A novel finite element formulation for frictionless contact problems

This article advocates a new methodology for the finite element solution of contact problems involving bodies that may undergo finite motions and deformations. The analysis is based on a decomposition of the two-body contact problem into two simultaneous sub-problems, and results naturally in geometrically unbiased discretization of the contacting surfaces. A proposed two-dimensional contact element is specifically designed to unconditionally allow for exact transmission of constant normal traction through interacting surfaces.     

A Newton-squaring algorithm for computing the negative invariantsubspace of a matrix

By combining Newton's method for the matrix sign function with a squaring procedure, a basis for the negative invariant subspace of a matrix can be computed efficiently. The algorithm presented is a variant of multiplication-rich schemes for computing the matrix sign function, such as the well-known inversion-free Schulz method, which requires two matrix multiplications per step. However, by avoiding a complete computation of the matrix sign and instead concentrating only on the negative invariant subspace, the final Newton steps can be replaced by steps which require only one matrix squaring each. This efficiency is attained without sacrificing the quadratic convergence of Newton's method     

Frequency-domain order reduction methods applied to a hydro power system

Contents Two frequency-domain reduction (approximation) methods are introduced, for the purpose of applying them to high-degree transfer functions of single-input single-output linear time-invariant systems and obtain corresponding adequate reduced order models. These two reduction methods (i.e. Pade and Routh) are coupled with fitting of time moments and Markov parameters matching techniques to better evaluate the adequacy of the obtained reduced order model. The mentioned methods have been applied successfully to the 6th-order transfer function of a practical hydro power system.

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Anwendung von Methoden zur Reduzierung der Ordnung des Frequenzbereichs in einem hydraulischen Leistungs-system

Übersicht Es werden zwei Methoden zur Reduzierung und Annäherung des Frequenzbereichs zwecks ihrer Anwendung in Übertragungsfunktionen höherer Ordnung von Systemen mit einem Eingang und einem Ausgang vorgestellt. Dies führt zu entsprechenden Modellen von niedrigerer Ordnung. Diese zwei Methoden (Pade- und Routh-Methoden) werden miteinander gekoppelt zwecks besserer Ausrechnung der Angemessenheit der obengenannten Modelle. Beide Methoden wurden erfolgreich auf die Übertragungsfunktion 6. Ordnung eines Wasserkraft-Energieerzeugers angewandt.

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List of principal symbols ₁₁, ₁₃, ₂₁, ₂₃ linearized hydroturbine model parameters - H inertia constant - f frequency deviation - K integral control loop gain - P _g gate power - P _m mechanical power - P _L inoremental (step) load change - s Laplace operator - transient speed droop coefficient - permanent speed droop coefficient - _g gate servomotor time constant - _p pilot valve time constant - _r dashpot time constant - _w water time constant     

The distribution of the elastic strain-energy density at the crack tip for fracture modes I and II

The study of the distribution of the elastic-strain energy density at the tip of a crack presents a special interest for the understanding of the mechanism of fracture. In this investigation an exhaustive theoretical analysis of the distribution of the elastic-strain energy density at the tip of a crack for cracks in isotropic elastic media deformed under modes I and II was developed.This study was complemented by an experimental solution of this type of problem, based on the method of reflected caustics. According to this method the distribution of the elastic-strain energy density was evaluated along a circular closed curve, defining the singular core around the crack-tip, which was dominated by the singularity at the crack tip and whose diameter is very small, not exceeding a few millimeters. In this way the situation at the crack-tip singularity was directly depicted on the energy density distribution. Interesting results concerning the factors influencing the mode of the strain energy distribution around the crack and the type of the crack path were derived.

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Résumé L'étude de la distribution de la densité d'énergie de déformation élastique à l'extrémité d'une fissure présente une intéret particulier pour comprendre le mécanisme de la rupture. Dans cette étude, on a développé une analyse théorique exhaustive de la distribution de la densité d'énergie de déformation élastique à l'extrémité d'une fissure dans le cas de fissures situées dans un milieu élastique isotrope déformé selon les models I et II. Cette étude a été complétée par une solution expérimentale de ce type de problème basée sur la méthode des caustiques réfléchies. Selon cette méthode, la distribuiton de la densité d'énergie de déformation élastique est évaluée le long d'une courbe fermée circulaire définissant un noyau singulier autour de l'extrémité de la fissure qui est influencé par la singularité à l'extrémité de la fissure, et dont le diamètre est tres petit et n'excède pas quelques millimètres. Par cette voie, la situation à la singularité de lextrémité de la fissure a été dépeinte directement sur base de la distribution de densité d'énergie. Des résultats intéressants concernant les facteurs qui influencent le mode de distribution d'énergie de déformation au voisinage de la fissure ainsi que du type de chemin de fissuration ont pu être dérivés.

     

The effect of low‐filler volume fraction on the elastic modulus and thermal expansion coefficient of particulate composites simulated by a multiphase model

The aim of this investigation is to determine the effect of low‐filler volume fraction on the elastic modulus and the thermal expansion coefficient of particulate composites. In the theoretical part, theoretical model valid for low‐filler volume fractions is used to evaluate these two magnitudes. In the experimental part, low‐percentage filler contents of 3, 5, 7, and 10% are used. The density for these epoxy resin‐iron particle composites is also determined. At the same time, an attempt to explain some of the disagreements observed between theoretical values and experimental data on a qualitative basis is also made. This attempt is in part assisted by scanning electron microscopy (SEM) observations concerning structural inhomogeneities and fractographical data. The comparison of the theoretical values derived from the present model with experimental results and with theoretical values derived from other workers appears satisfactory in many cases, but in some others the discrepancies among them are considerable. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Toxicological and ecotoxic impact of secondary and tertiary treated sewage effluents

Secondary sewage effluents are discharged in significant quantities in aquatic environments delivering pollutants that were not removed during treatment; yet advanced treated effluents are not lacking of contaminants. In this study, biochemical biomarkers were measured in liver and kidney of rainbow trout (Oncorynchus mykiss) exposed to unchlorinated, chlorinated and tertiary treated secondary sewage effluents. In addition, organic matter, nitrogen and suspended solids were assayed, while a common bioassay, Daphnia magna 21d reproduction test was also applied in order to examine potential relation between the performed bioassay and the biomarkers. Processes using oxidative conditions, such as ozonation and chlorination, resulted in significantly increased breeding rate (up to 74%) of the organism. Biomarkers measurements incorporated the determination of total glutathione (GSH), glutathione S-transferases (GST), glutathione peroxidase (GPX), lipid peroxidation (LPO) and an innovative biomarker in such applications, haem peroxidase. In general, the response of biomarkers was dependent upon the treatment method and it was tissue specific. Secondary effluents inhibited liver GST and haem peroxidase, while GSH levels and LPO were significantly provoked in liver. Ozonation provoked hepatic peroxidation, in terms of haem peroxidase and LPO, and GST; while the protective (to Reactive Oxidant Species – ROS) GSH was depleted, suggesting extended ROS attack to the organism. Similar response of biomarkers (but to a lesser extend) was observed after exposure of trout to effluents submitted to both coagulation and ozonation, emphasizing the significance of removing the residual organic matter by other methods than oxidative ones. Ozonation also enhanced renal LPO and GPX; however the former employment of coagulation limited the peroxidation phenomena. Chlorination mainly affected the levels of total GSH in both tissues.     

Scalable Distributed Sensor Fault Diagnosis for Smart Buildings

The enormous energy use of the building sector and the requirements for indoor living quality that aim to improve occupants’ productivity and health, prioritize Smart Buildings as an emerging technology. The Heating, Ventilation and Air-Conditioning (HVAC) system is considered one of the most critical and essential parts in buildings since it consumes the largest amount of energy and is responsible for humans comfort. Due to the intermittent operation of HVAC systems, faults are more likely to occur, possibly increasing eventually building’s energy consumption and/or downgrading indoor living quality. The complexity and large scale nature of HVAC systems complicate the diagnosis of faults in a centralized framework. This paper presents a distributed intelligent fault diagnosis algorithm for detecting and isolating multiple sensor faults in large-scale HVAC systems. Modeling the HVAC system as a network of interconnected subsystems allows the design of a set of distributed sensor fault diagnosis agents capable of isolating multiple sensor faults by applying a combinatorial decision logic and diagnostic reasoning. The performance of the proposed method is investigated with respect to robustness, fault detectability and scalability. Simulations are used to illustrate the effectiveness of the proposed method in the presence of multiple sensor faults applied to a 83-zone HVAC system and to evaluate the sensitivity of the method with respect to sensor noise variance.      

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring

Here, a single‐step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self‐organized nanopillar structures on fused silica (SiO₂). The laser‐induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar‐like morphology, as well as the remarkable antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser‐processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s–p linearly polarized configurations. However, in the near‐infrared spectrum, the laser‐textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.