University of Paderborn

About: University of Paderborn is a education organization based out in Paderborn, Nordrhein-Westfalen, Germany. It is known for research contribution in the topics: Computer science & Context (language use). The organization has 6684 authors who have published 16929 publications receiving 323154 citations.

Shape-optimized mesh partitioning and load balancing for parallel adaptive FEM

Abstract: We present a dynamic distributed load balancing algorithm for parallel, adaptive Finite Element simulations in which we use preconditioned Conjugate Gradient solvers based on domain-decomposition The load balancing is designed to maintain good partition aspect ratio and we show that cut size is not always the appropriate measure in load balancing Furthermore, we attempt to answer the question why the aspect ratio of partitions plays an important role for certain solvers We define and rate different kinds of aspect ratio and present a new center-based partitioning method of calculating the initial distribution which implicitly optimizes this measure During the adaptive simulation, the load balancer calculates a balancing flow using different versions of the diffusion algorithm and a variant of breadth first search Elements to be migrated are chosen according to a cost function aiming at the optimization of subdomain shapes Experimental results for Bramble's preconditioner and comparisons to state-of-the-art load balancers show the benefits of the construction

Elastic-plastic fracture behavior of an aluminum alloy under mixed mode loading

Abstract: M ixed mode elastic-plastic fracture tests were carried out by using A5083-O aluminum alloy compacttension-shear specimens, in which the angle between the loading axis and the crack surface was varied from 90° (mode I) to 0° (mode II) by employing a special loading device. A fractographic investigation was made at various stages of crack tip blunting and stable crack growth. Under mixed mode loadings, opposite deformations appeared at the crack tip: sharpening and blunting. With increasing mode II component, cracks due to shear type fracture initiated at the sharpened corner of the crack tip near the surfaces of the specimen, and then another crack due to dimple type fracture occurred at the blunted corner of the crack tip near the midthickness of the specimen. These experimental results were explained qualitatively by taking account of the stress and strain singularities and triaxial stress distributions near the surface and midthickness of the specimen. Fracture mechanics parameters appropriate for mixed mode elastic-plastic fractures were also studied based on the experimental data.

Frequency tuning of piezoelectric energy harvesters by magnetic force

Abstract: A piezoelectric energy harvester is an electromechanical device that converts ambient mechanical vibration into electric power. Most existing vibration energy harvesting devices operate effectively at a single frequency only, dictated by the design of the device. This frequency must match the frequency of the host structure vibration. However, real world structural vibrations rarely have a specific constant frequency. Therefore, piezoelectric harvesters that generate usable power across a range of exciting frequencies are required to make this technology commercially viable. Currently known harvester tuning techniques have many limitations, in particular they miss the ability to work during harvester operation and most often cannot perform a precise tuning. This paper describes the design and testing of a vibration energy harvester with tunable resonance frequency, wherein the tuning is accomplished by changing the attraction force between two permanent magnets by adjusting the distance between the magnets. This tuning technique allows the natural frequency to be manipulated before and during operation of the harvester. Furthermore the paper presents a physical description of the frequency tuning effect. The experimental results achieved with a piezoelectric bimorph fit the calculated results very well. The calculation and experimental results show that using this tuning technique the natural frequency of the harvester can be varied efficiently within a wide range: in the test setup, the natural frequency of the piezoelectric bimorph could be increased by more than 70%.

Tuning of Copper(I)–Dioxygen Reactivity by Bis(guanidine) Ligands

Abstract: A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)-, bis(dipiperidinoguanidino)-, and bis(dimethylpropyleno)propane (btmgp, DPipG 2 p and DMPG 2 p, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG 2 p), which has both N a m i n e atoms of each guanidine functionality connected by a short ethylene bridge, as a member. From this series, a family of novel bis(guanidine)copper(i) compounds - [Cu 2 (btmgp) 2 ][PF 6 ] 2 (1), [Cu 2 (DPipC 2 p) 2 ](PF 6 ] 2 (2), [Cu2(DMPG 2 p) 2 ][PF 6 ] 2 (3), and [Cu 2 (DMEG 2 p) 2 ][PF 6 ] 2 .2MeCN (4) - has been synthesised. Single-crystal X-ray analysis of 1-4 demonstrated that these compounds contain dinuclear complex cations that contain twelve-membered heterocyclic Cu 2 N 4 C 6 rings with the Cu atoms being more than 4 A apart. Each copper atom is surrounded by a set of two N-donor functions from different ligands, resulting in linear N-Cu-N coordination sites. Depending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller-like twists of the amino groups around their N-C i m i n e bonds. The influence of these groups on the reactivity of the corresponding complexes 1-4 with dioxygen was investigated at low temperatures by means of UV/Vis spectroscopy. The reaction products can be classified into μ-η 2 :η 2 -peroxodicopper(II) or bis(μ-oxo)dicopper(III) complex cations that contain the [Cu 2 O 2 ] 2 + core portion as different isomers. The electronic properties of the specific bis(guanidine) ligands are discussed from the viewpoint of their σ-donor and π-acceptor capabilities, and it is shown that μ-η 2 :η 2 -peroxodicopper(II) complexes are stabilised relative to the bis(μ-oxo)dicopper(III) ones if π conjugation within the guanidine moieties is optimised.

Demonstration of coherent time-frequency Schmidt mode selection using dispersion-engineered frequency conversion

Abstract: Time-frequency Schmidt (TFS) modes of ultrafast quantum states are naturally compatible with high-bit-rate integrated quantum communication networks. Thus they offer an attractive alternative for the realization of high-dimensional quantum optics. Here, we present a quantum pulse gate based on dispersion-engineered ultrafast frequency conversion in a nonlinear optical waveguide, which is a key element for harnessing the potential of TFS modes. We experimentally retrieve the modal spectral-temporal structure of our device and demonstrate a single-mode operation fidelity of 80%, which is limited by experimental shortcomings. In addition, we retrieve a conversion efficiency of 87.7% with a high signal-to-noise ratio of 8.8 when operating the quantum pulse gate at the single-photon level.