57 research outputs found
Data Set of PLOS Computational Paper PCOMPBIOL-D-18-02181R1
No full textFigures Data of PLOS Computational paper:Modeling of the axon plasma membrane structure and its effects on protein diffusionAuthors: Yihao Zhang, Anastasios V. Tzingounis, and George LykotrafitisCorresponding Author: George Lykotrafitis, Ph.D.University of ConnecticutStorss, CT UNITED STATES</div
Analysis of a wind farm with doubly fed induction generator based wind turbines interconnected to the grid
No full text123 σ.Η αυξανόμενη χρήση της αιολικής ενέργειας στη σημερινή εποχή σε συνδυασμό με τη ραγδαία ανάπτυξη των τεχνολογιών ανεμογεννητριών έχει προκαλέσει το έντονο ενδιαφέρον για τη βέλτιστη αξιοποίηση του ανέμου σε ευρύ φάσμα ταχυτήτων. Στο πλαίσιο αυτό, βασική απαίτηση των αιολικών συστημάτων αποτελεί η αδιάλλειπτη παροχή μέγιστης ισχύος στο ηλεκτρικό δίκτυο ακόμα και σε συνθήκες διαταραχών.
Στόχος της παρούσας διπλωματικής εργασίας είναι η ανάπτυξη κατάλληλου μοντέλου για την ανάλυση και τον έλεγχο ασύγχρονης γεννήτριας διπλής τροφοδότησης, που είναι πλέον πολύ διαδεδομένη στα αιολικά συστήματα. Αρχικά, περιγράφονται τα επιμέρους υποσυστήματα της ανεμογεννήτριας που μελετήθηκε. Συγκεκριμένα, γίνεται αναφορά στο αεροδυναμικό μέρος, αναλύεται μαθηματικά η ασύγχρονη γεννήτρια διπλής τροφοδότησης και επεξηγείται η λειτουργία του back – to – back μετατροπέα με λεπτομερή ανάλυση των επιμέρους τμημάτων του.
Στη συνέχεια, εξετάζεται το σύστημα ελέγχου και ο τρόπος λειτουργίας του. Στο σημείο αυτό, περιγράφεται η τεχνική του διανυσματικού ελέγχου και αναλύεται ο έλεγχος της ανεμογεννήτριας τόσο από την πλευρά του δρομέα όσο και από την πλευρά του δικτύου. Ακόμη, περιγράφεται η λειτουργία του συστήματος ανεύρεσης σημείου παροχής μέγιστης ισχύος με δεδομένη χαρακτηριστική καμπύλη.
Παρουσιάζονται τα χαρακτηριστικά αιολικού πάρκου και μελετώνται διάφορες περιπτώσεις λειτουργίας ώστε να αξιολογηθεί η εναρμόνισή του με τις τεχνικές απαιτήσεις του διαχειριστή του ελληνικού συστήματος. Στο πλαίσιο αυτό, προσομοιώνονται περιπτώσεις κανονικής λειτουργίας αλλά και διαταραχών στην πλευρά του δικτύου.
Επιπλέον, συγκρίνονται οι αποκρίσεις του συστήματος της παρούσας εργασίας με εκείνες αντίστοιχου συστήματος της βιβλιογραφίας, προκειμένου να επιβεβαιωθεί η ακρίβεια των αποτελεσμάτων του προτεινόμενου μοντέλου.
Τέλος, διερευνάται η επίπτωση στις αποκρίσεις του συστήματος ελέγχου με τυπικά κέρδη ανάδρασης ολοκληρωτικού σφάλματος με εκείνες που προκύπτουν μετά από βελτιστοποίησή τους με τη μέθοδο των Ziegler – Nichols.The fast development of wind energy in the current era combined with the rapid improvement of wind turbine technology has attracted the interest to optimize their use in wide wind speed ranges. In this context, an essential requirement for wind turbine systems is the uninterrupted supply of maximum power to the grid even under fault conditions.
The aim of this thesis is the development of appropriate simulation models for analysis and control of doubly fed asynchronous generators, which are now widely used in wind systems. In a first step, the subsystems of the studied wind turbines are described. More specifically, the aerodynamic part of a wind turbine is examined, the doubly fed asynchronous generator is analyzed and the operation of the back – to – back converter is explained while the constitutive parts are detailed.
In a second step, the control and its operating characteristics are examined. At this point, the vector control technique is described while the control actions of the wind turbine are analyzed both at the rotor side and at the grid side. The maximum power point tracking system (MPPT) based on a specific optimum power-speed characteristic is also described.
In the followings, a wind farm is simulated under various operating conditions in order to assess fulfillment of the technical requirements of the Greek system administrator. In this context, both normal operating conditions as well as fault cases at the grid side are examined.
Moreover, the response results of the studied grid interconnected wind farm system are compared to the corresponding ones of a system found in the literature, in order to validate the proposed model accuracy.
Finally, the impact on the controller efficiency is examined when tuning the PI feedback gains by using the Ziegler – Nichols optimization technique.Αναστάσιος Δ. Ουλής-Ρούση
Enhanced DC voltage control strategy for fault management of a VSC-HVDC connected offshore wind farm
No full textThis paper proposes a DC voltage control strategy for fault management taking into advantage the operation of the master controller located in the offshore AC substation platform. The issue resolved via the proposed controller relates to over-voltages caused in the HVDC links when the power transfer onshore is disrupted due to faults occurring at the AC side of the onshore grid. The control strategy presented in this paper proposes an effective way of maintaining the DC over-voltage within safety limits via reducing the connected wind farm power output. The operation of the aforementioned control strategy requires small computational power and no communication
COVID-19-associated Guillain–Barrè Syndrome and Urinary Dysfunction: A case report
No full textSevere acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral infection can cause multiple systemic and neurological complications, including Guillain–Barrè Syndrome (GBS). In this report we describe for the first time, urinary dysfunction in a patient with COVID-19. We reported a 41-years-old female patient with complaints of an increased generalized muscular weakness associated with progressive difficulty in walking. Four days earlier, patient complained of fever, diarrhea, and general weakness, and the RT-PCR was positive for COVID-19 infection. Due to the worsening of neurological symptoms, a neurophysiological examination on nervous conduction was performed and the diagnosis was suggestive of GBS. Two weeks later, patient developed two consecutive episodes of acute urinary retention that requested the placement of indwelling transurethral catheter. Patient started assuming selective alpha-1 adrenergic antagonist in association with 4 clean intermittent catheterization/die. Four months later, women continued the therapy and the ultrasound evaluation revealed non-pathologic post-void residual volume. Therefore, patient started to void spontaneously again and alpha-blockers were discontinued. We report for the first time a case of severe voiding disorder in a patient with COVID-19 associated GBS. Timely bladder drainage should be adopted to avoid irreversible detrusor damage
Progressive Damage Accumulation Process of CFRP Cross-Ply Laminates during the Early Fatigue Life
No full textThe present work aims at investigating the progressive damage accumulation process of CFRP laminates in an interactive scheme, with a special focus on the early fatigue life where mainly matrix-dominant damage accumulates and stiffness degrades significantly. An in-situ damage monitoring system, containing edge observation, digital image correlation and acoustic emission techniques, was established to characterize and quantify the accumulation of transverse cracks and delamination. Two cross-ply configurations ([0/902]s and [02/904]s) and different stress levels were involved in the experimental campaign. Dependent crack ratio was proposed to reflect the interaction among transverse cracks, and saturated crack density was used to represent the interactive level between transverse cracks and delamination. Results showed that generation of transverse cracks and their interaction govern the early fatigue damage accumulation of the [0/902]s laminates, while not only the interaction among cracks but also the interaction between both damage mechanisms were observed for the [02/904]s laminates.Structural Integrity & Composite
Where, when and in which social context do good ideas evolve in the distributed work environment of knowledge workers?
Get PDFThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Architecture, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 158-160).The nature of work is changing. Until recently, the majority of people worked in fixed, team-based collaborations in collocated settings for fixed periods of time. Currently we are experiencing a major shift towards distributed work. D-work is multi-tasking (workers participate in many projects and teams that often change), multi-locational (work is conducted by people located in different divisions, firms, organizations and time-zones) and mobile (people conduct work while transiting). D-Work changes the definition of the traditional office and blurs the boundaries between home, workplace and the city. Homes will have to accommodate work, businesses must adapt their policies and office spaces to D-work and cities have to adapt to new patterns of mixed work-live units. At the moment, workers, managers and designers have become less aware of where, when, with whom and during which activities, does the most productive and creative work take place. In an effort to tackle the above mentioned issue, we developed a methodology that combines Context-Aware Experience Sampling with traditional ethnographic tools. Our system is composed of a Bluetooth-based positioning system, a context-aware self-report survey administered on mobile phones and traditional questionnaires. The methodology was tested via a four week case study on innovation that was conducted in a marketing firm based in Helsinki. During the study we collected data from eleven participants about the occurrence of work-related ideas and barriers inside and outside the office space.(cont.) All participants provided us with information about their work habits by filling out a questionnaire prior to the beginning of the study. By juxtaposing their answers to their actual work-life data that we collected, similarities and discrepancies between the two emerged that helped us to understand and assess their work behavior. General results as well as personal reports that were compiled for three subjects are presented and analyzed. An overall assessment of the system and suggested improvements based on results and participant feedback are also discussed.by Anastasios Dimas.S.M
Micromechanical modeling of rate-dependent off-axis failure in thermoplastic composites
No full textA micromechanical framework for modeling failure in unidirectional (UD) thermoplastic composites under rate-dependent off-axis loading is presented, with the aim to predict and analyze transverse matrix cracking under various load conditions. The onset of global softening in the micromodel corresponds to macroscopic matrix crack initiation. The problem addressed in this study is to include matrix plasticity and microcracking in the failure analysis of UD composites. A thin slice representative volume element (RVE) with periodic boundary conditions is used, which enables representation of 3D stress states. The testing conditions of a constant prescribed strain-rate and an off-axis uniaxial stress state are reproduced in the model with a dedicated arclength control method. The studied material system is carbon/PEEK composite material, where plasticity in the matrix is represented with the Eindhoven Glassy Polymer (EGP) constitutive law, while the fibers are modeled as transversely isotropic elastic material. In order to account for microcracking in the matrix, a cohesive surface methodology is applied. Cohesive elements are added on the fly with a stress-based initiation criterion. For this purpose, a power law microcrack initiation criterion is proposed. After initiation, the microcracking process is governed by a mixed-mode damage cohesive law. Geometric nonlinear effects are also included in the cohesive model, such that cohesive forces include material as well as geometric contributions. The model is validated with experimental data from tensile tests on UD material at different off-axis angles and strain-rates. The obtained maximum stress levels are used to generate Tsai-Hill failure envelopes for macroscopic transverse crack initiation. Additional capabilities of the model are demonstrated through examples with different fiber-volume ratios and temperature conditions.Applied Mechanic
Multiple sets of solutions for harmonic elimination PWM bipolar waveforms: Analysis and experimental verification
Get PDFMultiple sets of solutions for the selective harmonic elimination pulse-width modulation method for inverter control exist. These sets present an independent solution to the same problem but further investigation reveals that certain sets may offer an improved overall harmonic performance. In this paper, a minimization method is discussed as a way to obtain these multiple sets of switching angles. A simple distortion harmonic factor that takes into account the first two most significant harmonics present in the generated waveform is considered in order to evaluate the performance of each set. The bipolar waveform is thoroughly analyzed and two cases are considered; single-phase patterns which eliminate all odd harmonics and three-phase counterparts which eliminate only the nontriplen odd harmonics from the line-to-neutral pattern but such harmonics are naturally eliminated from the line-to-line waveform. Experimental results support the theoretical considerations reported in the paper
Reversible Tremor and Myoclonus Associated With Topiramate-Fluvoxamine Coadministration
No full textThe antiepileptic agent topiramate has proved its efficacy in a variety of other conditions as well, including several kinds of tremor and migraine prophylaxis. We report on the case of a 42-yearold depressive female patient with comorbid migraine attacks, whereby the adjunction of topiramate as an antimigraine agent at the dosage of 50 mg/d to her antidepressive treatment with fluvoxamine at 300 mg/d triggered-the prima facie paradoxical for topiramate-side effects of tremor and myoclonus. Topiramate was immediately discontinued, and patient’s abnormal movements subsided completely within 24 to 72 hours. Topiramate was possibly the cause of patient’s abnormal movements enhanced by fluvoxamine’s potential to induce also tremor and mynclonus. Therefore, clinicians should be aware of the potentially severe adverse reactions that might occur during concomitant treatment with fluvoxamine and topiramate
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