131 research outputs found
Fracture of fillet welds under extreme loading
No full textThesis (Nav. E.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1996, and Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Sciences and Engineering, 1996.Includes bibliographical references (leaves 114-120).by Athanasios G. Sampos.M.S.Nav.E
Peak power reduction algorithms in asymmetric digital subscriber line modems
No full textThesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references (leaves 94-96).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.This thesis investigates peak-to-average ratio (PAR) reduction techniques for multicarrier modulation systems, such as discrete multitone (DMT) modems and orthogonal frequency-division multiplexed (OFDM) terrestrial broadcast transmitters. Through simulation and test implementation on a state-of-the-art programmable ADSL development platform, this thesis pursues a suitable solution for minimizing PAR given the resources of a programmable platform. This solution is integrated as a prototype implementation into a fully-functional ADSL modem and optimized for maximum PAR reduction performance within modem complexity constraints.by Athanasios Dimitri Dousis.M.Eng
Optical breakdown acoustics : transduction and sensing underwater
No full textThis electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 191-199).In the sea, infrastructures such as ships, pipelines, and wind turbines are exposed to harsh conditions that can wear down the structures through wave loading and corrosion. Because of these wear mechanisms, maritime structures require regular inspections to identify early signs of damage or fatigue. Currently, inspections are performed visually or with contact acoustic transducers, often by a human diver. However, these methods are slow and costly, and can be hindered by surface irregularities like biofouling. Therefore, new sensing techniques are needed to meet the rising demand for offshore infrastructure monitoring. In this thesis, I develop optical breakdown as an acoustic source for non-contact measurements of underwater structures. Optical breakdown occurs when a high-power laser is focused to a small spot, causing nonlinear interactions between the light and water. A compact plasma forms at the focus and expands explosively, radiating a loud, broadband pressure wave.Since this source is compact, laser-controlled and broadband, it provides unique sensing capabilities that overcome challenges faced by traditional transducers. First, I demonstrate how the breakdown source can be used to remotely measure the internal properties of submerged plates. The source is used to excite leaky Lamb waves in the plates, and broadband elastic dispersion spectra are measured using hydrophones in the water. The dispersion spectra are used to calculate the thicknesses and sound speeds in aluminum, steel, bronze and glass plates of varying thickness. Second, I characterize how the source can be controlled and scaled up by combining acoustic measurements with high-speed images of the breakdown plasma. In general, breakdown produces a loud (>100kPa at 10cm), ultra-broadband (5kHz-5MHz) source, whose characteristics depend on measurement orientation and laser properties.This transduction behavior is explained by modeling the breakdown plasma as an array of laser-driven explosions. When the laser is tightly focused, the plasma is compact, producing a loud and omnidirectional signal. However, for weak focusing and high energies, the plasma lengthens and becomes erratic, producing a weaker signal with less consistent behavior. These results reveal design challenges, tradeoffs and opportunities when adapting the breakdown source for dierent applications.by Athanasios G. Athanassiadis.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineerin
Parallel pulsed jets for precise underwater propulsion
No full textThesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.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 (pages 95-98).A significant limitation for underwater robots is their ability to maneuver in tight spaces or for complex tracking tasks. Next generation vehicles require thrusters that can overcome this problem and efficiently provide high maneuverability at low speeds. Recently, thruster design has begun to draw inspiration from nature's swimmers, applying the principles of pulsed jet propulsion to robotic thrusters. Although most developments have focused on single jet actuators, nature provides some indications that multi-jet systems can provide propulsive benefits -- marine invertebrates called sales connect into chains of individual animals that each eject short jets to collaboratively move the entire chain efficiently around the ocean. However, despite the promise of multi-jet propulsion, there are no existing models or empirical data that explain the physics of multi-jet propulsion. As a result, there are no physically motivated rules to guide the design of man-made multi-jet thrusters. In this thesis, I experimentally investigate how interactions between neighboring jets in a multi-jet thruster will affect the system's propulsive performance. I use high-speed fluorescence imaging to investigate the mutual influence of two pulsed jets under conditions relevant to low-speed maneuvering in a vehicle (Re ~ 350). Using a new force estimation technique developed in this thesis, I analyze the video data to evaluate how thrust and efficiency are affected by the jet spacing. This analysis reveals that, compared to non-interacting jets, the efficiency and thrust generated by the pair of interacting jets can fall by nearly 10% as the jets are brought into close proximity. Based on this data, I develop a model of vortex interactions to explain the thrust and efficiency drop. The data and model described in this thesis contribute new insights to understand vortex formation in pulsed jets, and these results can be used to guide the design of multi-jet underwater propulsion systeby Athanasios G. Athanassiadis.S.M
Experimental Investigation of Membrane Materials used in Multilayer Surfacing Systems for Orthotropic Steel Deck Bridges
No full textIn the Netherlands asphaltic surfacings on orthotropic steel deck bridges (OSDB) mostly consist of two structural layers. The upper layer consists of what is known as very open porous asphalt (ZOAB) for noise reduction. For the lower layer Guss Asphalt (GA) is used. Earlier investigations have shown that the bonding characteristics of membrane layers to the surrounding materials have a very strong influence on the overall response of the steel bridge decks. Rijkswaterstaat, an agency of the Dutch Ministry of Infrastructure and the Environment, has commissioned Delft University of Technology to investigate and rank the performance of various commercially available membranes. In order to obtain insight into the response of membranes and their interaction with the surrounding materials on orthotropic steel decks, a project of evaluation of the performance of modern surfacing systems on OSDBs has been undertaken. Currently, there are various kinds of membranes provided by various companies. Thereby it was necessary to examine the bonding strength of these membrane products and to develop a ranking methodology. The research project focused on membrane performance and the effects hereof on the bridge deck as a whole. The methodology used was a multi-phase approach, which consisted of three main phases. In Phase 1, a Membrane Adhesion Test (MAT) device was developed at Delft University of Technology for the characterization of the adhesive bonding strength of membranes with the surrounding materials on OSDBs on the basis of a fundamentally sound, mechanistic methodology. Several membrane products were tested monotonically in this phase. In Phase 2, the MAT device was utilised for investigation of the fatigue response of the various membrane products on various substrates and under two different temperature conditions and three different cyclic load levels. A ranking methodology consisting of a combination of experimental (via MAT) and computational investigations was also developed and utilized for the ranking of the various membrane products. In Phase 3 of the project, four typical Dutch multilayer surfacing systems, constructed with five selected membrane products from Phase 1 and 2, were studied by means of five-point bending (5PB) beam tests and FE simulations. The findings of the 5PB beam tests were used for calibration and validation of the finite element predictions and for further ranking of the performance of the various membranes in Dutch OSDBs. On the basis of the project results, the top two ranking membranes were selected for subsequent testing by means of the LINTRACK facility available at Delft University of Technology
Ageing Behavior of Porous and Dense Asphalt Mixtures in the Field
No full textBitumen ageing is one of the principal factors causing the deterioration of asphalt pavements. As bitumen ages, the pavement loses its ability to relax stresses during loading/unloading and thermal cooling process, thus the risk of cracking increases. Oxidation and ultraviolet (UV) radiation are believed to be the main factors that can cause bitumen ageing during pavement service life. The aim of this study is to evaluate the mechanical behavior of porous and dense asphalt pavements during field ageing. Pavement test sections were constructed in 2014 and are being exposed to actual environmental conditions since then. To investigate the effect of UV radiation on ageing, UV reflective glass-plates were utilized to cover part of the pavement surface. To study the evolution of the pavements’ mechanical properties, asphalt cores were collected from the test sections periodically (at one-year intervals). The changes in the stiffness modulus of the mixtures were determined via cyclic indirect tensile tests. The results show that the effect of mineral aggregate packing, and hence of air-void distribution and connectivity, on the ageing sensitivity (both thermal and UV ageing sensitivity) of the pavements with time was found to be significant, as the changes of the stiffness of the porous mixtures were greater than that of dense mixtures.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Pavement Engineerin
Corrigendum to “Delta SARS-CoV-2 variant is entirely substituted by the omicron variant during the fifth COVID-19 wave in Attica region” [Sci. Total Environ., 856(Pt 1) (2023)/159062] (Science of the Total Environment (2023) 856(P1), (S0048969722061617), (10.1016/j.scitotenv.2022.159062))
No full textThe authors state that the printed version of the above article missed the contribution of an author, which was that the third author had contributed to the writing of the original draft in addition to methodology. The correct and final version follows. CRediT authorship contribution statement Aikaterini Galani: Methodology, Validation, Writing – original draft. Athina Markou: Supervision, Writing – review & editing, Project administration. Lampros Dimitrakopoulos: Methodology, Writing – original draft. Aikaterini Kontou: Validation. Marios Kostakis: Validation. Vasileios Kapes: Methodology. Marios A. Diamantopoulos: Formal analysis, Software. Panagiotis G. Adamopoulos: Formal analysis. Margaritis Avgeris: Formal analysis, Writing– review & editing. Evi Lianidou: Writing – review & editing. Andreas Scorilas: Formal analysis. Dimitrios Paraskevis: Writing – review & editing. Sotirios Tsiodras: Writing – review & editing. Meletios-Athanasios Dimopoulos: Funding acquisition, Writing – review & editing. Nikolaos Thomaidis: Conceptualization, Project administration, Visualization, Resources. © 2022 Elsevier B.V
Mechanisms in Healing of Bitumen and the Impact of Normal Force
No full textDamage in pavements is known to reduce over time when the material is left to rest, this phenomenon is known as healing. It has been shown that healing is an important influence factor in pavement performance. However, an accepted method to assess the healing capability of a pavement is currently not available. Healing of cracks is assumed to be the sum of two processes, cracked surfaces coming into contact (wetting) and strength gain of surfaces in contact (intrinsic healing). The paper describes influencing parameters of these two processes. The healing potential of bitumen is assessed using a novel test method. In this method two pieces of bitumen are brought together and left to heal under controlled condi-tions. After healing the amount of healing is assessed by testing the specimens us-ing a direct tensile test. From the results it can be seen that normal force has a sig-nificant impact on the observed healing, indicating that the process of two surfaces coming into contact (wetting) has a significant impact on healing behavior of the bitumen.Pavement Engineerin
THE MINISTER FACED WITH DELICATE MARITAL CRISES: THE CASE OF A HOMOSEXUAL HUSBAND
No full textThis article deals with the case of a homosexual husband, whose wife came to seek assistance from the author of this paper. She found herself at a painful crossroads after realizing her husband's sexual orientation, followed by meeting another man whom she felt fulfilled her needs. During their session the author, as a pastoral minister, attempted to determine the steps that she should take in order to gain control of her life again on both a spiritual and psychological level. At the same time, the minister sought to help the woman remove the hurdles that prevented her from attaining wholeness. There are many complicated questions, theological as well as psychological, involved in this situation, such as homosexuality and divorce, all generating feelings of inadequacy and guilt. Reflecting on what happened, the author claims that he would nowadays handle the situation differently. In addition, he decided that, given the insoluble impasse of her marriage, she should continue her life with the responsibility of her choice. Another important issue to be considered is the handling of the painful feelings that prompted her to see a professional, who could help her recover from the emotional trauma which had been ailing her. Her counselor’s pastoral approach would also be important to help her realign herself with her church community and its teachings. Finally, through recourse to theological quotations an assessment is made concerning the implications of the divorce on a theological level, including the importance of confession, penance, and holy communion as sacramental aides towards healing and wholeness. Finally, it is explicitly pointed out that the Church allows divorce under certain conditions by exercising discernment, thus helping people to get out of desperate marital impasses and follow the way of Christ
A gauge-invariant unique continuation criterion for waves in asymptotically Anti-de Sitter spacetimes
No full textWe reconsider the unique continuation property for a general class of
tensorial Klein-Gordon equations of the form \begin{align*} \Box_{g} \phi +
\sigma \phi = \mathcal{G}(\phi,\nabla \phi) \text{,} \qquad \sigma \in
\mathbb{R} \end{align*} on a large class of asymptotically anti-de Sitter
spacetimes. In particular, we aim to generalize the previous results of
Holzegel, McGill, and the second author [14,15,24] (which established the
above-mentioned unique continuation property through novel Carleman estimates
near the conformal boundary) in the following ways: (1) We replace the
so-called null convexity criterion (the key geometric assumption on the
conformal boundary needed in [24] to establish the unique continuation
properties) by a more general criterion that is also gauge invariant. (2) Our
new unique continuation property can be applied from a larger, more general
class of domains on the conformal boundary. (3) Similar to [24], we connect the
failure of our generalized null convexity criterion to the existence of certain
null geodesics near the conformal boundary. These geodesics can be used to
construct counterexamples to unique continuation. Finally, our gauge-invariant
criterion and Carleman estimate will constitute a key ingredient in proving
unique continuation results for the full nonlinear Einstein-vacuum equations,
which will be addressed in a forthcoming paper of Holzegel and the second
author [16].Comment: 46 pages, 5 figures, minor changes, added references. Matches the
published versio
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