A Computational Study on Bounding the Makespan Distribution in Stochastic Project Networks

Due to the practical importance of stochastic project networks (PERT-networks), many methods have been developed over the past decades in order to obtain information about the random project completion time. Of particular interest are methods that provide (lower and upper) bounds for its distribution, since these aim at balancing efficiency of calculation with accuracy of the obtained information.We provide a thorough computational evaluation of the most promising of these bounding algorithms with the aim to test their suitability for practical applications both in terms of efficiency and quality. To this end, we have implemented these algorithms and compare their behavior on a basis of nearly 2000 instances with up to 1200 activities of different test-sets. These implementations are based on a suitable numerical representation of distributions which is the basis for excellent computational results. Particularly a distribution-free heuristic based on the Central Limit Theorem provides an excellent tool to evaluate stochastic project networks.     

Simulation of Rate Dependent Plasticity of Polymers

Polycarbonate is an amorphous polymer which exhibits nonlinear deformation before failure. It shows a pronounced strength-differential effect between compression and tension. Strain rate influences the mechanical response of the polycarbonate. In particular, the yield stress is increased with increasing strain rate. The concept of stress mode dependent weighting function is used in the proposed model to simulate the asymmetric effects for different loading speeds. In this concept, an additive decomposition of the flow rule is assumed into a sum of weighted stress mode related quantities. The characterization of the stress modes is obtained in the octahedral plane of the deviatoric stress space in terms of the mode angle, such that stress mode dependent scalar weighting functions can be constructed. The resulting evolution equations are updated using a backward Euler scheme and the algorithmic tangent operator is derived for the finite element equilibrium iteration. The numerical implementation of the resulting set of constitutive equations is used in a finite element program for parameter identification. The proposed model is verified by showing a good agreement with the experimental data. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vortex wake mitigation of an airfoil with winglets by active rudders

The wake vortex hazard and the required separation distances between aircraft at airports constitute an impediment to the growth of air traffic at major hubs. One approach to reduce separation distances is the exploitation of the inherent instability in vortex systems, leading to an accelerated breakdown. In this paper we show the impact of excitation by different control surfaces on the stability behavior of the wake of a rectangular wing with winglets. The experiment shows that the excitation through winglet rudders and ailerons yields the best results. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Efficient parallel simulation of biphasic materials in biomechanics

Biphasic materials are widely used for the modelling and the numerical simulation of articular cartilage within joints. In combination with contact, the numerical solution of the arising discrete systems is a demanding task, in particular for realistic geometries. We consider the stability and efficiency of different multilevel approaches for the numerical solution of the resulting non–smooth systems. Moreover, multilevel methods are derived which allow for the efficient numerical simulation of biphasic materials in contact. Numerical examples on problem specific geometries in three space dimensions are given. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A thermodynamic framework for coupled multiphase field and diffusion models for lower bainite transformation

The lower bainite transformation is characterized by a displacive transformation from austenite to bainitic ferrite and a subsequent separation of carbon within the new supersaturated phase. At accumulations of carbon carbides precipitate. To model this complex process a framework considering phase changes and carbon diffusion is required. In this work we present a thermodynamic framework based on the theory of microforce balances considering multiphase Ginzburg-Landau equations coupled with Cahn-Hilliard diffusion. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two accuracy improvements on nonuniform transformation field analysis for plasticity coupled to softening

A reduced order homogenization scheme for the case of plasticity coupled with softening effects is proposed. This is based on a straightforward extension of the so-called nonuniform transformation field analysis (NTFA, [2]). Two related new methods, denoted as uneven NTFA and adaptive NTFA accounting for accuracy improvements, are also presented, which are based on the ideas of parameter identification and adaptive modeling, respectively. A complementary numerical study is given. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)