Tribological research on the development of White Etching Cracks (WECs)

The aim of the presented research activities was to identify mechanical, thermal, and chemical factors possibly linked to the formation of WECs (White Etching Cracks). By means of a systematic variation of various influencing parameters, the significance of each of those was investigated. It is hoped that, once the parameters promoting WECs have been identified, the physical and chemical mechanisms responsible for WEC can be thoroughly understood in the near future. This would allow to prevent costly premature bearing failures, e.?g. given in wind turbines.Four research centers in Kaiserslautern, Münster and Hannover (Institute of Machine Elements, Gears and Transmissions (MEGT), Technische Universität Kaiserslautern, Materials Testing Group (AWP), Technische Universität Kaiserslautern, Institute of Physics, Westfälische Wilhelms-Universität Münster, and Institute for Machine Design and Tribology (IMKT), Leibniz University Hannover) provide their expertise and laboratory facilities for this purpose.At IMKT full bearing investigations with cylindrical roller thrust bearings and model tests with a special ring-roller-ring tribometer were performed under varied test conditions. In a theoretical work package, the stresses induced to the bearing surface were simulated.At “Physikalisches Institut” in Münster surfaces of the bearing washers (and rolling elements) were analysed applying Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) to determine the actual respective chemical composition of the tribofilms.At MEGT component tests were carried out on a three-axis dynamic test rig with radially loaded cylindrical roller bearings. The internal bearing dynamics and, above all, the associated distribution of frictional energy were analysed by means of multi-body simulation.The AWP concentrated on synthesized multi-axial dynamic stresses aimed at mirroring the stresses in a rolling contact apllied on laboratory test specimens to investigate crack initiation and growth in depth.     

Mass balance model of Lake Volta fisheries: The use of Ecopath model

A mass balance model of trophic interactions among ten key functional producer and consumer groups in Lake Volta was constructed using the Ecopath model to study the energy flows and species interactions in the lake. The present study was based on secondary and primary data on fish catch, diet composition, phytoplankton and zooplankton biomasses, collected in 2015 and 2016. Additional information on growth parameters of major species required for balancing the Ecopath model was obtained from sampling and FishBase. The functional groups were detritus, phytoplankton, zooplankton, benthos, prey fish, Tilapia, Bagrus, Chrysichthys, Alestes and Synodontis species. Four trophic levels were identified in the Lake Volta ecosystem, with the energy flow occurring mainly within the first three trophic levels. The calculated ecotrophic efficiency value of the primary producers (phytoplankton: 0.17; detritus: 0.22) indicated they were least exploited, compared to the secondary producers, zooplankton (0.80) and benthos (0.50). All secondary consumers had ecotrophic efficiency values higher than the primary producers, indicating they are exploited in the ecosystem. The main energy flows in the lake were from phytoplankton and detritus at trophic level I, and Bagrus species, the top predator, at a level of 3.30. The network analysis, illustrating a connectance index of 0.43 and an omnivory index of 0.06, in the lake system indicated the ecosystem is unstable, somewhat immature and still in a developing stage.     

Replacing the ({beta}{alpha})-unit 8 of E.coli TIM with its chicken homologue leads to a stable and active hybrid enzyme

In order to investigate how structural modifications interferewith protein stability, we modified a (ß)-unit inE.coli triosephosphate isomerase (TIM), a typical (ß)-barrelprotein, assuming that the pseudosymmetrical ß-barrelcan be divided into eight successive loop/ß-strand/loop/-helixmotifs. We replaced the eighth (ß)-unit of E.coliTIM with the corresponding chicken (ß)-unit. The substitution,involving the replacement of 10 of the 23 residues of this (ß)-unit, was evaluated first by modelling, then experimentally.Modelling by bomology suggests how the amino add replacementsmight be accommodated in the hybrid E.coli/chicken TIM (ETCM8CHI).Both natural and hybrid recombinant TIMs, overproduced in E.coli,were purified to homogeneity and characterized as to their stabilityand kinetics. Our kinetic studies show that the modificationperformed here leads to an active enzyme. The stability studiesindicate that the stability of ETIM8CHI is comparable to thatof the wild type TIM.     

Baseline layout and design of a 0.8 GW reference wind farm in the North Sea

The paper presents a model of a reference wind farm. The model considers the wind and wave climatologies for a specific site from which two different wind farm layouts are derived. These layouts are examined through the effective wake‐enhanced turbulence intensity at the hub height for a given climatology, and a simple model for the influence on capital expenditures is proposed. An electrical design is presented, the cable losses are calculated and the energy yield is determined. An operation and maintenance model is established, and the associated operating expenditure is obtained. All of the models are then summarized in terms of a levelized cost of energy using a numerical simulation tool, which allows the layouts to be compared. The data and models are freely available online for others to use and may serve as a baseline for benchmarking and allow researchers to compare and discuss their results. Copyright © 2017 John Wiley & Sons, Ltd.     

Semantic acid properties in multidatabases using remote procedure calls and update propagations

Global ACID properties (Atomicity, Consistency, Isolation and Durability) may be implemented by using a DDBMS (Distributed Data Base Management System.) However, in this solution data availability is low. Further, data may be blocked, i.e. if some data are locked from a remote location, the data cannot always be unlocked when the connection to the data fails. This is not a problem when client/server technology is used because client/server technology only uses local locks, a reason why multidatabases and client/server technology are widely used in real-life distributed systems. However, the trouble with such systems is that they have no inherent global ACID properties. The objective of this paper is to illustrate how global semantic ACID properties, enforced by the transactions themselves, may be implemented on top of client/server technology. This is done to preserve high data availability. The global atomicity property is implemented by using retriable and compensatable subtransactions. The global consistency property must be implemented by the transactions themselves. The global isolation property is implemented by using countermeasures to isolation anomalies. The global durability property is implemented by using the durability property of the local DBMSs. The largest bank in Denmark, Den Danske Bank, has implemented all its applications using methods described in this paper. © 1998 John Wiley & Sons, Ltd.     

MgH2–Nb2O5 investigated by in situ synchrotron X-ray diffraction

In situ real time synchrotron radiation powder X-ray diffraction (SR-PXD) experiments are utilized to study changes in the crystalline compounds under dynamic hydrogenation and dehydrogenation reactions of MgH₂ ball milled with 8 mol% Nb₂O₅. The ball milling conditions were systematically varied to prepare three samples with different reactivity. Up to eight full cycles of hydrogen release and uptake were investigated for each sample, which reveal that Nb₂O₅ reacts with Mg forming a ternary oxide, Mg_xNb_1−xO. The PXD data for the ternary oxide is similar to that observed for the isostructural compounds MgO and NbO although shifted to lower Bragg diffraction angles revealing an expansion of the unit cell. Rietveld refinements suggest that Mg_xNb_1−xO has a limiting composition of x ∼ 0.6 after eight cycles of hydrogen release and uptake. At elevated temperatures Nb(II) is reduced to metallic Nb(0) and extracted from the ternary oxide and forms in a reaction with Mg. This work suggests that a ternary solid solution Mg_xNb_1−xO is the active material responsible for the prolific kinetic properties for the additive Nb₂O₅. Mg_0.6Nb_0.4O has a ∼4.6% larger unit cell volume as compared to the binary oxides, MgO and NbO, which may lead to formation of cracks and hydrogen diffusion pathways in dense magnesium oxide surface layers.     

Fatigue distribution optimization for offshore wind farms using intelligent agent control

A novel control approach is proposed to optimize the fatigue distribution of wind turbines in a large‐scale offshore wind farm on the basis of an intelligent agent theory. In this approach, each wind turbine is considered to be an intelligent agent. The turbine at the farm boundary communicates with its neighbouring downwind turbines and organizes them adaptively into a wind delivery group along the wind direction. The agent attributes and the event structure are designed on the basis of the intelligent agent theory by using the unified modelling language. The control strategy of the intelligent agent is studied using topology models. The reference power of an individual wind turbine from the wind farm controller is re‐dispatched to balance the turbine fatigue in the power dispatch intervals. In the fatigue optimization, the goal function is to minimize the standard deviation of the fatigue coefficient for every wind turbine. The optimization is constrained such that the average fatigue for every turbine is smaller than what would be achieved by conventional dispatch and such that the total power loss of the wind farm is restricted to a few percent of the total power. This intelligent agent control approach is verified through the simulation of wind data from the Horns Rev offshore wind farm. The results illustrate that intelligent agent control is a feasible way to optimize fatigue distribution in wind farms, which may reduce the maintenance frequency and extend the service life of large‐scale wind farms. Copyright © 2012 John Wiley & Sons, Ltd.