bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nonlinear Control of Planar Parallel Robots with Redundant Servopneumatic Actuators
Authors: Hahn H. , Klier W. , Leimbach K.‐D. 
Published: March 2011

AbstractIn this paper a nonlinear control algorithm of a planar parallel robot with a redundant actuator configuration will be derived, based on exact linearization techniques. This controller achieves apart from standard degree of freedom decoupling and stabilization of the robot dynamics additional desired overconstraint control of the redundant actuators. This result has been achieved by suitably extending the output vector of the robot and by including an orthogonality relation into the controller design. Computer simulations prove the validity of this control concept.
Robust Control of an Aircraft Model
Authors: Werner H. 
Published: March 2011

AbstractA new multimodel approach to robust controller design is illustrated by a practical application; for a laboratory aircraft model, a robust controller is designed simultaneously for normal operating conditions and for propeller failure. Based on a linear model for each operating mode, an LMI formulation of the problem and convex programming are used to search for a state feedback controller which achieves the objective. This state feedback design is then realized simultaneously in both operating modes by a controller which is based on fast output sampling. Robust performance is demonstrated by experimental results.
Periodic Solutions of Conservative Non‐Smooth Dynamical Systems
Authors: Kunze M. 
Published: March 2011

AbstractWe consider conservative non‐smooth dynamical systems of the type ẍ+e+a sgn(x) = p(t) with an w‐periodic forcing p, and we first prove the existence of periodic solutions in the non‐resonant case for the corresponding differential inclusion. Moreover, in the resonant case w = 2π, we gave necessary and sufficient conditions on a and the first Fourier coefficient of p for periodic solutions to exist.
Numerical simulation of shock‐turbulence interaction
Authors: Adams N. A. 
Published: March 2011

AbstractWe outline results of numerical and theoretical investigation of shock‐turbulence interaction. Following the strategy of increasing complexity we first consider isotropic turbulence interacting with a normal shock. In the presence of mean‐shear and shock‐obliqueness, turbulence‐structure becomes dependent on additional parameters such as shock‐angle and strain‐rate. Presently, the most complex situation achievable by direct numerical simulation is the interaction of a turbulent boundary layer with a shock along a compression ramp. Based on recent results for a low‐Reynolds number direct simulation it is found that the simplified cases predict correct trends for turbulent structure. The DNS results indicate that explicit compressibility effects such as pressure dilatation and dilatation dissipation are non‐negligible in the interaction region.
Recent Developments in the Statistical Modelling of Turbulent Flows
Authors: Heinz S. 
Published: March 2011

AbstractStatistical methods are often used to describe transport processes in turbulent reacting flows. However, in many cases of practical relevance one has to consider not only such transports but also their influence on the turbulent reacting flow. Just the simulation of such interactions could not be provided by approaches applied previously. This led, e.g., in respect to the atmospheric chemistry to considerable problems because nonlinear chemical conversion processes, which appear nearly in all cases of practical interest, between emissions and species that are distributed in the ambient flow could only be treated approximately. To overcome these problems, methods are needed that are able to describe not only transport processes but additionally the turbulence. The developments of such techniques requires solutions for three problems: First, suited equations have to be derived that are in consistency with the hydrodynamic equations, second, the capability of such methods has to be demonstrated to reflect the features of complex turbulent flows, and third, it has to be shown that the computational costs of such methods are acceptable. It is described here, in which way these problems could be solved recently, and which advantages arise from the application of such advanced statistical methods.
Dynamic Behavior of Damaged Brittle Solids
Authors: Zhang C. H. , Gross D. 
Published: March 2011

AbstractThe macroscopic dynamic behavior of damaged brittle solids is investigated. Special attention is devoted to the analysis of wave attenuation and dispersion in micro‐cracked solids. Several methods are discussed and numerical examples are presented, to show the dependence of the wave attenuation coefficient and the effective wave velocity in damaged brittle solids on the wave frequency, crack‐size and crack‐density.
Adaptive Finite Element Discretization in Elasticity and Elastoplasticity by Global and Lokal Error Estimators using Local Neumann‐Problems
Authors: Stein E. , Ohnimus S. , Walhorn E. 
Published: March 2011

AbstractEquivalent to the direct Residual Error Method (REM), the Posterior Equilibrium Method (PEM) is presented for absolute global error estimates, yielding an upper bound with application to elasticity and elastoplasticity. Furthermore, a local residual error estimator is treated, solving local Neumann‐problems.
Mortar Finite Element Methods for Discontinuous Coefficients
Authors: Wohlmuth Barbara 
Published: March 2011

AbstractIn this paper, we consider a mortar finite element method for second order elliptic boundary value problems with discontinuous coefficients. At the interface where the coefficient is discontinuous, different triangulations and/or discretizations are coupled by means of Lagrange multipliers. The numerical algorithm is based on the algebraic saddle point formulation. The coupling of P1 conforming finite elements with P1 conforming and nonconforming Crouzeix‐Raviart elements as studied. Finally, we present the performance of the adaptive refinement process.
Stability of Levitrons
Authors: Dullin Holger R. , Easton Robert W. 
Published: March 2011

AbstractThe Levitron is a magnetic spinning top which can levitate in the constant field of a repelling base magnet. The present work develops an exact theory of the Levitron with six degrees of freedom. The stability of the symmetric relative equilibrium is calculated. The main result is a complete classification of possible Levitrons that allow for an interval of stable spin rates. Stability of the relative equilibrium is lost in Hamiltonian Hopf bifurcations if either the span rate is too large or too small.
Lösungsverhalten der FEM für das ebene Couette‐ und Poiseuille‐Problem eines Oldroyd‐B Fluides
Authors: Reichenbach J. , Aksel N. 
Published: March 2011

AbstractEs wird ein viskoelastisches Fluid betrachtet, das durch ein Oldroyd‐B‐Modell beschrieben wird. Am Beispiel ebener Schichtenströmungen (Couette‐ und Poiseuille‐Strömung) wird das Verhalten der Galerkin‐FEM untersucht. Der Schlüssel zur Charakterisierung des Lösungsverhaltens des entstehenden nichtlinearen Gleichungssystems ist die Matrix der partiellen Ableitungen (Jacobi‐Matrix). Während für das Couette‐ und Poiseuille‐Problem die bekannten Lösungen reguläre Lösungen des stetigen Dgl. problems sind, besitzen die durch FEM erhaltenen diskreten Systeme für größere Werte der Deborah‐Zahl De stets singuläre Lösungen. Für das Poiseuille‐Problem kann mit wachsender De‐Zahl ein oszillatorisches Verhalten der Fehler der diskreten Näherungen festgestellt werden, dessen genaue Ursache bisher nicht bekannt ist.
A thermodynamic approach to elastic and plastic anisotropy
Authors: Svendsen B. 
Published: March 2011

AbstractIn the framework of a recent thermodynamic formulation of rate‐independent material behaviour as based on a loading rate function, possible effects of the plastic deformation on elastoplastic material behaviour at large deformation are discussed. In particular, attention is focused on the possible effect that such deformation may have on the material symmetry, i.e., anisotropy, of the material behaviour.
Sekundärwirbel in Nachlauf‐ und freien Scherschichten in der Transition
Authors: Brede Martin 
Published: March 2011

AbstractIn freien Scherschichten werden bei der Transition zur Turbulenz in Strömungsrichtung orientierte Sekundärwirbel beobachtet, die deutlich kleiner sind als die Primärwirbel. Die gleichen Strukturen zeigen sich im Zylindernachlauf. Hier treten Sekundärwirbel zusätzlich zu den Kármánwirbeln auf. In beiden Fällen sind die Proportionen der Primär und Sekundärwirbel ähnlich. Experimentelle und numerische Ergebnisse aus Nachlauf‐ und freien Scherschichten werden vorgestellt, um die Übereinstimmungen und Unterschiede der Wirbelstrukturen zu demonstrieren. Zwei Wirbelmodelle werden aus einer experimentellen Untersuchung des Kreiszylindernachlaufs abgeleitet, die die grundlegenden Mechanismen der Transition und die beteiligten Instabilitäten beider Strömungsformen aufzeigen.
Über die Strouhal‐Reynoldszahl‐Gesetzmäßigkeit im Nachlauf eines Kreiszylinders
Authors: Fey U. , Eckelmann H. 
Published: March 2011

AbstractAufgrund der Ergebnisse umfangreicher Messungen im Nachlauf von Kreiszylindern wird eine neue Gesetzmäßigkeit für die Wirbelablösefrequenz vorgeschlagen, die erstmals in geschlossener Form die Strouhalzahl Sr als Funktion der Reynoldszahl vom Beginn der Wirbelablösung bei Re = 47 bis zum laminar‐turbulenten Umschlag der Zylindergrenzschicht bei Re > 2 × 105 mit wenigen Koeffizienten als Sr = Sr* + m/ √ beschreibt. In dieser Darstellung zeigen sich mit steigender Reynoldszahl die unterschiedlichen Wirbelbildungsprozesse durch stückweise lineare Zusammenhänge zwischen Sr und dern Kontrollparameter 1/ √ Re Die ‘kritischen’ Reynoldszahlen sind gekennzeichnet durch Schnittpunkte der Geraden. Die vorgeschlagene Gesetzmäßigkeit ist in guter Übereinstimmung mit neueren Messungen anderer Autoren und es zeigt sich, daß für Re > 5000 eine zunehmende Abweichung von der Faustregel 'Sr ≈ 0,21' auftritt.
Enclosing the solution of linear Fredholm integral equations of the second kind
Authors: Gienger A. 
Published: March 2011

AbstractWe propose a new method to enclose the solution X of a linear Fredholm integral equation of the second kind. Given a certain tolerance ϵ we compute an approximation ẍ for X so that || X — ẍ ||∞ < ϵ holds. The method uses a very efficient algorithm to determine ẍ and interval arithmetic to compute an upper bound for ||x ẍ||∞ On some numerical examples we demonstrate the efficiency of our method.
Globale Optimierung nichtdifferenzierbarer Funktionen unter Einsatz von Intervallsteigungen
Authors: Ratz Dietmar 
Published: March 2011

AbstractVon großer Bedeutung für die Effizienz von Intervallverfahren zur globalen Optimierung sind Techniken zur Elimination von Teilen des Suchbereichs. Hier finden häufig Techniken unter Verwendung von Ableitungen (wie z. B. der Monotonietest) Anwendung. Für nichtdifferenzierbare Probleme kann eine entsprechende Technik unter Verwendung von Intervallsteigungen angegeben werden. Wir beschreiben eine solche Pruning‐ Technik zur Elimination von Teil‐bereichen. Zur Verdeutlichung der Effizienz dieser Technik vergleichen wir zwei Varianten eines Modell‐Algorithmus ausgestattet mit Monotonietest bzw. Pruning‐Schritt anhand von differenzierbaren Beispielen. Abschließend dokumentieren wir dann numerische Resultate für ein nichtdifferenzierbares Optimierungsproblem.
On the relevance of “molecular chaos” for granular flows
Authors: Luding Stefan 
Published: March 2011

AbstractWith numerical simulations of dissipative, spherical particles the assumption is tested, that the velocities and the positions of colliding particles are uncorrelated (“molecular chaos”). This assumption is a basic ingredient of all theoretical approaches based on a kinetic theory or a pseudo‐Liouville operator formalism. The numerical model is an event‐driven method for the simulation of rigid spherical particles in two dimensions. In elastic systems, the impact parameter is uniformly distributed or, with other words, the molecular chaos assumption is valid independent of the density. In freely cooling systems, the molecular chaos assumption fails as soon as dissipation becomes strong enough so that convective shear modes are created.
Second‐order and affine estimates for the behavior of viscous polycrystals
Authors: M. Bornert , R. Masson , Castañeda P. Ponte 
Published: March 2011

AbstractThe recently proposed second‐order and affine estimates for the effective behavior of nonlinear heterogeneous materials rely on the same linearization procedure and refer to some “thermoelastic comparison composite”. But both approaches differ in the way the overall nonlinear answer is defined, either through average stress‐strain relations or energy considerations. When combined with the linear self‐consistent model, they provide new tools for the predictions of the behavior of viscoplastic polycrystals (steady state creep). They are compared to each other and to other more classical estimates (incremental and tangent) in the following situations.: a 2D antiplane model problem for which efficient rigorous upper bounds are available, the creep of isotropic aggregates of FCC crystals and HCP crystals with different critical stresses on the basal and prismatic slip systems.
Dynamic Analysis of Rolling Contact Problem by the Space‐Time Element Method
Authors: R. Bogacz , C. Bajer 
Published: March 2011

AbstractHigher speed of rolling and heavy loads that subject the contact zone in rail‐wheel contact require dynamic analysis with wave effects taken into account. In such a case we can observe polygonization and corrugation of wheels and rails. This phenomenon occurs and increases in nature with the increase of speed and load. Even the steady motion of the roller center along the flat surface without friction results in oscillation of the contact force. Its amplitude increases with the speed while the number of corrugations decreases in the same time. The simulation showed the rotation of the wheel material around the axle. The space‐time element method was applied to refine the time step of integration only for elements being in contact. The spatial refinement allows to determine precisely the contact zone.
Contact in Multibody Systems
Authors: Bremer H. , Ch Glocker 
Published: March 2011

AbstractMulti Body Systems (MBS) consist of a finite number of interconnected elastic and/or rigid bodies. If its number of d.o.f. varies due to contact it is advantageous to calculate dynamics in terms of subsystems. Partitioning into subsystems is fluent but at least reasonable at points of possible contact. Corresponding equations are then suitably structurized in terms of element matrices yielding a (state dependent) Finite Element structure. When contact(s) take place one ist left either with a (often unsolvable) combinatorical problem or with the question of how to solve the contact case uniquely and in an optimal way. The access to the latter is obtained with Helmholtz 's auxiliary equation leading to Gauss ' principle of least constraints.
Vibration Control by Interface Dissipation in Semi‐Active Joints
Authors: Gaul L. , Nitsche R. 
Published: March 2011

AbstractLarge space structures often consist of truss elements which are connected by bolted joints. The interface dissipation in these junctions turns out to be the major source of damping. For vibration suppression, the joints are designed such that the normal force in the frictional interface is controlled for improving damping performance. The norma1 contact is varied by active control using a bolt with a piezoelectric washer. A system consisting of two elastic beams connected by a single active joint is considered. The friction phenomena are described by a friction model with velocity dynamics. The effect of displacement‐ and velocity‐induced friction dynamics is considered in the design of the control law. The result is a dynamic controller which prevents frictional energy, stored as potential energy in the bristles, from being returned into the system.
Phonon contribution to friction stress in an atomically flat contact of crystalline solids at low temperature
Authors: Popov V. 
Published: March 2011

AbstractIt has recently been shown both experimentally and theoretically that there is no static friction in a contact of atomically flat crystalline solids provided the periods of their lattices are incommensurate and their interaction does not exceed some critical value. In dielectric materials, the only mechanism of friction in this case is generation of phonons. In the present paper, it is shown that, at low temperatures, the phonon contribution to the friction force can be very small by virtue of the quantum mechanical nature of the elementary excitations in a solid. Incommensurate dielectric crystals could therefore slip at low temperatures practically without friction.
Wave interaction with unilateral and adhesive cracks. Application on impact‐echo
Authors: Stavroulakise G. 
Published: March 2011

AbstractBreathing, unilateral cracks close under the influence of a compressive loading. Nonsmooth mechanics' tools are used for the study of unilateral, frictional and adhesive cracks. In this framework, a frictionless unilateral crack leads to a variational inequality or to a linear complementarity problem, while frictional or adhesive problems lead to nonconvex variational or hemivariational inequalities. The numerical solution is based here on a convolutional boundary element method. An application on impact‐echo modeling and the influence of unilateral cracks on the classical impact‐echo technique are presented.
Wave propagation in complex structures modelled by medium with internal variables
Authors: Belyaevk A. 
Published: March 2011

AbstractGoverning equations for complex engineering structures are derived by means of the Hamilton variational principle under the assumption that the vibration field has a slowly varying component of an evolutionary type. This function is identified as the displacement of the primary structure, whereas the internal degrees of freedom model substructural vibrations. In the frequency domain of high modal overlap the secondary systems act as dynamic vibration absorbers with respect to primary structure. Wave propagation in the primary structures of complex structure is shown to be modelled by hyperbolic equations with source terms, the latter describing the backward effect of vibrating substructures on the primary structure. In order to take into account uncertain mechanical and spectral properties of real structures propagation of a wave packet in the structure is considered. The evolution of the amplitude of the propagating wave packet is shown to be governed by a boundary value problem of parabolic type. The approach developed presents an alternative to the vibrational conductivity approach and other techniques utilising various sorts of parabolic equations for describing energy propagation in complex structures.
Control of Propagating Waves in Rods by Piezoelectric Actuators and Sensors
Authors: Kurt Schlacher , Andreas Kugi 
Published: March 2011

AbstractThis paper deals with active control of propagating waves in rods. The mechanical models under consideration are characterized by hyperbolic equations with source terms. The latter ones describe the control or disturbance inputs. Piezoelectric layers are used as actuators and sensors to affect the dynamic behavior of the system, The controller design is based on the theory of Lagrangian systems with inputs. One can show that a special pairing of the actuators and sensors simplifies the controller design significantly, but this approach requires a specific design of the actuators and sensors. Smart structures based on piezoelectricity allow to construct spatially distributed devices, which are the prerequisites for the realization of so designed control laws.
On the Influence of the Matrix Compressibility in Two‐Phase Models
Authors: Diennels S. 
Published: March 2011

AbstractIn the theory of porous media the volume fractions are introduced as internal variables. Therefore, additional evolution equations for the volume fractions are required an general. In the present contribution, a binary model consisting of a compressible porous skeleton saturated by a compressible pore fluid is formulated. From this general model, the so‐called hybrid model and the incompressible model are derived. The different behavior of the models is illustrated in two examples.
On Porous Soil Materials Saturated with a Compressible Pore‐fluid Mixture
Authors: Ehlers W. , Blome P. 
Published: March 2011

AbstractGeomaterials can be understood as consisting of a porous solid skeleton, with the pores saturated by a fluid. The fluid may be water, air or, a water‐air mixture. The mechanical behaviour of the whole soil body, consisting of the soil skeleton and the pore‐fluid, is governed by the properties of its constituents. Since the exact structure of the pore‐space is not known, a homogenization over a representative elementary volume is required in order to obtain a macroscopic approach. The description of the motion of a multiphase‐mixture of immiscible constituents can be performed by the Theory of Porous Media (TPM). Based on the classical mixture theory, the TPM deals with superimposed continua. Additionally, the information of the structure is included via the concept of volume fractions [1–3].Here, the soil skeleton material is assumed to be incompressible, whereas the pore‐fluid mixture is compressible according to its composition of water and air by variable volume fractions. The pressure‐density‐function of the pore‐fluid is derived within the framework of the TPM. Furthermore, the viscous pore‐fluid allows for a linear momentum exchange (interaction force) between the constituents, which, under special assumptions, leads to the well‐known Darcy law.
Study of structure of porous biomaterials. Application of ultrasonic method
Authors: Kubik J. , Kaczmarek M. , Pakula M. 
Published: March 2011

Abstractlt is commonly accepted that, in modelling of the mechanical behavior of biological tissues (e.g. cortical and trabecular bones) and porous bone implants “working” in their natural environment, one should take into account the fact that such biomaterials have a multiphase nature and they are composed of solid porous matrix filled with interstitial fluid. Spatial arrangement of matrix structure strongly influences mechanical properties, transport phenomena, and acoustic waves propagating in saturated biomaterials.The well known two‐phase model of dynamics of saturated porous materials developed by Biot, [1], and extended by other authors includes three pore structure macro‐parameters (porosity, permeability, and tortuosity). The attenuation and dispersion of wave velocity predicted by the model results mainly from relative motion of phases.On the other hand, a disease like osteoporosis is identified in both cortical and trabecular bone by an increase of pore space and related changes in the microscopic pore characteristic that does not appear in Biot's model. Such micro‐quantity (characteristic pore or grain size) plays an important role for scattering phenomena at microinhomogeneities in the two‐phase porous materials when the ratio of waves length and average size of pores / grain is not too large.Taking these facts into account, and using the model which incorporates the relative motion of phases and scattering effects, both the micro‐ and macro‐structure parameters an be determined from wave propagation analysis using the ultrasonic technique and applying wide band spectroscopy.