One- and two-equation turbulence models for the prediction of complex cascade flows using unstructured grids

One- and two-equation, low-Reynolds eddy-viscosity turbulence models are employed in the context of a primitive variable, finite volume, Navier-Stokes solver for unstructured grids. Through the study of the complex flow in a controlled-diffusion compressor cascade at off-design conditions, the ability of the models under consideration to predict the laminar separation bubble close to the leading edge and the boundary layer development is investigated. In order to control the unphysical growth of turbulent kinetic energy near the leading edge stagnation point, appropriate modifications to the conventional models are employed and tested. All of them improve the leading edge flow patterns and significantly affect the size of the predicted laminar separation bubble. The use of an adequately refined mesh around the airfoil, that is formed by triangles placed in a quasi-structured way, allows for the generation of grid elements of moderate aspect ratios. This helps to readily overcome any relevant problems of accuracy; a second-order upwind scheme without flux limiters or least squares approximations is successfully employed for the gradients. The test case includes quasi-3D effects by considering the streamtube thickness variation in the governing equations.     

A front tracking method on unstructured grids

A numerical method is developed for tracking discontinuities which is integrated in a generalized finite-volume solution framework for systems of conservation laws on unstructured grids of arbitrary element type. The location, geometry and the movement of the discontinuities are described by a local level set method on a restricted, dynamic definition range. Special algorithms based on least square methods are developed for handling the transport and renormalization of the level set function within the restricted range. An additional error correction is employed to minimize topological errors of the tracked front geometry. The jump conditions at the front are updated by one-sided extrapolation which define the local front velocity and the Riemann problem. A flux separation concept enables the treatment of the discontinuity within the finite-volume concept. The front tracking method is demonstrated by a number of computational examples for shock wave problems.     

Computation of complex turbulent flow using matrix-free implicit dual time-stepping scheme and LRN turbulence model on unstructured grids

In this study, a matrix-free implicit dual time-stepping method has been developed. It is implemented, together with a low-Reynolds-number q-ω turbulence model, in a high-order upwind finite-volume solver on unstructured grids. Semi-implicit treatment of the source terms of the q and ω equations is also introduced to further stabilize the numerical solution. It has been found that these techniques provide strong stabilization in the computation of a supersonic flow with complex shock-boundary-layer interactions in a channel with a backward-facing step. The proposed method has a low-memory overhead, similar to an explicit scheme, while it shows good stability and computational efficiency as an implicit scheme. The method developed has been validated by comparing the computed results with the corresponding experimental measurements and other calculated results, which shows good agreement. Research is being done to extend the method to calculate unsteady turbulent flows.     

Parallel computation of unsteady three-dimensional incompressible viscous flow using an unstructured multigrid method

The development and validation of a parallel unstructured tetrahedral non-nested multigrid (MG) method for simulation of unsteady 3D incompressible viscous flow is presented. The Navier-Stokes solver is based on the artificial compressibility method (ACM) and a higher-order characteristics-based finite-volume scheme on unstructured MG. Unsteady flow is calculated with an implicit dual time stepping scheme. The parallelization of the solver is achieved by a MG domain decomposition approach (MG-DD), using the Single Program Multiple Data (SPMD) programming paradigm. The Message-Passing Interface (MPI) Library is used for communication of data and loop arrays are decomposed using the OpenMP standard. The parallel codes using single grid and MG are used to simulate steady and unsteady incompressible viscous flows for a 3D lid-driven cavity flow for validation and performance evaluation purposes. The speedups and efficiencies obtained by both the parallel single grid and MG solvers are reasonably good for all test cases, using up to 32 processors on the SGI Origin 3400. The parallel results obtained agree well with those of serial solvers and with numerical solutions obtained by other researchers, as well as experimental measurements.     

非结构几何下二阶自共轭中子输运方程中的简化球谐函数方法研究

针对球谐函数方法求解中子输运方程时计算量大的缺点,将简化球谐函数(SPN)方法用于离散二阶自共轭(SAAF)中子输运方程的角度变量,同时应用有限元方法在非结构几何下进行空间离散,研究了简化球谐函数方法在SAAF方程中的应用.数值计算结果表明,SPN方法在保证计算精度的条件下,具有比PN方法快的计算速度,而且对于维数越高、网格数越多、展开阶数越高的问题,节省时间的效果越好.     

Parallel generation of unstructured surface grids

In this paper, a new grid generation system is presented for the parallel generation of unstructured triangular surface grids. The object-oriented design and implementation of the system, the internal components and the parallel meshing process itself are described. Initially in a rasterisation stage, the geometry to be meshed is analysed and a smooth distribution of local element sizes in 3-D space is set up automatically and stored in a Cartesian mesh. This background mesh is used by the advancing front surface mesher as spacing definition for the triangle generation. Both the rasterisation and the meshing are MPI-parallelised. The underlying principles and strategies will be outlined together with the advantages and limitations of the approach. The paper will be concluded with examples demonstrating the capabilities of the presented approach.

Fully automatic and fast mesh size specification for unstructured mesh generation

A fully automatic surface mesh generation system is presented in this paper. The automation is achieved by an automatic determination of a consistent mesh size distribution, which is based on geometry rasterisation. The user specifies a minimal and maximal allowed mesh size, and a maximal allowed curvature angle for the complete geometry, or, rather, parts of it. Now, these local curvature and local characteristic lengths of the geometry are computed, which determine the local mesh size. These local mesh sizes are stored and smoothed in a Cartesian background mesh. Afterwards, the triangulation is generated by an advancing front triangulator: the local resolution of the surface triangulation is determined by the mesh sizes stored in the Cartesian background mesh. The object-oriented design and implementation is described. The complete system is very fast due to an efficient parallelisation based on MPI for computer systems with distributed memory.     

非结构化信息管理

随着信息的发展,出现了越来越多的非结构化信息。并且非结构化信息在政府和企业等的决策中占有重要的位置。如何将非结构化数据有效的管理起来,能够进行数据和知识挖掘,提取当中的隐含信息,对决策进行支持成为当今亟待解决的主要问题。本文介绍了几种非结构化信息管理方式,并对未来非结构化信息管理方式进行探讨。     

Reducing network traffic in unstructured P2P systems using Top-k queries

A major problem of unstructured P2P systems is their heavy network traffic. This is caused mainly by high numbers of query answers, many of which are irrelevant for users. One solution to this problem is to use Top-k queries whereby the user can specify a limited number (k) of the most relevant answers. In this paper, we present FD, a (Fully Distributed) framework for executing Top-k queries in unstructured P2P systems, with the objective of reducing network traffic. FD consists of a family of algorithms that are simple but effective. FD is completely distributed, does not depend on the existence of certain peers, and addresses the volatility of peers during query execution. We validated FD through implementation over a 64-node cluster and simulation using the BRITE topology generator and SimJava. Our performance evaluation shows that FD can achieve major performance gains in terms of communication and response time. Recommended by: Sunil Prabhakar Work partially funded by the ARA Massive Data of the Agence Nationale de la Recherche.     

High accuracy flow simulations: Advances and challenges for future needs in aeronautics

Computational Fluid Dynamics (CFD) methods, as practical aeronautical tools, is used routinely to complement the wind tunnel in the design of aircraft [1,2], and therefore plays a crucial role in the industrial context for strengthening the competitiveness of the Research and Development (R &; D) teams, by reducing developments costs and delay. The field of high-order methods has seen a surge of research activity over the last decade, particularly due to its potential in applications such as LES [30–34], DNS [35–37], Computational Aero-Acoustics (CAA) [38,39], turbulent combustion [40] and vortex dominated flows [41]. This paper presents the challenging issues of the high-order methods for the aeronautics community.