178 research outputs found
<i>H<sup>p</sup></i>-bounds for spectral multipliers on Riemannian manifolds
No full textAbstractLet M be a Riemannian manifold which satisfies the doubling volume property. Let Δ be the Laplace–Beltrami operator on M and m(λ), λ∈R, a multiplier satisfying the Mikhlin–Hörmander condition. We also assume that the heat kernel satisfies certain upper Gaussian estimates and we prove that there is a geometric constant p0<1, such that the spectral multiplier m(Δ) is bounded on the Hardy spaces Hp for all p∈(p0,1]
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
Key environmental stress biomarker candidates for the optimisation of chemotherapy treatment of leukaemia
No full textThe impact of fluctuations of environmental parameters
such as oxygen and starvation on the evolution of leukaemia
is analysed in the current review. These fluctuations may occur
within a specific patient (in different organs) or across patients
(individual cases of hypoglycaemia and hyperglycaemia). They
can be experienced as stress stimuli by the cancerous population,
leading to an alteration of cellular growth kinetics, metabolism
and further resistance to chemotherapy. Therefore, it is of high
importance to elucidate key mechanisms that affect the evolution
of leukaemia under stress. Potential stress response mechanisms
are discussed in this review. Moreover, appropriate cell biomarker
candidates related to the environmental stress response and/or
further resistance to chemotherapy are proposed. Quantification
of these biomarkers can enable the combination of macroscopic kinetics
with microscopic information, which is specific to individual
patients and leads to the construction of detailed mathematical
models for the optimisation of chemotherapy. Due to their nature,
these models will be more accurate and precise (in comparison
to available macroscopic/black box models) in the prediction of
responses of individual patients to treatment, as they will incorporate
microscopic genetic and/or metabolic information which is
patient-specific.peer-reviewe
Spectral multipliers on spaces of distributions associated with non-negative self-adjoint operators
No full textWe consider spaces of homogeneous type associated with a non-negative self-adjoint operator whose heat kernel satisfies certain upper Gaussian bounds. Spectral multipliers are introduced and studied on distributions associated with this operator. The boundedness of spectral multipliers on Besov and Triebel–Lizorkin spaces with full range of indices is established too. As an application, we obtain equivalent norm characterizations for the spaces mentioned above. Non-classical spaces as well as Lebesgue, Hardy, (generalized) Sobolev and Lipschitz spaces are also covered by our approach
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
Embeddings between Triebel-Lizorkin Spaces on Metric Spaces Associated with Operators
No full textWe consider the general framework of a metric measure space satisfying the doubling volume property, associated with a non-negative self-adjoint operator, whose heat kernel enjoys standard Gaussian localization. We prove embedding theorems between Triebel-Lizorkin spaces associated with operators. Embeddings for non-classical Triebel-Lizorkin and (both classical and non-classical) Besov spaces are proved as well. Our result generalize the Euclidean case and are new for many settings of independent interest such as the ball, the interval and Riemannian manifolds
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
Fourier multipliers on anisotropic mixed-norm spaces of distributions
No full textA new general Hörmander type condition involving anisotropies and mixed norms is introduced, and boundedness results for Fourier multipliers on anisotropic Besov and Triebel-Lizorkin spaces of distributions with mixed Lebesgue norms are obtained. As an application, the continuity of such operators is established on mixed Sobolev and Lebesgue spaces too. Some lifting properties and equivalent norms are obtained as well
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