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Tour planning for an unmanned air vehicle under wind conditions

JOURNAL OF GUIDANCE CONTROL AND DYNAMICS 2007年第5期30卷 1299-1306页

作者： McNeely, Rachelle L. Iyer, Ram V. Chandler, Phillip R. Texas Tech Univ Dept Math & Stat Lubbock TX 79409 USA USAF Res Lab Wright Patterson AFB Wright Patterson AFB OH 45433 USA

A very important subproblem in the task assignment problem for unmanned air vehicles is the evaluation of costs for the state transitions of a directed graph. Usually a Dubins vehicle flying in the absence of wind is considered in the computation of costs. However, when a prevailing wind vector field is considered, the costs taken on very different values and the task assignment problem can have very different solutions. In this paper, we consider the problem of constructing minimum-time trajectories for a Dubins vehicle in the presence of a time-varying wind vector field. We present results on the existence and uniqueness of minimum-time solutions for a Dubins vehicle flying in a general time-varying wind vector field under some technical conditions. These results extend the conclusions of the well-known Dubins theorem. We also propose an algorithm for obtaining the minimum-time solution for an unmanned air vehicle and prove its convergence. We also present the results of numerical experiments that show that the importance of considering wind vector fields while planning the tour for an unmanned air vehicle.

关键词： Air Vehicle Micro Air Vehicle numerical Stability Coordinate Transformations Pontryagin's Maximum Principle Air Force Research Laboratory algorithms Convergence Boundary Value Problems numerical algorithms Euler Equations

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The adjustment-stabilization method for constrained systems

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COMPUTER PHYSICS COMMUNICATIONS 2007年第6期177卷 500-505页

作者： Han, Wen-Biao Liao, Xin-Hao Chinese Acad Sci Shanghai Astron Observ Shanghai 200030 Peoples R China Chinese Acad Sci Grad Sch Beijing 100049 Peoples R China

For constrained system which has several independent first integrals, we give a new stabilization method which named adjustment-stabilization method. It can stabilize all known constants of motion for a given dynamical system very well instead of the stabilization and post-stabilization methods which only conserves one of all first integrals. Further more, new method can improve numerical accuracy too. We also point out the post-stabilization is just a simplest case of the new method. (c) 2007 Elsevier B.V. All rights reserved.

关键词： numerical algorithms conservation stabilization least squares adjustment

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Direct transcription solution of high index optimal control problems and regular Euler-Lagrange equations

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JOURNAL OF COMPUTATIONAL AND APPLIED MATHEMATICS 2007年第2期202卷 186-202页

作者： Campbell, Stephen L. Maerz, Roswitha N Carolina State Univ Dept Math Raleigh NC 27695 USA Humboldt Univ Inst Math D-10099 Berlin Germany

Direct transcription methods are used to solve optimal control problems in many industrial settings. Models for physical systems often take the form of differential algebraic equations (DAEs). The index of the DAE traditionally is viewed as an important factor in deciding whether a particular numerical approach should be used. Recently it has been observed that what the user thinks is the index of the DAE may not be the same as the index available to the optimization software. An investigation of this fact is underway in order to develop guidelines to assist users of various numerical optimal control packages. 'This paper develops some theoretical results that will be needed for this development. (c) 2006 Elsevier B.V. All rights reserved.

关键词： direct transcription optimal control control selection numerical algorithms differential algebraic equations

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Size effects and dislocation patterning in two-dimensional bending

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JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2007年第6期55卷 1182-1195页

作者： Scott Weingarten, N. Selinger, Robin L. B. Catholic Univ Amer Dept Phys Washington DC USA Kent State Univ Chem Phys Interdisciplinary Program Kent OH USA Kent State Univ Inst Liquid Crystal Kent OH USA

We perform atomistic Monte Carlo simulations of bending a Lennard-Jones single crystal in two dimensions. Dislocations nucleate only at the free surface as there are no sources in the interior of the sample. When dislocations reach sufficient density, they spontaneously coalesce to nucleate grain boundaries, and the resulting microstructure depends strongly on the initial crystal orientation of the sample. In initial yield, we find a reverse size effect, in which larger samples show a higher scaled bending moment than smaller samples for a given strain and strain rate. This effect is associated with source-limited plasticity and high strain rate relative to dislocation mobility, and the size effect in initial yield disappears when we scale the data to account for strain rate effects. Once dislocations coalesce to form grain boundaries, the size effect reverses and we find that smaller crystals support a higher scaled bending moment than larger crystals. This finding is in qualitative agreement with experimental results. Finally, we observe an instability at the compressed crystal surface that suggests a novel mechanism for the formation of a hillock structure. The hillock is formed when a high angle grain boundary, after absorbing additional dislocations, becornes unstable and folds to form a new crystal grain that protrudes from the free surface. (c) 2007 Elsevier Ltd. All rights reserved.

关键词： dislocations crystal plasticity grain boundaries numerical algorithms atomistic simulation

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A Finite Element approach with patch projection for strain gradient plasticity formulations

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INTERNATIONAL JOURNAL OF PLASTICITY 2007年第4期23卷 690-710页

作者： Han, Chung-Souk Ma, Anxin Roters, Franz Raabe, Dierk N Dakota State Univ Dept Civil Engn Fargo ND 58105 USA Max Planck Inst Eisenforsch GmbH D-40237 Dusseldorf Germany

Several strain gradient plasticity formulations have been suggested in the literature to account for inherent size effects on length scales of microns and submicrons. The necessity of strain gradient related terms render the simulation with strain gradient plasticity formulation computationally very expensive because quadratic shape functions or mixed approaches in displacements and strains are usually applied. Approaches using linear shape functions have also been suggested which are, however, limited to regular meshes with equidistanced Finite Element nodes. As a result the majority of the simulations in the literature deal with plane problems at small strains. For the solution of general three dimensional problems at large strains an approach has to be found which has to be computationally affordable and robust. For this goal a strain gradient Finite Element approach is suggested where elements with linear shape functions are applied in combination with a patch projection technique well known from error indication and adaptive mesh procedures. This approach is applied to a strain gradient crystal plasticity formulation where strain gradients are incorporated in terms of geometrically necessary dislocation densities. Simulation results of size dependent problems, including laminates in simple shear and a three dimensional contact problem, are presented and discussed to assess the performance of the suggested approach. (c) 2006 Elsevier Ltd. All rights reserved.

关键词： dislocations crystal plasticity finite elements numerical algorithms size dependent deformation

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FE-analysis of sheet metal forming processes using continuous contact treatment

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INTERNATIONAL JOURNAL OF PLASTICITY 2007年第3期23卷 544-560页

作者： Kim, Tae-Jeong Yang, Dong-Yol Korea Adv Inst Sci & Technol Dept Mech Engn Taejon 305701 South Korea Hyundai Motors Co R&D Div Gyunggi 445705 South Korea Kia Motors Corp Gyunggi 445705 South Korea

Discrete meshes cause stepwise propagation of the contact nodes of a sheet despite the fact that the contact region in the actual forming process is altered very smoothly. This can cause problems of convergence and accuracy in contact-sensitive processes, such as a bending process. In this study, a scheme for a continuous contact treatment is proposed in order to consider the more realistic behavior of the contact phenomena during the forming process. For verification of the proposed method, the contact pressures and forming load are evaluated during the compression forming of a tube. The analysis of a hemispherical dome formed without a blank holder is also presented in order to investigate the effects of the proposed algorithm. The results show that the precise deformation mode is predicted by the utilization of the proposed method. (c) 2006 Elsevier Ltd. All rights reserved.

关键词： finite elements numerical algorithms contact anisotropic material

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Computation of a science orbit about Europa

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JOURNAL OF GUIDANCE CONTROL AND DYNAMICS 2007年第1期30卷 259-263页

作者： Lara, Martin Russell, Ryan Real Observ Armada San Fernando 11110 Spain CALTECH Jet Prop Lab Navigat & Control Sect Pasadena CA 91109 USA

THE instrument requirements for a scientific mission about Europa constrain the orbit design to a subset of limited values of the orbital elements. Near-circular, low-altitude, high-inclination orbits are normally required for mapping missions, and space mission designers try to minimize the altitude variation of the satellite over the surface of the body by searching for orbits with small eccentricity and with a fixed argument of periapsis. These orbits are usually called frozen orbits [1-3].

关键词： Europa Satellites Synchronous Orbit Giant Planet Periapsis Computing numerical algorithms Spherical Harmonics Spacecraft Orbits Frozen Orbit

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Specific weight:: A challenge for a fuel-cell-powered electric helicopter

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JOURNAL OF AIRCRAFT 2007年第6期44卷 2073-2076页

作者： Guinea, D. M. Roura, M. Garcia-Alegre, M. C. Ribeiro, A. Guinea, D. Ceram & Glass Inst Spanish Council Sci Res Glass Dept Madrid 28049 Spain Univ Politecn Madrid Tech Sch Aeronaut Engineers Inst Univ Microgravedad Ignacio da Riva E-28040 Madrid Spain Ind Automat Inst Spanish Council Sci Res Dept Syst Madrid 28500 Spain

The article describes the fuel-cell powered vehicles, specifically helicopters, focusing on the cost and specific weight of their fuel cells. The cost/functionality of fuel-cell powered vehicles have been extensively analyzed. Fuel-cell cars have higher global costs than conventional cars under base-case conditions but they have the lowest externality costs. Some fuel cells have been deisgned for helicopters which are created to fly on Mars. Fuel cells are an interesting option for aircraft because they can provide much energy per unit of mass with limited explosion risks.

关键词： Polymer Electrolyte Membrane Fuel Cells Helicopters Lift Forces MATLAB Probability Distribution Functions Data Acquisition Supercapacitors Atmospheric Conditions Industrial Automation numerical algorithms

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Testing the accuracy of a data assimilation algorithm

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INTERNATIONAL JOURNAL OF COMPUTATIONAL SCIENCE AND ENGINEERING 2007年第4期3卷 305-313页

作者： Zlatev, Z. Brandt, J. Aarhus Univ Natl Environm Res Inst Frederiksborgvej 399POB 358 DK-4000 Roskilde Denmark

Variational data assimilation techniques have been successfully used in connection with comprehensive research projects in different fields of science and engineering. The use of the variational data assimilation approach is becoming more and more popular also in the attempts to improve the accuracy of the results obtained by running large-scale air pollution models. Different tests were carried out in order both to check the ability of the data assimilation procedures to improve the initial concentrations and to start building up a benchmark for testing the performance of these procedures on different modern high-speed computers. Selected results from these tests will be presented and discussed in this paper.

关键词： air pollution models data assimilation numerical algorithms ordinary differential equations ODEs partial differential equations splitting techniques

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Computational aspects of prospect theory with asset pricing applications

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Computational Economics 2007年第3-4期29卷 267-281页

作者： Giorgi, Enrico Hens, Thorsten Mayer, János NCCR-FINRISK University of Zurich Zurich Switzerland Swiss Finance Institute University of Lugano Lugano Switzerland Norwegian School of Economics and Business Administration Bergen Norway Swiss Banking Institute University of Zurich Zurich Switzerland Institute for Operations Research University of Zurich Zurich Switzerland

We develop an algorithm to compute asset allocations for Kahneman and Tversky's (Econometrica, 47(2), 263-291, 1979) prospect theory. An application to benchmark data as in Fama and French (Journal of Financial Economics, 47(2), 427-465, 1992) shows that the equity premium puzzle is resolved for parameter values similar to those found in the laboratory experiments of Kahneman and Tversky (Econometrica, 47(2), 263-291, 1979). While previous studies like Benartzi and Thaler (The Quarterly Journal of Economics, 110(1), 73-92, 1995), Barberis, Huang and Santos (The Quarterly Journal of Economics, 116(1), 1-53, 2001), and Grüne and Semmler (Asset prices and loss aversion, Germany, Mimeo Bielefeld University, 2005) focussed on dynamic aspects of asset pricing but only used loss aversion to explain the equity premium puzzle our paper explains the unconditional moments of asset pricing by a static two-period optimization problem. However, we incorporate asymmetric risk aversion. Our approach allows reducing the degree of loss aversion from 2.353 to 2.25, which is the value found by Tversky and Kahneman (Journal of Risk and Uncertainty, 5, 297-323, 1992) while increasing the risk aversion from 1 to 0.894, which is a slightly higher value than the 0.88 found by Tversky and Kahneman (Journal of Risk and Uncertainty, 5, 297-323, 1992). The equivalence of these parameter settings is robust to incorporating the size and the value portfolios of Fama and French (Journal of Finance, 47(2), 427-465, 1992). However, the optimal prospect theory portfolios found on this larger set of assets differ drastically from the optimal mean-variance portfolio. © Springer Science+Business Media, LLC 2007.

关键词： Asset pricing Equity premium puzzle Global optimization Non-smooth problems numerical algorithms Prospect theory

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