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Search Results: 1 - 10 of 4671 matches for " Wolfgang Witteveen "
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On the Modal and Non-Modal Model Reduction of Metallic Structures with Variable Boundary Conditions  [PDF]
Wolfgang Witteveen
World Journal of Mechanics (WJM) , 2012, DOI: 10.4236/wjm.2012.26037
Abstract: Vibration mode based model reduction methods like Component Mode Synthesis (CMS) will be compared to methods coming from control engineering, namely moment matching (MM) and balanced truncation (BT). Conclusions based on the theory together with a numerical demonstration will be presented. The key issues on which the paper is focused are the reduction of metallic structures, the sensitivity of the reduced model to varying boundary conditions, full system response, accurate statics and the possibility to determine “a priori” the number of needed modes (trial vectors). These are important topics for the use of reduction methods in general and in particular for the implementation of FE models in multi body system dynamics where model reduction is widely used. The intention of this paper is to give insight into the methods nature and to clarify the strengths and limitations of the three methods. It turns out, that in the considered framework CMS delivers the best results together with a clear strategy for an “a priori” selection of the modes (trial vectors).
Efficient Model Order Reduction for the Dynamics of Nonlinear Multilayer Sheet Structures with Trial Vector Derivatives
Wolfgang Witteveen,Florian Pichler
Shock and Vibration , 2014, DOI: 10.1155/2014/913136
Abstract: The mechanical response of multilayer sheet structures, such as leaf springs or car bodies, is largely determined by the nonlinear contact and friction forces between the sheets involved. Conventional computational approaches based on classical reduction techniques or the direct finite element approach have an inefficient balance between computational time and accuracy. In the present contribution, the method of trial vector derivatives is applied and extended in order to obtain a-priori trial vectors for the model reduction which are suitable for determining the nonlinearities in the joints of the reduced system. Findings show that the result quality in terms of displacements and contact forces is comparable to the direct finite element method but the computational effort is extremely low due to the model order reduction. Two numerical studies are presented to underline the method’s accuracy and efficiency. In conclusion, this approach is discussed with respect to the existing body of literature. 1. Introduction Some examples of permanent joints between two solid bodies are screwed or bolted joints, crimp connections, and spot-welded seams. The construction of such joints normally permits large relative displacements between the contacting bodies. This leads to a time invariant potential contact area , which is identical to the spatial distribution of the joint. However, due to small structural displacements there are areas inside the joint where the surfaces are in contact and other areas where the surfaces are not in contact. Consider The argument represents this structural displacement and indicates that the contacting and gapping areas are displacement dependent. In dynamic cases, this implies an indirect dependency on time. The state dependent contact forces, which avoid penetration, and the friction forces act between the two contacting surfaces. Such friction forces are a function of the local contact forces and the relative in-plane displacements of the contacting surfaces. Due to the joint’s construction, such relative displacements are small and are, therefore, often referred to as “microslip” or “small sliding” displacements. These friction forces lead to energy dissipation. In summary, it can be said that such joints lead to nonlinear stiffness and damping effects. The latter observations are well documented in the literature (see [1–7] for more detailed information on the subject of experiments and simulations). Depending on some parameters, such as the spatial extension of the joint, the aforementioned nonlinearities can have an
Pilot study on the Non-Invasive Detectability of Femoral Neck Fractures with Frequency Response Functions  [PDF]
Wolfgang Witteveen, Carina Wagner, Patrick Jachs, Stefan Froschauer, Harald Sch?ffl
World Journal of Mechanics (WJM) , 2014, DOI: 10.4236/wjm.2014.47022
Abstract:

A suspicion of a femoral neck fracture is a frequently recurring situation, especially in nursing homes. For the clarification of such a suspicion normally imaging techniques are used. Such equipment is expensive and therefore is located in hospitals. In addition to the costs, a transport causes stress for the patient. This pilot study is devoted to the question whether the detection of a femoral neck fracture with vibration measurements is possible in principal. In such a case, the clarification could be done on-site by an ordinary person using much cheaper equipment. This would reduce the stress for the patient and save money. For this purpose vibration measurements on a dead body with intact, with partially fractured and with complete cut femoral neck have been performed. Two different methods for the vibration initiation have been investigated, the so called impact testing and the shaker testing. The frequency response function has been determined for all combinations on both sides of the body. It turned out that there is a clear difference in the frequency response functions of the fractured bone with respect to the intact bone when shaker testing is used. This indicates that the method could have the potential to be a cost-saving alternative to imaging techniques. However, in a next step a statistically reliable clinical survey on living persons needs to be done.

Learning about “wicked” problems in the Global South. Creating a film-based learning environment with “Visual Problem Appraisal”
Loes Witteveen,Rico Lie
MedieKultur : Journal of Media and Communication Research , 2012,
Abstract: The current complexity of sustainable development in the Global South calls for the design of learning strategies that can deal with this complexity. One such innovative learning strategy, called Visual Problem Appraisal (VPA), is highlighted in this article. The strategy is termed visual as it creates a learning environment that is film-based. VPA enhances the analysis of complex issues, and facilitates stakeholder dialogue and action planning. The strategy is used in workshops dealing with problem analysis and policy design, and involves the participants “meeting” stakeholders through filmed narratives. The article demonstrates the value of using film in multi stakeholder learning environments addressing issues concerning sustainable development.
On the cost-complexity of multi-context systems
Peter Novák,Cees Witteveen
Computer Science , 2014,
Abstract: Multi-context systems provide a powerful framework for modelling information-aggregation systems featuring heterogeneous reasoning components. Their execution can, however, incur non-negligible cost. Here, we focus on cost-complexity of such systems. To that end, we introduce cost-aware multi-context systems, an extension of non-monotonic multi-context systems framework taking into account costs incurred by execution of semantic operators of the individual contexts. We formulate the notion of cost-complexity for consistency and reasoning problems in MCSs. Subsequently, we provide a series of results related to gradually more and more constrained classes of MCSs and finally introduce an incremental cost-reducing algorithm solving the reasoning problem for definite MCSs.
Higher Period Stochastic Bifurcation of Nonlinear Airfoil Fluid-Structure Interaction
Jeroen A. S. Witteveen,Hester Bijl
Mathematical Problems in Engineering , 2009, DOI: 10.1155/2009/394387
Abstract: The higher period stochastic bifurcation of a nonlinear airfoil fluid-structure interaction system is analyzed using an efficient and robust uncertainty quantification method for unsteady problems. The computationally efficient numerical approach achieves a constant error with a constant number of samples in time. The robustness of the method is assured by the extrema diminishing concept in probability space. The numerical results demonstrate that the system is even more sensitive to randomness at the higher period bifurcation than in the first bifurcation point. In this isolated point in parameter space the clear hierarchy of increasing importance of the random nonlinearity parameter, initial condition, and natural frequency ratio, respectively, even suddenly reverses. Disregarding seemingly less important random parameters based on a preliminary analysis can, therefore, be an unreliable approach for reducing the number of relevant random input parameters.
Polynomial Chaos Expansion for general multivariate distributions with correlated variables
Maria Navarro,Jeroen Witteveen,Joke Blom
Mathematics , 2014,
Abstract: Recently, the use of Polynomial Chaos Expansion (PCE) has been increasing to study the uncertainty in mathematical models for a wide range of applications and several extensions of the original PCE technique have been developed to deal with some of its limitations. But as of to date PCE methods still have the restriction that the random variables have to be statistically independent. This paper presents a method to construct a basis of the probability space of orthogonal polynomials for general multivariate distributions with correlations between the random input variables. We show that, as for the current PCE methods, the statistics like mean, variance and Sobol' indices can be obtained at no significant extra postprocessing costs. We study the behavior of the proposed method for a range of correlation coefficients for an ODE with model parameters that follow a bivariate normal distribution. In all cases the convergence rate of the proposed method is analogous to that for the independent case. Finally, we show, for a canonical enzymatic reaction, how to propagate experimental errors through the process of fitting parameters to a probabilistic distribution of the quantities of interest, and we demonstrate the significant difference in the results assuming independence or full correlation compared to taking into account the true correlation.
Sequences and Limits  [PDF]
Wolfgang Mueckenheim
Advances in Pure Mathematics (APM) , 2015, DOI: 10.4236/apm.2015.52007
Abstract: It is widely held that irrational numbers can be represented by infinite digit-sequences. We will show that this is not possible. A digit sequence is only an abbreviated notation for an infinite sequence of rational partial sums. As limits of sequences, irrational numbers are incommensurable with any grid of decimal fractions.
Lectoraat professionalisering van de agogische beroepen en vaktherapeuten in de gezondheidszorg
Mari?l van Pelt,Hanneke van Wijgerden,Ursula Witteveen
Journal of Social Intervention : Theory and Practice , 2008,
Abstract:
Flaporhexis: Alternative Way to Make a Femtosecond Laser Corneal Flap  [PDF]
Wolfgang A. Pfaeffl
Surgical Science (SS) , 2013, DOI: 10.4236/ss.2013.43042
Abstract:

Flaporhexis, a technique to make the femtosecond laser corneal flap was developed primarily to reduce the mechanical trauma associated with separating the corneal tissue by taking advantage of the cornea’s natural anatomy and was first described in a peer reviewed artice in 2008. The anterior part of the cornea tends to be stronger, thereby allowing tearing (Greek “rhexis”) along the preformed cuts of the femtosecond photo disruption. Flaporhexis differs from Binder’s technique of “hinge opening as published in 2006 by opening the flap from the opposite side. If a femtosecond flap lift is performed correctly, the surgeon will find a virgin, uniform, dry stromal bed, which had not been manipulated by any surgical instrument and is therefore perfect for wavefront guided laser surgery as “Sub-Bowmans Keratomileusis”.

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