检索结果-内蒙古大学图书馆

Maximum Torque and Momentum Envelopes for Reaction-Wheel Arrays

JOURNAL OF GUIDANCE CONTROL AND DYNAMICS 2010年第5期33卷 1606-1614页

作者： Markley, F. Landis Reynolds, Reid G. Liu, Frank X. Lebsock, Kenneth L. NASA Goddard Space Flight Ctr Attitude Control Syst Engn Branch Greenbelt MD 20771 USA Millennium Space Syst Inc Torrance CA 90503 USA Stinger Ghaffarian Technol Inc Greenbelt MD 20770 USA Orbital Sci Corp Greenbelt MD 20770 USA

Spacecraft reaction-wheel maneuvers are limited by the maximum torque and/or angular momentum that the wheels can provide. The torque or momentum envelope for an n-wheel configuration can be obtained by projecting the n-dimensional hypercube, representing the domain boundary of individual wheel torques or momenta, into three-dimensional space via the 3 x n matrix of wheel axes. This paper elucidates the properties of the projected hypercube and presents algorithms for determining this maximal torque or momentum envelope for general wheel configurations and for distributing a prescribed torque or momentum among the n wheels. We show that these algorithms provide 22, 27, and 33% more capability than the more conventional pseudoinverse algorithm for configurations of four, five, and six wheels, respectively. Analysis of a representative slew using six wheels shows that these algorithms can provide either a 25% reduction in maximum wheel momentum or a 30% reduction in slew time when compared with the pseudoinverse algorithm.

关键词： Reaction Wheels James Webb Space Telescope Gamma Ray Bursts Spacecraft Structures Resonant Frequencies Earth Mathematical Analysis numerical algorithms Attitude Control Lagrange Points

来源：评论

学校读者我要写书评

暂无评论

Decentralized estimation and control of graph connectivity for mobile sensor networks

引用

AUTOMATICA 2010年第2期46卷 390-396页

作者： Yang, P. Freeman, R. A. Gordon, G. J. Lynch, K. M. Srinivasa, S. S. Sukthankar, R. Northwestern Univ Dept Elect Engn & Comp Sci Evanston IL 60208 USA Northwestern Univ Dept Mech Engn Evanston IL 60208 USA Carnegie Mellon Univ Sch Comp Sci Pittsburgh PA 15213 USA Intel Res Pittsburgh PA 15213 USA

The ability of a robot team to reconfigure itself is useful in many applications: for metamorphic robots to change shape, for swarm motion towards a goal, for biological systems to avoid predators, or for mobile buoys to clean up oil spills. In many situations, auxiliary constraints, such as connectivity between team members or limits on the maximum hop-count, must be satisfied during reconfiguration. In this paper, we show that both the estimation and control of the graph connectivity can be accomplished in a decentralized manner. We describe a decentralized estimation procedure that allows each agent to track the algebraic connectivity of a time-varying graph. Based on this estimator, we further propose a decentralized gradient controller for each agent to maintain global connectivity during motion. (C) 2009 Elsevier Ltd. All rights reserved.

关键词： Connectivity Decentralized control Graph theory numerical algorithms Autonomous mobile robots

来源：评论

学校读者我要写书评

暂无评论

Probabilistic modeling of microstructure evolution using finite element representation of statistical correlation functions

引用

INTERNATIONAL JOURNAL OF PLASTICITY 2012年 30-31卷 62-80页

作者： Sundararaghavan, V. Kumar, A. Univ Michigan Ann Arbor MI 48109 USA

A probabilistic finite element scheme is presented for simulating evolution of polycrystalline microstructures during deformation. The microstructure is described using conditional orientation correlation function (COCF), defined as the probability density of occurrence of a crystal orientation g' at a distance r from a given orientation g. The COCF is represented using three interconnected layers of finite element meshes in the g' r and g spaces. As the microstructure evolves, the reoriented neighborhood and strain fields close to an orientation (g) are captured by updating probability fields in these finite element meshes. For this purpose, a novel total Lagrangian approach has been developed that allows evolution of probability densities while satisfying normalization constraints, probability interdependencies and symmetries. The improvement in prediction of texture and strains achieved by the COCF approach over ODF-based methods is quantified through deformation analysis of a planar polycrystalline microstructure. (C) 2011 Elsevier Ltd. All rights reserved.

关键词： Microstructural descriptors numerical algorithms Crystal plasticity Microstructures Probability and Statistics

来源：评论

学校读者我要写书评

暂无评论

Strain-rate effects in rheological models of inelastic response

引用

INTERNATIONAL JOURNAL OF PLASTICITY 2003年第8期19卷 1097-1118页

作者： Lubarda, VA Benson, DJ Meyers, MA Univ Calif San Diego Dept Mech & Aerosp Engn La Jolla CA 92093 USA

Stress-strain response under constant and variable strain-rate is studied for selected models of inelastic behavior. The derived closed-form solutions for uniaxial loading enable simple evaluation of the strain-rate effects on the material response. The effect of an abrupt change of strain-rate is also examined. Non-Newtonian viscosity which decreases with an increasing strain-rate is incorporated in the analysis. Parabolic and hyperbolic hardening are used to describe the plastic response in monotonic loading. A three-dimensional generalization of an elastic-viscoplastic model is employed to study the stress relaxation in simple shear. A combined isotropic-kinematic hardening and the concept of overstress are used in the analysis. The unloading nonlinearity of the stress-strain curve is then discussed. (C) 2002 Published by Elsevier Science Ltd.

关键词： creep stress relaxation constitutive behavior polymeric materials viscoplastic material numerical algorithms

来源：评论

学校读者我要写书评

暂无评论

Fuel-Optimal Rocket Landing with Aerodynamic Controls

引用

JOURNAL OF GUIDANCE CONTROL AND DYNAMICS 2019年第1期42卷 65-77页

作者： Liu, Xinfu Beijing Inst Technol Sch Aerosp Engn Beijing 100081 Peoples R China

Aerodynamic forces are not negligible for a reusable rocket returning back to Earth. How the aerodynamic controls and propulsion should be coordinated to realize fuel-optimal precise landing is addressed in this paper. To this end, a model-based optimal control problem is formulated with the rocket's angle of attack and thrust as control inputs, and constraints on the controls are included to reflect the capabilities of the vehicle. Precise landing requires the (highly nonlinear and nonconvex) problem to be solved onboard in real time. This ability of online computation is becoming increasingly desired in aerospace guidance and control for autonomous missions. Hence, this paper presents how to solve the rocket landing problem via convex optimization that has guaranteed polynomial-time complexity. Specifically, a novel methodology of handling the rocket nonlinear dynamics is introduced, and a relaxation technique used to convexify nonconvex constraints is theoretically proved to be valid. High efficiency of the proposed method, with potential for online computation, is demonstrated by numerical examples and comparisons with other methods.

关键词： Rocket Trajectories Fuels Lift Coefficient Aerodynamic Force Nonlinear Programming Trajectory Optimization Nonlinear Dynamics Earth Flight Path Angle numerical algorithms

来源：评论

学校读者我要写书评

暂无评论

A STOCHASTIC FRAMEWORK FOR ATOMISTIC FRACTURE\ast

引用

SIAM JOURNAL ON APPLIED MATHEMATICS 2022年第2期82卷 526-548页

作者： Buze, Maciej Woolley, Thoms E. Mihai, Angela Cardiff Univ Sch Math Cardiff CF24 4AG Wales

We present a stochastic modeling framework for atomistic propagation of a Mode I surface crack, with atoms interacting according to the Lennard--Jones interatomic potential at zero temperature. Specifically, we invoke the Cauchy--Born rule and the maximum entropy principle to infer probability distributions for the parameters of the interatomic potential. We then study how uncertainties in the parameters propagate to the quantities of interest relevant to crack propagation, namely, the critical stress intensity factor and the lattice trapping range. For our numerical investigation, we rely on an automated version of the so-called numerical-continuation enhanced flexible boundary NCFlex algorithm.

关键词： Key words elasticity interatomic potential stress fracture stochastic modelling numerical algorithms

来源：评论

学校读者我要写书评

暂无评论

A novel approach to study dislocation density tensors and lattice rotation patterns in atomistic simulations

引用

JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2012年第4期60卷 711-722页

作者： Begau, C. Hua, J. Hartmaier, A. Ruhr Univ Bochum ICAMS D-44801 Bochum Germany Xian Univ Architecture & Technol Dept Mech Sch Sci Xian 710055 Shaanxi Peoples R China

Crystal plasticity caused by the nucleation and interaction of dislocations is an important aspect in crystal deformation. Recent nanoindentation experiments in single crystals of copper or aluminum revealed large deviations in the lattice rotation and an inhomogeneous distribution of the dislocation density in the plastic zone under the indenter tip. Molecular dynamics simulations offer the possibility to study the origin of these phenomena on an atomistic scale, but require sophisticated analysis routines in order to deal with the massive amount of generated data. Here a new efficient approach to analyze atomistic data on the fly during the simulation is introduced. This approach allows us to identify the dislocation network including Burgers vectors on the timescale of picoseconds and below. This data does not only reveal the evolution of dislocation structures, but it offers the possibility to quantify local dislocation density tensors calculated on an atomic level. The numerical results are compared with experimental data from the literature. The presented approach provides useful insight into the active deformation mechanisms during plastic deformation that will help us to bridge simulations on atomic scales and continuum descriptions. (C) 2012 Elsevier Ltd. All rights reserved.

关键词： Dislocations Indentation and hardness numerical algorithms

来源：评论

学校读者我要写书评

暂无评论

Novel Approach to Improve Stability and Convergence of Flowfield Solution Processes: Mode Multigrid

引用

AIAA JOURNAL 2023年第8期61卷 3496-3506页

作者： Wang, XuKun Liu, Yilang Zhang, Weiwei Northwestern Polytech Univ Sch Aeronaut Natl Key Lab Sci & Technol Aerodynam Design & Res Xian 710072 Peoples R China

In the field of computational fluid dynamics, stability and convergence problems are often encountered when solving the governing equation. This paper studies the effect of the mode multigrid on the stability and convergence of iterative algorithms. By further analyzing the mechanism for accelerating the convergence of mode multigrid, a new adaptive mode multigrid (AMMG) is proposed, and an adaptive selection criterion is formulated for the parameters of dynamic modal decomposition, which can accurately identify and filter out the unstable modes in the flowfield, thus efficiently obtaining the accurate steady solution. In the cases of laminar flow past circular cylinder and turbulent flow past airfoil, the AMMG significantly improves the stability of the iterative algorithm;in the case of transonic flow past airfoil, the AMMG solves the problem of shock wave shaking during the iteration, and the convergence is significantly improved. In summary, the AMMG method significantly enhances the stability and convergence of the iterative algorithm, and can be used as an effective convergence-preserving computation strategy.

关键词： Computational Fluid Dynamics Airfoil Finite Volume Formulation Reynolds Averaged Navier Stokes numerical algorithms algorithms Convergence numerical Stability

来源：评论

学校读者我要写书评

暂无评论

ARTIFICIAL DISSIPATION MODELS FOR THE EULER EQUATIONS

引用

AIAA JOURNAL 1986年第12期24卷 1931-1940页

作者： PULLIAM, TH NASA Ames Research Center Moffett Field California

Various artificial dissipation models that are used with central difference algorithms for the Euler equations are analyzed for their effect on accuracy, stability, and convergence rates. In particular, linear and nonlinear models are investigated using an implicit approximate factorization code (ARC2D) for transonic airfoils. Fully implicit application of the dissipation models is shown to improve robustness and convergence rates. The treatment of dissipation models at boundaries will be examined. It will be shown that accurate, error free solutions with sharp shocks can be obtained using a central difference algorithm coupled with an appropriate nonlinear artificial dissipation model. © 1986 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.

关键词： Rankine Hugoniot Relation Total Variation Diminishing Supersonic Flow Viscosity NACA 0012 Navier Stokes Equations Solution Algorithm Angle of Attack numerical algorithms Specific Heat

来源：评论

学校读者我要写书评

暂无评论

Estimation of the full Nye's tensor and its gradients by micro-mechanical stereo-inference using EBSD dislocation microscopy

引用

INTERNATIONAL JOURNAL OF PLASTICITY 2013年 50卷 146-157页

作者： Hardin, Thomas J. Adams, Brent L. Fullwood, David T. Wagoner, Robert H. Homer, Eric R. Brigham Young Univ Dept Mech Engn Provo UT 84602 USA Ohio State Univ Dept Mat Sci & Engn Columbus OH 43210 USA

A novel method of micromechanical stereoinference is reported which yields components and gradients of Nye's GND tensor which are inaccessible by surface EBSD. In particular, it determines the Nye's tensor gradients going into the sample bulk. The method overcomes limitations imposed by metal's electron opacity by combining experimentally-accessible Nye's tensor components and measured infinitesimal elastic distortion tensors with a solution to the underlying stress equilibrium equations. The full Nye's tensor can be transformed into a crystal coordinate frame and interpreted in the context of slip systems, a more physical sense than in the sample or experimental frame. A demonstration of the method is given for a simulated microstructure. The method is largely robust to random experimental noise but may be sensitive to pattern-center errors. (C) 2013 Elsevier Ltd. All rights reserved.

关键词： Dislocations Elastic material Electron microscopy numerical algorithms

来源：评论

学校读者我要写书评

暂无评论

建议与咨询留下您的常用邮箱和电话号码，以便我们向您反馈解决方案和替代方法

时间限定

文献类型

馆藏选择

核心期刊

语言

文献类型

帮助

文字说明：

检索规则说明：

检索范例：

分类表

所选分类

限定检索结果

文献类型

馆藏范围

日期分布

学科分类号

主题

机构

作者

语言

请选择保存的检索档案：

请选择收藏分类：

通借通还

建议与咨询 留下您的常用邮箱和电话号码，以便我们向您反馈解决方案和替代方法

时间限定

文献类型

馆藏选择

核心期刊

语言

文献类型

帮助

文字说明：

检索规则说明：

检索范例：

分类表

所选分类

限定检索结果

文献类型

馆藏范围

日期分布

学科分类号

主题

机构

作者

语言

请选择保存的检索档案： 新增检索档案 确定 取消

请选择收藏分类： 新增自定义分类 确定 取消

通借通还

建议与咨询留下您的常用邮箱和电话号码，以便我们向您反馈解决方案和替代方法

请选择保存的检索档案：

请选择收藏分类：