142 research outputs found
Effects of multijet coupling on propulsive performance in underwater pulsed jets
No full textDespite the importance of pulsed jets for underwater propulsion, the effect of multiple-jet interactions remains poorly understood. We experimentally investigate how interactions between parallel jets in a pulsed-jet thruster affect the thruster's propulsive performance. Using high-speed fluorescence imaging, we investigate the mutual influence of two pulsed jets under conditions relevant to low-speed maneuvering in a vehicle (Re ≈ 350, L/D ≤ 2). Thrust production and propulsive efficiency are evaluated for different nozzle spacings using a new force estimation technique based on the fluorescence data. This analysis reveals that compared to noninteracting jets, the efficiency and thrust generated by the pair of interacting jets can fall by as much as 10% when the jets are brought into close proximity. Empirically, the thrust T falls off with the nondimensional jet spacing [~ over Δ] as T = T[bar over ∞](1 − Co[~ over Δ][superscript −6]) for a thrust coupling coefficient Co = 2.04 ± 0.11. Finally, we predict this dependence of thrust on spacing using a model that relates the thrust and efficiency drop to streamline curvature and vortex induction at the nozzles.Lincoln LaboratoryUnited States. Office of Naval Research (Award 7000308296
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
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
Broadband leaky Lamb waves excited by optical breakdown in water
No full textOptical breakdown of water is used as a sound source to excite a broadband set of leaky Lamb waves in submerged aluminum plates. The source is shown to simultaneously excite guided modes spanning 0.1–5 MHz in frequency and 0–0.8 mm−1 in wavenumber. The measured response overlaps well with dispersion curves for Lamb waves in the plates, revealing strong coupling to both symmetric and antisymmetric modes. The strongest responses arise when a mode's phase velocity approximately equals the plate's compressional wave velocity. These results are shown to arise from an interplay of the sensing geometry, guided wave speeds, and signal processing. Finally, implications for non-contact sensing are discussed.United States. Office of Naval Research (Award N00014-18-1-2066
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
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