bluuebunny/crossref_metadata_2025_split huggingface dataset:
Approximation of a nonlinear degenerate parabolic equation via a linear relaxation scheme
Authors: Slodička Marián 
Published: June 2004

AbstractWe study a nonlinear degenerate parabolic equation of the type

accompanied by an initial datum and mixed boundary conditions. The symbol [ · ]+ denotes the usual cutoff function. The problem represents a model of a reactive solute transport in porous media. The exponent p fulfills p ∈ (0, 1). This limits the regularity of a solution and leads to inconveniences in the error analysis. We design a new robust linear numerical scheme for the time discretization. This is based on a suitable combination of the backward Euler method and a linear relaxation scheme. We prove the convergence of relaxation iterations on each time point ti. We derive the error estimates in suitable function spaces for all values of p ∈ (0, 1). © 2004 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2005.
Green element method for 2D Helmholtz and convection diffusion problems with variable velocity coefficients
Authors: Onyejekwe Okey Oseloka 
Published: June 2004

AbstractComputation of 2D Helmholtz and transient convection diffusion problems with linear reaction and variable velocity components are implemented with the Green element method (GEM). GEM's fundamental solution which is derived from the diffusion differential operator simplifies the numerical procedure considerably, and together with the Green's second identity, an element to element treatment of the inhomogeneous terms is guaranteed. The reported numerical experiments reveal that the method can be relied on to yield faithful results. © 2004 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2005
Mixed finite element methods for generalized Forchheimer flow in porous media
Authors: Park Eun‐Jae 
Published: June 2004

AbstractMixed finite element methods are analyzed for the approximation of the solution of the system of equations that describes the flow of a single‐phase fluid in a porous medium in ℝd, d ≤ 3, subject to Forchhheimer's law—a nonlinear form of Darcy's law. Existence and uniqueness of the approximation are proved, and optimal order error estimates in L∞(J; L2(Ω)) and in L∞(J; H(div; Ω)) are demonstrated for the pressure and momentum, respectively. Error estimates are also derived in L∞(J; L∞(Ω)) for the pressure. © 2004 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2005
Laser surface hardening using proper orthogonal decomposition for a threedimensional example
Authors: Hömberg Dietmar , Volkwein Stefan 
Published: June 2004

AbstractLaser surface hardening of steel is formulated in terms of an optimal control problem with bilateral control constraints. The state equations are composed of a semilinear heat equation and an ordinary differential equation, which describes the evolution of the high temperature phase. In this work a three‐dimensional example is solved numerically by utilizing model reduction based on proper orthogonal decomposition (POD).
Balanced truncation model reduction for descriptor systems
Authors: Stykel Tatjana 
Published: June 2004

AbstractWe present a generalization of a balanced truncation model reduction method for descriptor systems. This method is closely related to the proper and improper controllability and observability Gramians and Hankel singular values that can be computed by solving projected generalized Lyapunov equations. We demonstrate the application of balanced truncation model reduction to the semidiscretized Stokes equation.
Feedback Control Of Industrial Robots By Means Of Stochastic Optimization Techniques
Authors: Aurnhammer Andreas , Marti Kurt 
Published: June 2004

AbstractThe standard engineering approach in control of industrial robots assumes all model parameters are known exactly and therefore can be modeled by their nominal values. Due to stochastic variations of the material, manufacturing errors, modeling errors and most important stochastic variations of the workspace environment (e.g. stochastic payload mass) this assumption does not hold. Hence, an approach that models these parameters by means of random variables was proposed, cf. [1]. Solving this substitute optimal control problem by means of direct optimal control discretization techniques allows the numerical computation of approximate optimal stochastic reference trajectories for the on‐line control process. These optimal reference trajectories are ‐ according to the chosen probability distribution ‐ valid for a variety of parameter configurations and are more stable toward parameter uncertainties. However, due to the stochastic parameters in the robots dynamic model, in the on‐line control process still deviations from the reference trajectories appear. Hence, a method for the construction of feedback controllers based on feedback linearization is presented.
On the motion of a shape memory cantilever subject to harmonic end displacement and proposal for a microdamper
Authors: Rizzoni Raffaella 
Published: June 2004

AbstractIn [8] a model for the description of the dynamics of martensitic materials has been proposed. The model is specifically intended for materials whose free energy grows steeply away from the energy wells. Essentially, it is the multi‐well analog of ordinary rigid body mechanics with three additional features: the stress is not treated as arbitrary (the average of its normal component on the interfaces enters the theory in a non trivial way), a certain component of the local balance of the linear momentum is used, and kinetic laws for interfacial motion are introduced based on ideas of Eshelby [6] and Abeyaratne and Knowles [1, 2].In this contribution the model is specialized to a cantilever subject to harmonic end displacement and made of two different variants of martensite separated by a system of parallel interfaces. The equations governing the motion of the cantilever when the displacement at its free extremity is prescribed are obtained. This very preliminary study could serve as a basis for the design of a microdamper.
Impact of correlation structure on drainage in open rough walled fractures
Authors: Neuweiler Insa , Sorensen Ivan , Kinzelbach Wolfgang 
Published: June 2004

AbstractThe spatial correlation structure of heterogeneous parameters plays an important role in upscaling of many flow and transport processes in porous media and fractures. We analyze upscaling of a slow drainage process in a horizontal open rough walled fracture. The applicability of upscaled continuum models for this kind of flow process is discussed. We consider two different fracture aperture distributions with different spatial correlation properties. One is described by a slowly decaying, the other one by a Gaussian autocovariance. The flow process is modelled using an invasion percolation model, which takes the in plane curvature of the fluid cluster into account. Results are compared to laboratory experiments in artificial plexiglas fractures. Different averaging procedures are tested in order to assess the applicability of continuum models. Independently of the correlation structure of the fracture aperture field, spatial averages do not converge to a continuum formulation. Ensemble averages do converge, however, they only give probabilistic information and are not applicable to model drainage in a fracture.
Spline‐Based Elastic Image Registration
Authors: Rohr Karl 
Published: June 2004

AbstractFor the geometric alignment of image data we propose spline‐based schemes utilizing anatomical point landmarks. We have developed three different types of elastic registration approaches which have different properties and thus differ w.r.t. their suitability for certain registration applications. All approaches are both computationally efficient and can be applied for multimodal images. Our approaches have been successfully applied for the registration of 2D and 3D tomographic images of the human brain.
On time‐dependent behaviour of controlled turbulent flow with separation and reattachment
Authors: Neumann J. 
Published: June 2004

AbstractResults from numerical simulations of separating and reattaching turbulent boundary layer flow over a backwardfacing step are analysed with respect to the time‐dependent flow behaviour. Beside the well‐known roll‐up of the separated shear‐layer (“Kelvin‐Helmholtz instability”) and the ejection of subsequently formed large‐scale structures (denoted as “shedding”), another unsteady phenomenon which is commonly called “flapping” can be observed. Additionally, the effects of open‐loop passive and active flow control methods are investigated.
Testing the Exactness of Convex Relaxations for Robust LMI Problems
Authors: Scherer Carsten 
Published: June 2004

AbstractWe consider non‐convex robust linear matrix inequality problems and general families of convex relaxations based on multipliers that cover a whole variety of known concrete versions. In this framework we provide new tests for when these relaxations are exact that go beyond a recently suggested rank‐one principle, and we reveal applicability to an asymptotically exact relaxation sequence.
Homogenization of bone elasticity based on tissue‐independent (‘universal’) phase properties
Authors: Hellmich Christian , Ulm Franz‐Josef , Dormieux Luc 
Published: June 2004

AbstractAs candidates for tissue‐independent phase properties of cortical and trabecular bone we consider (i) hydroxyapatite, (ii) collagen, (iii) ultrastructural water and non‐collagenous proteins, and (iv) marrow (water) filling the Haversian canals and the intertrabecular space. From experiments reported in the literature, we assign stiffness properties to these phases (experimental set I).On the basis of these phase definitions, we develop, within the framework of continuum micromechanics, a two step homogenization procedure: (i) At a length scale of 100 – 200 nm, hydroxyapatite (HA) crystals build up a crystal foam ('polycrystal'), and water and non‐collagenous organic matter fill the intercrystalline space (homogenization step I); (ii) At the ultrastructural scale of mineralized tissues, i.e. 5 to 10 microns, collagen assemblies composed of collagen molecules are embedded into the crystal foam, acting mechanically as cylindrical templates. At an enlarged material scale of 5 to 10 mm, the second homogenization step also accommodates the micropore space as cylindrical pore inclusions (Haversian and Volkmann canals, inter‐trabecular space), that are suitable for both trabecular and cortical bone. The input of this micromechanical model are tissue‐specific volume fractions of HA, collagen, and of the micropore space. The output are tissue‐specific ultrastructural and microstructural (=macroscopic=apparent) elasticity tensors.A second independent experimental set (composition data and experimental stiffness values) is employed to validate the proposed model. We report a a good agreement between model predictions and experimentally determined macroscopic stiffness values. The validation suggests that hydroxyapatite, collagen, and water are tissue‐independent phases, which define, through their mechanical interaction, the elasticity of all bones, whether cortical or trabecular.
Towards a consistency proof for immiscible lattice BGK
Authors: Kehrwald Dirk 
Published: June 2004

AbstractEmanating from the incompressible Navier‐Stokes equations, a widely used mathematical model of immiscible twophase flow is rigorously derived. Once the model is fixed, it is analysed to what extend the immiscible lattice BGK method approximates the solution of the model equations. Using equivalent moment analysis and some elementary tools from differential geometry we are able to show weak consistency if certain well founded but unproven presumptions hold.
Dynamics of Ship–Motion
Authors: Kreuzer Edwin , Pick Marc‐André 
Published: June 2004

AbstractThe ship capsizing problem is one of the major challenges in naval architecture. The IMO criterion regarding capsize stability is still the righting lever curve of static stability calculated for calm water. For the prediction of large–amplitude motions the dynamic loads have to be included. The capsizing of a ship in regular waves is resulting from a sequence of bifurcations in the ship's motion: The determination of bifurcations is possible using path‐following techniques of nonlinear dynamics. Existing tools are, however, without adaption not readily applicable for the determination of bifurcations. The main and until now unsolved problem is the necessity of including memory integrals to describe the ship hydrodynamics. First results with simple algorithms show two different scenarios leading to capsizing due to increasing wave amplitudes.
Forced vibrations of gearboxes with time dependent meshing stiffnesses
Authors: Hajžman M. , Zeman V. , Byrtus M. 
Published: June 2004

AbstractThe contribution presents the efficient method of the mathematical modelling of gearboxes with time dependent meshing stiffnesses. A gearbox is considered including a housing and an interior rotating shaft system with gears. Further the paper deals with an analysis of vibrations excited by the important sources of the internal excitation in gear meshing. The theory is applied to a simple test‐gearbox.
FRF simulation of structural joints with uncertain parameters
Authors: Hanss Michael , Oexl Stefan , Gaul Lothar 
Published: June 2004

AbstractFor the simulation of systems with uncertain parameters, the application of fuzzy arithmetic proves to be a practical approach. The uncertainties in the model parameters can be taken into account by defining them as fuzzy numbers, and the uncertain system can then be simulated by applying fuzzy arithmetic based on the so‐called transformation method. As an example, a normally‐loaded joint which connects two rods in axial direction is discussed in the paper. A worst‐case scenario for the frequency response function (FRF) of the coupled rods with uncertainties in the joint can be obtained, and additionally, one can get information about how much each of the uncertain model parameters contributes to the overall uncertainty of the model output at different frequencies of excitation.
Dynamic analysis of sandwich beams with interlayer slip
Authors: Heuer Rudolf 
Published: June 2004

AbstractForce induced dynamic bending of sandwich beams composed of three symmetrically arranged layers are studied. Moderately thick elastic layers made of dissimilar transversal isotropic materials are considered. If rigid bond between the layers cannot be achieved, due to relative deformation of the connectors an interlayer slip occurs, that can significantly affect both strength and deformation of the layered structure. Analyzing two special cases of interlaminar connections, i.e., “perfect bond” and “elastic interlayer slip”, a complete analogy between the corresponding initialboundary value problems is derived. Hence, the dynamic response of the beam with elastic interlayer slip can be determined directly from the solution of the perfectly bonded sandwich beam with moderately thick faces.
The piecewise full decoupling method for dynamic problems
Authors: Kranjčević Nenad , Stegić Milenko , Vranković Nikola 
Published: June 2004

AbstractThe piecewise full decoupling method is a new developed numerical procedure of explicit integration based on piecing together local linear solutions. The method is applied for solving piecewise linear dynamic systems under periodic excitations. Close agreement is found between obtained results and published findings of a harmonic balance method and a finite element method in time domain.
Design of Friction Dampers for Mistuned Bladed Disks
Authors: Panning L. , Sextro W. , Popp K. 
Published: June 2004

AbstractFriction damping devices such as underplatform dampers are widely used in turbomachinery applications to reduce blade vibration amplitudes. In this paper, a model is presented briefly to calculate the forced response of mistuned bladed systems with friction dampers. Some results will give an insight in the vibrational behavior of frictionally damped bladed disks and how the damping gain is affected by the damper properties.
Avoidance of Stick‐Slip Motion by Vibration Control
Authors: Popp K. , Rudolph M. 
Published: June 2004

AbstractThis paper deals with the excitation mechanisms of friction induced vibrations and recently found methods to avoid these vibrations. It is remarkable, that mechanisms of excitation and avoidance are closely related and, infact, the presented idea and realization of vibration control is derived from theoretical investigation of excitation mechanism. The basic mechanism will be explained and two methods of vibration control will be demonstrated theoretically and by experiment. The concept of fluctuating normal forces is the basic idea behind the two methods of vibration control. In the first case the fluctuating normal forces are generated by an additional oscillator connected to the basic stick‐slip oscillator. In the second case, fluctuating normal forces are generated directly by a magnetic actuator.
Orbital Stability of Skyhooks
Authors: Steindl Alois 
Published: June 2004

AbstractThe sky‐hook, that is a string forming a connection from the surface of the Earth to a satellite in geostationary orbit, which may be used as track for an Earth to space elevator, is an old dream of mankind, originating about 100 years ago in Russia. Besides the question of feasibility from a technological point of view also the question concerning the stability of such a configuration has not yet been completely solved. Under the assumption that a proper material (e.g. “Bucky Tubes” with very high tensile strength and small density) is available, making the connection possible from the technological point of view, we address the question of stability of the radial relative equilibrium of a very long tapered string on a circular geosynchronous orbit around the Earth.
Eigenpath analyses of friction induced vibrations depending on the friction coefficient
Authors: Wagner N. , Gaul L. 
Published: June 2004

AbstractThe tracking of eigenvalues and eigenvectors for parameterized matrices is of major importance in stability problems. A continuation method for approximating complex eigenpaths of parameter‐dependent matrices is proposed. The presence of a multiple eigenvalue (especially when it is defective) is inherently connected with an ill‐conditioned Jacobian and calls for a special treatment of the approach. Therefore, an invariant subspace is used to circumvent the difficulties of passing possible branch points. The procedure is illustrated by means of an example coming from structural dynamics.
Possibilities of Optimal and Adaptive Control Laws Design in Piezoelectric Smart Structures
Authors: Nestorović‐Trajkov Tamara , Köppe Heinz , Gabbert Ulrich 
Published: June 2004

AbstractPossibilities of the controller design for piezoelectric smart structures are considered in the paper. The modelbased approach, which relies on the finite element modal analysis of structures, is used. As an alternative, system identification is suggested in cases when the real plant or its prototype is available for studying. Dynamics of piezoelectric active materials used as actuators and sensors is taken into account in the modelling procedure. The aim of control is vibration suppression in the sense of magnitudes reduction as well as the isolation of undesirable frequencies from the response spectra. For known plant models and available a priori information on the type of excitation or disturbances, optimal LQ tracking control systems perform good behavior. Owing to the approximation of the real structure's behavior by the finite element model and by modal reduction, the full infinite‐dimensional dynamics of an elastic structure cannot be completely captured. Introduced inaccuracies could be compensated for if they are viewed as the source of the model parameters variation and therefore by employing adaptive control laws. The adaptive control design is considered from this point of view. The application possibilities are illustrated by examples of piezoelectrically controlled structures.
Computer Algebra Algorithms for Control Related Tests of Implicit Dynamic Systems
Authors: Schlacher Kurt , Zehetleitner Kurt 
Published: June 2004

AbstractThis contribution is focused on control related tests for implicit dynamic systems, like accessibility, observability or input to output, input to state linearizability. Since the performance of these tests needs tedious symbolic calculations, computer algebra systems are the ideal tool to cope with this problem. Accessibility and observability are exemplarily used to present a new approach based on Lie groups. It is shown that non accessible or non observable systems admit Lie‐groups acting on their solutions such that distinguished parts remain unchanged. This fact allows us to apply this technique, as well as its realization by computer algebra algorithm, to several fundamental problems in control.
Stability of Linear Parameter Varying and Linear Switching Systems
Authors: Wirth Fabian 
Published: June 2004

AbstractWe consider stability of families of linear time‐varying systems, that are determined by a set of time‐varying parameters which adhere to certain rules. The conditions are general enough to encompass on the one hand stability questions for systems that are frequently called linear parameter varying systems in the literature and on the other hand also linear switching systems, in which parameter variations are allowed to have discontinuities. Combinations of these two sets of assumptions are also possible within the framework studied here. Under the assumption of irreducibility of the sets of system matrices, we show how to construct parameter dependent Lyapunov functions for the systems under consideration that exactly characterize the exponential growth rate. It is clear that such Lyapunov functions do not exist in general. But every system of our class can be reduced to a finite number of subsystems for which irreducibility holds.
Non‐linear Model Based Control for a Hydraulically Actuated Mobile Harbour Crane
Authors: Aschemann Harald , Hild Ingo , Hofer Eberhard P. 
Published: June 2004

AbstractActive damping of oscillations performed by the rope suspended payload becomes a more and more important feature for mobile harbour cranes characterised by their non‐linear kinematic structure. This contribution presents a nonlinear control scheme for a hydraulically actuated mobile harbour crane, which is based on a multibody model of the mechanical configuration. The decentralised control structure consists of a combined feedforward and feedback control law for each crane axis and includes a compensation for the most dominant coupling between the axes due to centrifugal acceleration. By this, crane operation can be simplified and handling performance can be increased in particular for less experienced crane operators and, moreover, automatised container handling becomes an attractive feature for operating mobile harbour cranes. The efficiency of the proposed control scheme is emphasized by selected simulation results.
Dynamic Simulation of Colliding Trains for Buffer Stop Dimensioning
Authors: Grabner Gerald , Kecskeméthy Andrés 
Published: June 2004

AbstractThis paper describes an application of multibody dynamics to the determination of global physical behavior patterns that mimic traditional empirical laws. The main idea is to sift out such patterns from virtual experiments carried out with a simulation package. The concrete investigation in this setting is the dynamics of trains colliding onto buffer stops. Using a rigid‐body model with nonlinear couplers featuring hysteresis, it is shown that, for typical train constellations, the highest load at the buffer stops is almost independent of the number of coaches, and that the value and the location of the maximum force within the train does not increase from a particular train length on. These results are compared with traditional formulas for dimensioning of buffer stops used in rail‐vehicle industry.