Review of Computational Aeroacoustics Algorithms

This paper presents a review of recent advancements in computational methodology for aeroacoustics problems. High-order finite difference methods for computation of linear and nonlinear acoustic waves are the primary focus of the review. Schemes for numerical simulation of linear waves include explicit optimized and DRP finite-difference operators, compact schemes, wavenumber extended upwind schemes and leapfrog-like algorithms. Both spatial approximations and time-integration techniques, which include low-dissipation low-dispersion Adams-Bashforth and Runge-Kutta (RK) methods, are examined. Wave propagation properties are analysed in the wavenumber and frequency space. Different approaches to eliminate short-wave spurious numerical waves are also reviewed. Methods for simulating nonlinear acoustic phenomena include essentially non-oscillatory (ENO) schemes, numerical adaptive filtering for high-order explicit and compact finite-difference operators, MacCormack and adaptive compact nonlinear algorithms. A literature survey of other CAA methods is provided in the introductory part.     

Radiation and Wall Boundary Conditions for Computational Aeroacoustics: A Review

A review of unsteady computational boundary conditions for computational aeroacoustics (CAA) problems is presented. This review is meant to serve as a general overview of previous work on solid wall, radiation and outflow boundary conditions that have been proposed and used in CAA calculations. Both the physical nature of the boundary condition problem as well as the numerical considerations affecting their implementation are discussed.     

Parallel computing of discrete element method on multi-core processors

This paper describes parallel simulation techniques for the discrete element method (DEM) on multi-core processors. Recently, multi-core CPU and GPU processors have attracted much attention in accelerating computer simulations in various fields. We propose a new algorithm for multi-thread parallel computation of DEM, which makes effective use of the available memory and accelerates the computation. This study shows that memory usage is drastically reduced by using this algorithm. To show the practical use of DEM in industry, a large-scale powder system is simulated with a complicated drive unit. We compared the performance of the simulation between the latest GPU and CPU processors with optimized programs for each processor. The results show that the difference in performance is not substantial when using either GPUs or CPUs with a multi-thread parallel algorithm. In addition, DEM algorithm is shown to have high scalability in a multi-thread parallel computation on a CPU.     

Parallel Implementation of DSMC Using Unstructured Mesh

The parallel implementation of the Direct Simulation Monte Carlo (DSMC) method on memory-distributed machines using unstructured mesh is reported. Physical domain decomposition is used to distribute the workload among multiple processors. A high-speed driven cavity flow is used as the benchmark problem for the validation of the parallel implementation. Three static partitioning techniques including simple coordinate partitioning, two-step partitioning (JOSTLE) and multi-level partitioning (METIS) are used for static domain decomposition, respectively. A cell renumbering technique is used to improve the memory management efficiency. Results of parallel efficiency show that two-step partitioning using JOSTLE performs the best, with 63% up to 25 processors, due to better load balancing among the processors. The powerful computational capability of the parallel implementation is demonstrated by computing a 2-D, near-continuum, hypersonic flow over a cylinder as well as a 3-D hypersonic flow over a sphere, respectively, both using 25 processors. Results compare reasonably well with previous simulated and experimental studies.     

Comparison of two numerical methods for the solution of non-Newtonian flow in ducts

Two numerical methods, the Galerkin finite element method (FEM) and the boundary-fitted co-ordinate transformation method (BFCTM), have been applied to solve inelastic non-Newtonian fluid flow in ducts of irregular cross-section. Three representative fluid models, namely the power-law, the Ellis and the Bingham models, have been analysed. The application of the FEM is straightforward, while for the BFCTM the accurate estimation of viscosity on the duct boundary and the proper mesh adjustment appear to be critical for generating convergent solutions. A detailed comparison of the two numerical methods in terms of volumetric flow rate, axial velocity, shear rate, viscosity and CPU time is given. Both methods can generate accurate solutions of velocity over a wide range of variables, but the FEM requires much less computing time to reach the same level of accuracy. Only the BFCTM can be used to approximate shear rate and viscosity with reasonable accuracy.     

Simulation of oscillatory flow in an aortic bifurcation using FVM and FEM: A comparative study of implementation strategies

Cardiovascular diseases are one of the major causes of long‐term morbidity and mortality in human beings. The nearly epidemic increase in prevalence of such diseases poses a serious threat to public health and calls for efficient methods of diagnosis and treatment. Non‐invasive diagnostic procedures such as MRI are often used in this context; however, these are limited in terms of spatial and temporal resolution and do not provide information on time‐dependent pressures and wall shear stresses—key quantities considered to be partially responsible for the formation and development of related pathologies. The present study is concerned with the numerical simulation of oscillatory flow through the abdominal aortic bifurcation. Computational fluid dynamics simulation of oscillatory flow in a branched geometry at high Reynolds numbers poses considerable challenges. The present study reports a detailed comparison of simulations performed with a finite volume and a finite element method, two approaches with significant differences in their discretization strategy, treatment of boundary conditions and other numerical aspects. Both solvers were parallelized, using loop parallelization of the BiCGStab linear solver for the finite volume and domain decomposition based on the Schur complement method for the finite element technique. The experience gained with these two approaches for the solution of flow in a bifurcation forms the focus of this study. Although similar results were obtained for both methods, the computation time required for convergence was found to be significantly smaller for the finite element approach. Copyright © 2010 John Wiley & Sons, Ltd.     

A fire field model implemented in a parallel computing environment

This paper describes the implementation of a fire field model in the parallel computing environment offered by multiple transputers. The fire model is built into the general purpose SIMPLE-based CFD code HARWELL-FLOW3D. The technique of domain decomposition has been applied tb convert the conventional serial version of FLOW3D into a code capable of efficiently utilizing an arbitrary number of transputers. Fire simulations consisting of up to 24 000 computational cells are performed on parallel systems with up to 15-processors. The run time for this simulation has been reduced from over 4 days on a single processor to just over 8 h on the 15-processor system. An interactive graphics system has also been developed which runs in parallel with the main computations.     

A Parallel 3D Unsteady Flow Analysis in an Asymmetrically Constricted Vessel

In this study, parallel computation of unsteady incompressible flow in an asymmetrically constricted 3D vessel has been presented. A time accurate cell centered finite volume method (FVM) in conjunction with pseudo-compressibility technique and Roe's flux difference splitting of nonlinear terms has been employed for solving the Navier-Stokes (NS) equations on the multiple instruction multiple data (MIMD) machine VPP700. The influence of Reynolds' number ( Re ) and the Strouhal number ( St ) on flow dynamic factors like wall pressure (WP), wall shear stress (WSS), central axis velocity (CAV), etc., have been analyzed. Three-dimensional (3D) features in the formation and detachment of separation zones, which are sensitive to both Re and St have been noticed on the diverging wall of the constriction.     

A comparison of coarse and fine grain parallelization strategies for the simple pressure correction algorithm

The primary aim of this work was to determine the simplest and most effective parallelization strategy for control-volume-based codes solving industrial problems. It has been found that for certain classes of problems, the coarse-grain functional decomposition strategy, largely ignored due to its limited scaling capability, offers the potential for significant execution speed-ups while maintaining the inherent structure of traditional serial algorithms. Functional decomposition requires only minor modification of the existing serial code to implement and, hence, code portability across both concurrent and serial computers is maintained. Fine-grain parallelization strategies at the ‘DO loop’ level are also easy to implement and largely preserve code portability. Both coarse-grain functional decomposition and fine-grain loop-level parallelization strategies for the SIMPLE pressure correction algorithm are demonstrated on a Silicon Graphics 4D280S eight CPU shared memory computer system for a highly coupled, transient two-dimensional simulation involving melting of a metal in the presence of thermal-buoyancy-driven laminar convection. Problems requiring the solution of a larger number of transport equations were simulated by including further scalar variables in the calculation. While resulting in slight degradation of the convergence rate, the functional decomposition strategy exhibited higher parallel efficiencies and yielded greater speed-ups relative to the original serial code. Initially, this strategy showed a significant degradation in convergence rate due to an inconsistency in the parallel solution of the pressure correction equation. After correcting for this inconsistency, the maximum speed-up for 16 dependent variables was a factor of 5·28 with eight processors, representing a parallel efficiency of 67%. Peak efficiency of 76% was achieved using five processors to solve for 10 dependent variables.     

云环境下的大规模线性有限元并行实现

针对Hadoop MapReduce框架实现迭代算法效率不高的问题,提出了基于Spark RDDs(Resilient Distributed Datasets)的大规模线性有限元并行算法,探索在云平台上有效地实现迭代算法。在Hadoop+Spark实验室集群上,通过空间桁架进行算例验证,并与基于Hadoop MapReduce的线性有限元并行算法进行性能比较。结果表明,在本文搭建的集群上,基于RDDs的并行算法能求解15000000个自由度的空间桁架问题,远大于Hadoop平台上的3000000个自由度;对于小模型,Spark可获得200倍以上的加速比,对于大模型,获得7~8倍加速比。     

A parallel fast multipole BEM and its applications to large-scale analysis of 3-D fiber-reinforced composites

In this paper, an adaptive boundary element method (BEM) is presented for solving 3-D elasticity problems. The numerical scheme is accelerated by the new version of fast multipole method (FMM) and parallelized on distributed memory architectures. The resulting solver is applied to the study of representative volume element (RVE) for short fiber-reinforced composites with complex inclusion geometry. Numerical examples performed on a 32-processor cluster show that the proposed method is both accurate and efficient, and can solve problems of large size that are challenging to existing state-of-the-art domain methods.The project supported by the National Natural Science Foundation of China (10472051).The English text was polished by Yunming Chen.     

A New Parameter Design Method of a 6-DOF Parallel Motion Simulator for a Given Workspace

This paper introduces a new 9-DOF motion simulator that consists of a 3-DOF parallel manipulator and a 6-DOF parallel manipulator. For the 6-DOF manipulator, a new ‘3–3’-PSS parallel mechanism and a new parameter design method for a given workspace are presented. With the kinematic study of this parallel mechanism and the method of Lagrange multipliers, we have found several key points that represent the worst performance of the manipulator within the given workspace. When a position workspace is given, by checking the manipulator's performances at these key points, one can quickly find out whether the manipulator with certain parameters meets the requirements within this workspace. Furthermore, several figures that can find the appropriate parameters are plotted. This key point method can make the design of parameters much quicker and easier for manipulators.     

计算力学非线性分析的现状与展望

本文对计算力学非线性分析近年来的发展做了简单回顾。文中说明了非线性数值分析日益受到重视并且还需要走很长的路,进而介绍了近廿年来非线性问题算法的两个最要方面:延续算法和单纯形算法。文末对数值方法未来十年的可能发展做了展望。     

A multiresolution continuum simulation of the ductile fracture process

With the advancement in computational science that is stepping into the Exascale era and experimental techniques that enable rapid reconstruction of the 3D microstructure, quantitative microstructure simulations at an unprecedented fidelity level are giving rise to new possibilities for linking microstructure to property. This paper presents recent advances in 3D computational modeling of ductile fracture in high toughness steels. Ductile fracture involves several concurrent and mutually interactive mechanisms at multiple length scales of microstructure. With serial sectioning tomographic techniques, a digital dataset of microstructure features associated with the fracture process has been experimentally reconstructed. In this study, primary particles are accurately and explicitly modeled while the secondary particles are modeled by a two scale multiresolution continuum model. The present numerical simulation captures detailed characteristics of the fracture process, such as zigzag crack morphology, critical void growth ratios, local stress triaxiality variation, and intervoid ligament structure. For the first time, fracture toughness is linked to multiscale microstructures in a realistic large 3D model.     

A Hybrid Domain Decomposition and Multigrid Method for the Acceleration of Compressible Viscous Flow Calculations on Unstructured Triangular Meshes

This paper is concerned with the formulation and the evaluation of a hybrid solution method that makes use of domain decomposition and multigrid principles for the calculation of two-dimensional compressible viscous flows on unstructured triangular meshes. More precisely, a non-overlapping additive domain decomposition method is used to coordinate concurrent subdomain solutions with a multigrid method. This hybrid method is developed in the context of a flow solver for the Navier-Stokes equations which is based on a combined finite element/finite volume formulation on unstructured triangular meshes. Time integration of the resulting semi-discrete equations is performed using a linearized backward Euler implicit scheme. As a result, each pseudo time step requires the solution of a sparse linear system. In this study, a non-overlapping domain decomposition algorithm is used for advancing the solution at each implicit time step. Algebraically, the Schwarz algorithm is equivalent to a Jacobi iteration on a linear system whose matrix has a block structure. A substructuring technique can be applied to this matrix in order to obtain a fully implicit scheme in terms of interface unknowns. In the present approach, the interface unknowns are numerical fluxes. The interface system is solved by means of a full GMRES method. Here, the local system solves that are induced by matrix-vector products with the interface operator, are performed using a multigrid by volume agglomeration method. The resulting hybrid domain decomposition and multigrid solver is applied to the computation of several steady flows around a geometry of NACA0012 airfoil.     

On the simultaneous estimation of model parameters used in constitutive laws for inelastic material behaviour

The paper deals with the simultaneous estimation of parameters used in constitutive laws for modeling inelastic material behaviour. Experimental data is obtained from specimens with a uniform distribution of stresses. Uniaxial evaluations of the constitutive laws are used. Due to the numerical stiffness of the constitutive equations, it is shown that it is necessary to use an implicit time-integration scheme. The model parameters are estimated from experimental data using a least-squares criterion. Stochastic and deterministic optimization methods are used. The algorithms are parallelized on the basis of the master-slave paradigm. Since a complete parameter estimation requires error estimates and confidence regions, statistic methods are implemented.     

Review of Coherent Structures in Turbulent Free Shear Flows and Their Possible Influence on Computational Methods

A review of selected experiments on coherent structures in turbulent shear flows is performed. Different experimental approaches (conditional averages, filtering techniques, wavelets, linear stochastic estimation and proper orthogonal decomposition, etc.) are illustrated and their links with computations (LES, DNS, SDM, etc.) is emphasized. It is particularly shown that some kind of universal behavior of the background turbulence can be retrieved from these various experimental methods. This revised version was published online in August 2006 with corrections to the Cover Date.