Chalmers Research

    Impedance Analysis of Modular Multi-level Converters Connected to Weak AC Grids

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    This paper investigates the stability of a modular multi-level converter (MMC)-based HVDC system connected to a weak ac grid. Impedance analysis is used to study the interactions between the weak ac grid and the converter. For this, the impedance of the MMC is first derived considering various outer- and inner-loop controllers. The Nyquist-stability criterion is then used to analyze the stability of the MMC connected to the weak ac grid for various case studies. The impact of circulating current controller on the stability of the MMC connected to the weak ac grid is also validated for two different types of outerloop controllers. Finally, time-domain simulations are carried out using MATLAB/Simscape Power System toolbox to validate the analytical results

    Demonstration of post-growth wavelength control of VCSELs using high-contrast gratings

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    We demonstrate post-growth wavelength setting of vertical-cavity surface-emitting lasers (VCSELs) using high-contrast gratings (HCGs). By fabricating HCGs with different duty-cycle and period, the HCG reflection phase can be varied, in effect giving different optical cavity lengths for HCGVCSELs with different grating parameters. This enables fabrication of monolithic multi-wavelength HCG-VCSEL arrays for wavelength-division multiplexing (WDM). The GaAs HCG is suspended in air by selective removal of an InGaP sacrificial layer. Electrically injected 980-nm HCGVCSELs with sub-mA threshold currents indicate high reflectivity from the GaAs HCGs. Lasing over a wavelength span of 15 nm was achieved, enabling a 4-channel WDM array with 5 nm channel spacing. Device design, fabrication and experimental proof-of-concept are presented

    In vitro and in vivo degradation of myo-inositol hexakisphosphate by a phytase from Citrobacter braakii

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    Phytases (EC 3.1.3) are widely used in animal feed to increase the availability of phosphorus and decrease the anti nutritive effect of myo-inositol hexakisphosphate (InsP(6)). The aim of this work was to investigate the stereospecific degradation of InsP(6) in vitro and in vivo by a phytase from Citrobacter braakii (C. braakii), and to study gastric survival of the phytase as well as the site of action in the gastrointestinal tract. The in vitro results showed that the C. braakii phytase belongs to the group of 6-phytases (EC 3.1.3.26). However, in approximately one out of 10 instances the phytase initiated hydrolysis at the D-3 (L-1) position, demonstrating that phytase specificity is not unambiguous. Following the main degradation pathway, InsP(6) was degraded by stepwise removal of the phosphate groups on positions 6/1/5. The stereospecificity was found to be similar under in vitro and in vivo conditions. The phytase was found to be stable in the gastric environment and to be active in the stomach and possibly also in the proximal small intestine. While InsP(4) was accumulated under in vitro conditions this was not the case in vivo, where both InsP(5) and InsP(4) were seen to be hydrolysed in the small intestine, possibly as a combined action of the C. braakii phytase and endogenous phosphatases present in the mucosa. The ability of the C. braakii phytase to focus its activity on degrading InsP(6) to InsP(4) is believed to be a favourable complement to the endogenous phosphatases

    Predictive simulations of impurity transport at JET

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    Impurity transport in the Joint European Torus is analysed using the coupling between the transport codes JETTO (for main ions) and SANCO (for impurities) for predictive simulations of dedicated impurity injection experiments. The experimentally injected trace levels of Ne, Ar and Ni in a 2% C background are considered together with numerical experiments covering a broad range of impurities from Be to W. Transport coefficients due to Ion-Temperature-Gradient (ITG) mode and Trapped-Electron (TE) mode turbulence are used together with neoclassical transport from NCLASS. The transport coefficients obtained using the Chalmers fluid model are compared with gyrokinetic results using the code GENE. An updated, multiple ion species version of the Chalmers model called EDWM (Extended Drift Wave Model) has been used for the transport coefficients. Self-consistent simulations of electron and ion temperatures, main ion and impurity densities and toroidal momentum are performed. The role of neoclassical impurity transport is evaluated and the dependence of the simulated profiles on impurity charge number Z, collisionality, ExB shearing, rotation gradient (roto-diffusion), and impurity charge fraction (Zeff) are discussed. For the NBI heated JET L-mode discharges considered, the predictive simulations show that the impurity transport in the bulk of the plasma is dominated by ITG driven transport resulting in impurity peaking factors substantially below the neoclassical predictions for low to intermediate charge numbers. For larger Z-values as well as in the inner core region (

    A generated property specification language for resilient multirobot missions

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    The use of robots is gaining considerable traction in several domains, since they are capable of assisting and replacing humans for everyday tasks. To harvest the full potential of robots, it must be possible to define missions for robots that are domain-specific, resilient, and collaborative. Currently, robot vendors provide low-level APIs to program such missions, making mission definition a task-specific and error-prone activity. There is a need for quick definition of new missions, by users that lack programming expertise, such as farmers and emergency workers. In this paper, we extend the existing FLYAQ platform to support the high-level specification of adaptive and highly-resilient missions. We present an extensible specification language that allows users to declaratively specify domain-specific constraints as properties of missions, thus complementing the existing FLYAQ mission language. This permits to move at runtime, the actual generation of low-level operations to satisfy the declaratively specified mission. We show how this specification language can be automatically generated from a domain-specific FLYAQ mission language by using the generative ProMoBox approach. Next, we show how mission goals are achieved taking mission properties into account, and how missions may change due to unexpected circumstances

    Microwave technology for detecting traumatic intracranial bleedings: tests on phantom of subdural hematoma and numerical simulations

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    Traumatic brain injury is the leading cause of death and severe disability for young people and a major public health problem for elderly. Many patients with intracranial bleeding are treated too late, because they initially show no symptoms of severe injury and are not transported to a trauma center. There is a need for a method to detect intracranial bleedings in the prehospital setting. In this study, we investigate whether broadband microwave technology (MWT) in conjunction with a diagnostic algorithm can detect subdural hematoma (SDH). A human cranium phantom and numerical simulations of SDH are used. Four phantoms with SDH 0, 40, 70 and 110 mL are measured with a MWT instrument. The simulated dataset consists of 1500 observations. Classification accuracy is assessed using fivefold cross-validation, and a validation dataset never used for training. The total accuracy is 100 and 82–96 % for phantom measurements and simulated data, respectively. Sensitivity and specificity for bleeding detection were 100 and 96 %, respectively, for the simulated data. SDH of different sizes is differentiated. The classifier requires training dataset size in order of 150 observations per class to achieve high accuracy. We conclude that the results indicate that MWT can detect and estimate the size of SDH. This is promising for developing MWT to be used for prehospital diagnosis of intracranial bleedings

    In Vitro Models for Studying Secondary Plant Metabolite Digestion and Bioaccessibility

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    There is an increased interest in secondary plant metabolites, such as polyphenols and carotenoids, due to their proposed health benefits. Much attention has focused on their bioavailability, a prerequisite for further physiological functions. As human studies are time consuming, costly, and restricted by ethical concerns, in vitro models for investigating the effects of digestion on these compounds have been developed and employed to predict their release from the food matrix, bioaccessibility, and assess changes in their profiles prior to absorption. Most typically, models simulate digestion in the oral cavity, the stomach, the small intestine, and, occasionally, the large intestine. A plethora of models have been reported, the choice mostly driven by the type of phytochemical studied, whether the purpose is screening or studying under close physiological conditions, and the availability of the model systems. Unfortunately, the diversity of model conditions has hampered the ability to compare results across different studies. For example, there is substantial variability in the time of digestion, concentrations of salts, enzymes, and bile acids used, pH, the inclusion of various digestion stages; and whether chosen conditions are static (with fixed concentrations of enzymes, bile salts, digesta, and so on) or dynamic (varying concentrations of these constituents). This review presents an overview of models that have been employed to study the digestion of both lipophilic and hydrophilic phytochemicals, comparing digestive conditions in vitro and in vivo and, finally, suggests a set of parameters for static models that resemble physiological conditions

    Efficient context-aware K-nearest neighbor search

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    We develop a context-sensitive and linear-time K-nearest neighbor search method, wherein the test object and its neighborhood (in the training dataset) are required to share a similar structure via establishing bilateral relations. Our approach particularly enables to deal with two types of irregularities: (i) when the (test) objects are outliers, i.e. they do not belong to any of the existing structures in the (training) dataset, and (ii) when the structures (e.g. classes) in the dataset have diverse densities. Instead of aiming to capture the correct underlying structure of the whole data, we extract the correct structure in the neighborhood of the test object, which leads to computational efficiency of our search strategy. We investigate the performance of our method on a variety of real-world datasets and demonstrate its superior performance compared to the alternatives

    5,6-Dioxo-1,10-phenanthrolin-1-ium chloride

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    Computational Fluid Dynamics Investigation of a Core-Mounted Target-Type Thrust Reverser - Part 1: Reverser Stowed Configuration

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    During the second half of the 90 s, NASA performed experimental investigations on six novel thrust reverser (TR) designs; core-mounted target-type thrust reverser (CMTTTR) design is one of them. To assess the CMTTTR efficiency and performance, NASA conducted several wind tunnel tests at sea level static (SLS) conditions. The results from these experiments are used in this paper series to validate the computational fluid dynamics (CFD) results. This paper is part one of the three-part series. Parts 1 and 2 discuss the CMTTTR in stowed and deployed configurations; all analyses in the first two papers are performed at SLS conditions. Part 3 discusses the CMTTTR in the forward flight condition. The key objectives of this paper are: first, to perform the three-dimensional (3D) CFD analysis of the reverser in stowed configuration; all analyses are performed at SLS condition. The second objective is to validate the acquired CFD results against the experimental data provided by NASA (Scott, C. A., 1995, "Static Performance of Six Innovative Thrust Reverser Concepts for Subsonic Transport Applications: Summary of the NASA Langley Innovative Thrust Reverser Test Program," NASA - Langley Research Centre, Hampton, VA, Report No. TM-2000-210300). The third objective is to verify the fan and overall engine net thrust values acquired from the aforementioned CFD analyses against those derived based on one-dimensional (1D) engine performance simulations. The fourth and final objective is to examine and discuss the overall flow physics associated with the CMTTTR under stowed configuration. To support the successful implementation of the overall investigation, full-scale 3D computer aided design (CAD) models are created, representing a fully integrated GE-90 engine, B777 wing, and pylon configuration. Overall, a good agreement is found between the CFD and test results; the difference between the two was less than 5%
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