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Higher-order characteristics-based method for incompressible flow computation on unstructured grids

AIAA JOURNAL 2001年第7期39卷 1280-1287页

作者： Zhao, Y Tai, CH Nanyang Technol Univ Sch Mech & Prod Engn Singapore 639798 Singapore

An algorithm for numerical simulation of incompressible hows on three-dimensional unstructured grids is presented. It is an upwind finite volume method based on the method of characteristics, which is made possible with the introduction of Chorin's artificial compressibility formulation (Chorin, A., "A numerical Method for Solving Incompressible Viscous Flow Problems," Journal of Computational Physics, Vol. 2, No. 1, 1967, pp. 12-26). Flow variables are calculated along characteristics, and their initial values are interpolated based on the signs of the corresponding characteristics. In addition, an upwind-biased interpolation method of third-order accuracy is used for interpolating flow variables on unstructured grids. With these inherent upwinding techniques for evaluating convection fluxes at control volume surfaces, no artificial viscosity is required. The discretized equations are solved by an explicit multistage Runge-Kutta time-stepping scheme, which is found to be efficient in terms of CPU and memory overheads. ii computer code has been developed using the numerical methods presented. A number of test cases, including two-dimensional/three-dimensional inviscid and viscous hows, have been calculated to validate the code and to evaluate the performance of the numerical algorithm. numerical results obtained are in good agreement with exact solutions and of her published experimental/numerical results. The convergence rate of numerical simulation is generally found to be satisfactory, The third-order characteristics-based scheme is found to be more accuarate than the second-order central scheme.

关键词： Unstructured Grid Incompressible Viscous Flow Courant Friedrichs Lewy Compressibility Freestream Conditions Streamlines Pattern Convection Tetrahedral Element numerical algorithms Continuity Equation

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Aeroacoustic modeling of turbulent airfoil flows

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AIAA JOURNAL 2001年第6期39卷 1057-1064页

作者： Shen, WZ Sorensen, JN Tech Univ Denmark Dept Energy Engn DK-2800 Lyngby Denmark Cent S Univ Res Ctr High Speed Trains Hunan 410075 Peoples R China

A numerical algorithm for acoustic noise generation is extended to handle turbulent Bows, The approach involves two steps comprising a viscous incompressible how part and an inviscid acoustic part. In the turbulent case, the flow is further split into a Reynolds-averaged component and a component corresponding to the turbulent small-scale fluctuations. The latter is modeled by an eddy-viscosity-based turbulence model and appears as a source term in the acoustic equations. The formulation is applied to acoustic noise generated by the Bow past a NACA 0015 airfoil at an incidence of 20 deg, First, acoustic noise generated by laminar Bow is considered as a validation of the acoustic model. The results are compared to solutions obtained using the Lighthill acoustic analogy (linearized wave equations) (Lighthill, M, J., "On Sound Generated Aerodynamically I: General Theory," Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences, Vol. 211,1952, pp, 563-587). The comparisons show that noise levels and frequency content are in good agreement. Next, the acoustic model is applied on a turbulent Bow in which the small-scale turbulence is modeled by a Reynolds-averaged turbulence model [Baldwin-Barth one equation model (Baldwin, B, S., and Earth, T J., "A One-Equation Turbulence Transport Model for High Reynolds Number Wall-Bounded Flows," NASA TM 102847, 1990)]. The computations show that the generated acoustic field is dominated by the Strouhal frequency and its harmonics. The acoustic noise level for the turbulent flow is of the same order as for the laminar Bow Because of the turbulence model used in the flow solver, only one frequency and its higher harmonics are seen. For capturing more frequencies, one should combine the acoustic model with large eddy simulation or direct Navier-Stokes simulation.

关键词： Freestream Velocity NACA 0015 Acoustic Noise numerical algorithms Turbulence Models Eddy Viscosity Vortex Shedding Boussinesq Approximation Large Eddy Simulation Acoustic Velocity

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A plasticity model for metal powder forming processes

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INTERNATIONAL JOURNAL OF PLASTICITY 2001年第12期17卷 1659-1692页

作者： Lewis, RW Khoei, AR Univ Coll Swansea Dept Mech Engn Swansea SA2 8PP W Glam Wales Sharif Univ Technol Dept Civil Engn Tehran Iran

In this paper, a double-surface plasticity model, based on a combination of a convex yield surface consisting of a failure envelope, such as a Mohr-Coulomb yield surface and, a hardening cap model, is developed for the nonlinear behaviour of powder materials in the concept of a generalized plasticity formulation for the description of cyclic loading. This model reflects the yielding, frictional and densification characteristics of powder along with strain and geometrical hardening which occur during the compaction process. The solution yields details on the powder displacement from which it is possible to establish the stress state in the powder and the densification is derived from consideration of the elemental volumetric strain. A hardening rule is used to define the dependence of the yield surface on the degree of plastic straining. Finally, an adaptive finite element model (FEM) analysis is employed by the updated Lagrangian formulation to simulate the compaction of a set of complex powder forming processes. (C) 2001 Elsevier Science Ltd. All rights reserved.

关键词： powder compaction elastic-plastic material constitutive behaviour numerical algorithms finite elements

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Dynamic ruptures in recent models of earthquake faults

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JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2001年第9期49卷 2209-2244页

作者： Ben-Zion, Y Univ So Calif Dept Earth Sci Los Angeles CA 90089 USA

We discuss several problems of dynamic rupture relevant to mechanics of earthquake faults, material sciences, and physics of spatially extended dissipative systems. The problems include dynamic rupture along an interface separating different elastic solids., dynamic rupture on a planar surface governed by strongly velocity-weakening friction, and elastodynamic calculations of long deformation history on a smooth fault in an elastic continuum. These separate problems share a number of methodological and conceptual issues that form recurring themes in the paper. An important methodological issue for computational schemes is dependency of numerical results on the used grid size. This arises inevitably in computer simulations when the assumed constitutive laws do not include a length scale (e.g., of shear or extensional displacement) over which material properties evolve. Such simulations do not have a stable underlying solution, to which they may converge with sufficient grid refinement. However, they may provide rough approximations-lacking at present a rigorous foundation-to the behavior of systems containing elements of discreteness (associated with abrupt fluctuations) at scales relevant to observations of interest. Related important conceptual issues are connections between. or when appropriate separation of, small scale phenomena (e.g., nucleation of rupture, processes at rupture front) and large scale features of the response (e.g., overall space-time dimensions of rupture, statistics of many events). Additional recurring conceptual topics are crack vs. pulse modes of dynamic rupture, the stress under which earthquake faults slip, and the origin of spatio-temporal complexities of earthquakes. These seemingly different issues probably have one or more common origins. Dynamic rupture on an interface between different solids. strongly velocity-weakening friction on a homogeneous fault, and strong fault zone heterogeneities can all produce narrow self-healing slip p

关键词： crack propagation and arrest dynamics friction layered material numerical algorithms

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Early exercise policies of American floating strike and fixed strike lookback options

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NONLINEAR ANALYSIS-THEORY METHODS & APPLICATIONS 2001年第7期47卷 4591-4602页

作者： Yu, H Kwok, YK Wu, LX Natl Univ Singapore Sch Comp Dept Informat Syst Singapore 119260 Singapore Hong Kong Univ Sci & Technol Dept Math Hong Kong Hong Kong Peoples R China

Using appropriate similarity transform, we present the partial differential equation formulation of both floating strike and fixed strike American lookback option models. We examine the early exercise policies of the floating strike and fixed strike American lookback options, while the realized extrmum of the asset price can be monitored continuously or discretely, The characterizations of the optimal exercise prices of American lookback options are also discussed. For the numerical valuation of the American lookback options, several approaches for deriving efficient and accurate numerical results are addressed.

关键词： American lookback options early exercise policies floating strike and fixed strike numerical algorithms

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A general methodology for investigating flow instabilities in complex geometries:: application to natural convection in enclosures

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INTERNATIONAL JOURNAL FOR numerical METHODS IN FLUIDS 2001年第2期37卷 175-208页

作者： Gadoin, E Le Quéré, P Daube, O CNRS LIMSI F-91403 Orsay France CEMIF Evry France

This paper presents a general methodology for studying instabilities of natural convection flows enclosed in cavities of complex geometry. Different tools have been developed, consisting of time integration of the unsteady equations, steady state solving, and computation of the most unstable eigenmodes of the Jacobian and its adjoint. The methodology is validated in the classical differentially heated cavity, where the steady solution branch is followed for vary large values of the Rayleigh number and most unstable eigenmodes are computed at selected Rayleigh values. Its effectiveness for complex geometries is illustrated on a configuration consisting of a cavity with internal heated partitions. We finally propose to reduce the Navier-Stokes equations to a differential system by expanding the unsteady solution as the sum of the steady state solution and of a linear combination of the leading eigenmodes. The principle of the method is exposed and preliminary results are presented. Copyright (C) 2001 John Wiley & Sons, Ltd.

关键词： Arnoldi-Krylov method flow instabilities natural convection Newton's method numerical algorithms

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Evolution strategies for automatic optimization of jet mixing

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AIAA JOURNAL 2001年第5期39卷 967-969页

作者： Koumoutsakos, P Freund, J Parekh, D Swiss Fed Inst Technol Inst Computat Sci CH-8092 Zurich Switzerland Univ Calif Los Angeles Dept Aerosp & Mech Engn Los Angeles CA 90095 USA Georgia Tech Res Inst Res Ctr Aerosp Transportat & Adv Syst Lab Atlanta GA 30080 USA

Provides information on a study which examined the evolution strategies for the optimization of active control parameters for enhancing jet mixing. Information on evolution strategies; Optimized excitation of compress... 详细信息

关键词： Strouhal Numbers Direct numerical Simulation Mach Number Shock Wave Attenuation Temperature Ratio Compressibility Thermal Conductivity numerical algorithms Turbulence Models Radial Velocity

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Optimization of launch vehicle ascent trajectories with path constraints and coast arcs

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JOURNAL OF GUIDANCE CONTROL AND DYNAMICS 2001年第2期24卷 296-304页

作者： Gath, PF Calise, AJ Georgia Inst Technol Sch Aerosp Engn Atlanta GA 30332 USA

This paper describes improvements made to a hybrid analytic/numerical algorithm for optimization of launch vehicle trajectories. Modifications are described that improve the accuracy of the solution. In addition, the algorithm has been extended to include path constraints for normal force and angle of attack, and the terminal constraints have been generalized to allow optimal attachment to a target orbit defined by inclination, apogee radius, and perigee radius. Singularities that occur for circular orbits and for equatorial orbits are identified. Finally, necessary conditions are derived and applied for the introduction of coasting arcs that are typically required for a great variety of missions. To verify the algorithm presented in this paper, results of a test mission to a 100-nm circular equatorial orbit have been compared with two other independent optimization codes.

关键词： Launch Vehicle Design Perigee Angle of Attack Equatorial Orbit numerical algorithms Transversality Condition Elliptical Orbit Boundary Value Problems Earth Sensitivity Analysis

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A class of thermo-hyperelastic -: viscoplastic models for porous materials:: theory and numerics

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INTERNATIONAL JOURNAL OF PLASTICITY 2001年第7期17卷 943-969页

作者： Mähler, L Ekh, M Runesson, K Frontec Res & Technol AB SE-41250 Gothenburg Sweden

Modelling and numerical issues in conjunction with large strain analysis of thermohyperelastic-viscoplastic models are discussed with emphasis placed on porous materials (metal powders). In order to account for the proper temperature effect on the rate-dependent response of metal powders, the concept of "viscoplastic admissibility" is employed as part of a model framework that includes a dynamic yield surface. A generic class of pressure-sensitive models is considered;a particular model based on quasistatic and dynamic yield surfaces, that are elliptic in the meridian planes in the principal stress space, is used for the subsequent numerical evaluation. The generic model also includes kinematic hardening, thus introducing non-coaxiality between the deformation, the back-stress and the stress tensors tin any given reference configuration). Implicit (backward Euler) integration is used and the corresponding algorithmic tangent stiffness (ATS) tenser is established in a setting that is valid for complete noncoaxiality. For the prototype problem of simple shear (using constant strain approximation), quadratic convergence in the equilibrium iterations is demonstrated. The paper is concluded by an investigation of a Hot-Isostatic-Pressing (HIP) problem, whereby experimental results from the literature are used for calibration of the model parameters. (C) 2001 Elsevier Science Ltd. All rights reserved.

关键词： sintering HIP powder compaction finite strain elastic-plastic elastic-viscoplastic porous material numerical algorithms

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Integrating synchronization with priority into a Kronecker representation

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PERFORMANCE EVALUATION 2001年第1-4期44卷 73-96页

作者： Donatelli, S Kemper, P Univ Dortmund D-44221 Dortmund Germany Univ Turin Dipartimento Informat I-10149 Turin Italy

The compositional representation of a Markov chain using Kronecker algebra, according to a compositional model representation as a superposed generalized stochastic Petri net or a stochastic automata network, has been studied for a while. In this paper we describe a Kronecker expression and associated data structures, that allows to handle nets with synchronization over activities of different levels of priority. New algorithms for these structures are provided to perform an iterative solution method of Jacobi or Gauss-Seidel type. These algorithms are implemented in the APNN Toolbox. We use this implementation in combination with GreatSPN and exercise an example that illustrates characteristics of the presented algorithms. (C) 2001 Elsevier Science B.V. All rights reserved.

关键词： stochastic Petri nets performance evaluation tools numerical algorithms

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