并给出了相关解释。通过对不同参数的比较给出了优化的参数。模拟结果表明,分级轮转速越高、叶片数越多,分级效果越好。     

An adaptive finite element method for transient compressible flows

An adaptive finite element method for the solution of time dependent strongly compressible flows in two dimensions is described. The computational domain is represented by an unstructured assembly of linear triangular elements and the mesh adaptation is achieved by local regeneration of the grid, using an error estimation procedure coupled to an automatic triangular mesh generator. Problems involving shock propagation are solved to illustrate the numerical performance of the proposed approach.     

An efficient parallel computation of unsteady incompressible viscous flow with elastic moving and compliant boundaries on unstructured grids

This paper presents the development and validation of a parallel unstructured‐grid fluid–structure interaction (FSI) solver for the simulation of unsteady incompressible viscous flow with long elastic moving and compliant boundaries. The Navier–Stokes solver on unstructured moving grid using the arbitrary Lagrangian Eulerian formulation is based on the artificial compressibility approach and a high‐order characteristics‐based finite‐volume scheme. Both unsteady flow and FSI are calculated with a matrix‐free implicit dual time‐stepping scheme. A membrane model has been formulated to study fluid flow in a channel with an elastic membrane wall and their interactions. This model can be employed to calculate arbitrary wall movement and variable tension along the membrane, together with a dynamic mesh method for large deformation of the flow field. The parallelization of the fluid–structure solver is achieved using the single program multiple data programming paradigm and message passing interface for communication of data. The parallel solver is used to simulate fluid flow in a two‐dimensional channel with and without moving membrane for validation and performance evaluation purposes. The speedups and parallel efficiencies obtained by this method are excellent, using up to 16 processors on a SGI Origin 2000 parallel computer. A maximum speedup of 23.14 could be achieved on 16 processors taking advantage of an improved handling of the membrane solver. The parallel results obtained are compared with those using serial code and they are found to be identical. Copyright © 2005 John Wiley & Sons, Ltd.     

Local Grid Refinement in Large-Eddy Simulation

A method for performing nested-grid calculations with a Large-Eddy Simulation (LES) code is described. The grid consists of a coarse mesh and a fine mesh which overlaps the coarse in some region. A standard finite-volume method is used on both meshes. By means of grid communication, the velocity and pressure at both meshes are matched. To check that large eddies which are already resolved on the coarse grid are not affected by the nesting procedure, a simple two-dimensional mixing layer is simulated. Several simulations of this flow have been carried out with a different number of grid points on the nested grid. It is found that, without much extra computational effort, the grid-nesting improves the turbulent statistics with respect to the results found on the coarse mesh. This improvement occurs first of all in the region where grid refinement is applied, but better results are also found on the coarse mesh outside the grid-refinement region. Furthermore, it is shown that the large-scale structures in this flow are not influenced by the boundary between the coarse and fine grid.     

An adaptive solution of the 3-D Euler equations on an unstructured grid

Summary A new algorithm to generate the unstructured grid on a curved surface is developed. The advancing front method is used to generate the tetrahedral meshes in the space. An adaptive grid technique is used to enhance the calculation efficiency. The AUSM⁺ (Advection Upstream Splitting Method) scheme which was developed on a structured grid has been extended to be used to the spatial discretization of a cell-centered finite volume formulation on the unstructured grid. A second order spatial accuracy is achieved by applying a novel cell reconstruction procedure which can prevent the solution from exhibiting spurious oscillations without adding a limiter. A 3-D Euler solver for an adaptive tetrahedral grid and numerical results for several cases are presented.