3,281 research outputs found
A direct A.C. to A.C. regenerative frequency and voltage converter
No full textIncludes bibliographical references.The reliable variable speed operation of an a. c. machine over a wide speed range is a problem that has received attention for some time. In this thesis a system to permit such operation from a fixed frequency fixed voltage supply is proposed, under the name of the asynchronous modulation converter. This converter is of the cycloconverter family, but is force-commutated and is not synchronised with the supply frequency. The power switching element comprises a power transistor in a diode bridge, coupled to the control circuitry by opto-isolators. The output of the a-mod converter is produced by a "chopper" modulation process and this results in the harmonics present being at frequencies well removed from the fundamental. A feedback system to ensure smooth commutation of the output current from the power switches is described. This feedback system operates by sensing the rate of rise of the inductive voltage surge on the load when a switch is opened to operate a freewheel path switch
Zane Grey with Orange A.C. baseball team
No full textThis photograph shows Ohio native Zane Grey with the Orange A.C. Baseball Team, ca. 1896-1904. Prior to becoming a well-known author of western novels, Zane Grey was also a dentist and a semi-professional baseball player, with teams including the Orange Athletic Club and the Newark (New Jersey) Colts
Experimental research on spatial distribution of overtopping
No full textThe overtopping empirical formulas calculate the discharge only at the top of the crest of a coastal protection structure. On the other hand, the tolerable overtopping discharges are defined at certain points behind the crest where the total overtopping is reduced. The scope of this thesis is to find an empirical formula to describe the distribution of overtopping at the space behind the crest. This thesis comes as a further investigation on the work conducted by v.Kester [2009] for regular waves. In this research, a physical model was developed on which irregular waves are tested. Because of the duration of the tests and the amount of collected water (significant lose of water during the test), a completely new measuring system was designed. Five influencing parameters (variables) are considered on this research: wave height, wave period/steepness, slope angle, crest freeboard and crest permeability. The entire overtopping process is analysed separately for the total overtopping discharge, the overtopping discharge directly behind the crest and the distribution of overtopping behind the structure. In the analysis of the data collected from the measurements, the impact of the varying parameters is investigated leading to useful conclusions and better understanding of the entire process. Additionally, the experimental findings are analysed and compared to the relative existing methods. Based on the TAW [2002] method which is proposed by the EurOtop Manual [2007], a prediction formula is developed. This formula is a generic version of TAW [2002] formula in which a new reduction factor ?c is introduced in order to describe the decay of the overtopping and thus predict the discharge at any certain distance behind the crest. Other relevant methods are also analysed (Juul Jensen [1984], Steenaard [2002], Besley [1999] and v.Kester [2009]) and conclusions for their applicability are drawn leading to suggested improvements or corrections. Apart from the distribution of overtopping, on this thesis the determination of crest freeboard (which is an ambiguous issue) is also investigated. Finally, suggestions of further research on this topic are discussed. The entire work has been perfomed in close cooperation with van Oord.Hydraulic EngineeringCivil Engineering and Geoscience
Flow over partially liquid filled cavity
No full textExperiments have been carried out to investigate the effect of liquid cavity filling on the behavior of the gas flow over a flat plate cavity. PIV measurements in the gas phase reveal that cavity filling can affect vortex shedding in the cavity mouth. Shear layer vortices can break-up into smaller vortices, thereby losing their periodic interaction with the aft wall and, hence, their sound producing potential. Expected is that this is one of the mechanisms causing sound mitigation in corrugated pipes with liquid addition, observed in literature
On modular lattices generated by chains
No full textDe La Maza, A.C. (reprint author). Instituto de Matematica y Fisica, Universidad de Talca. Casilla 747, Talca, Chile.We describe the free modular lattice generated by two chains and a single point, under the assumption that there are few meets
Quantized current steps due to the a.c. coherent quantum phase-slip effect
No full text| openaire: EC/H2020/862660/EU//QUANTUM E-LEAPS Funding Information: This work was supported by European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 862660/QUANTUM E-LEAPS and Engineering and Physical Sciences Research Council (EPSRC) grant no. EP/T004088/1. Publisher Copyright: © 2022, The Author(s), under exclusive licence to Springer Nature Limited.The a.c. Josephson effect predicted in 19621 and observed experimentally in 19632 as quantized ‘voltage steps’ (the Shapiro steps) from photon-assisted tunnelling of Cooper pairs is among the most fundamental phenomena of quantum mechanics and is vital for metrological quantum voltage standards. The physically dual effect, the a.c. coherent quantum phase slip (CQPS), photon-assisted tunnelling of magnetic fluxes through a superconducting nanowire, is envisaged to reveal itself as quantized ‘current steps’3,4. The basic physical significance of the a.c. CQPS is also complemented by practical importance in future current standards, a missing element for closing the quantum metrology triangle5,6. In 2012, the CQPS was demonstrated as superposition of magnetic flux quanta in superconducting nanowires 7. However, the direct flat current steps in superconductors, the only unavailable basic effect of superconductivity to date, was unattainable due to lack of appropriate materials and challenges in circuit engineering. Here we report the direct observation of the dual Shapiro steps in a superconducting nanowire. The sharp steps are clear up to 26 GHz frequency with current values 8.3 nA and limited by the present set-up bandwidth. The current steps were theoretically predicted in small Josephson junctions 30 years ago5. However, unavoidable broadening in Josephson junctions prevents their direct experimental observation8,9. We solve this problem by placing a thin NbN nanowire in an inductive environment.Peer reviewe
Stabiliteit van het achtertalud van een rubble mound golfbreker
No full textCivil Engineering and Geoscience
Interview with A.C. Salaam by Tessa Desmond, September 27, 2023
No full textA.C. Salaam reflects on her time attending medical school in the Caribbean and discusses how that experience led her to the practice of plant healing. She shares stories of plant medicines and how she uses them throughout the interview. She reflects on her family's food practices and the role of sufficiency. Salaam touches on the importance of sustenance, and connects this value to her work in the Farmville, NC, community garden
Measurement of the chromatic dispersion of gradient index materials by multiple wavelength A.C. interferometry
No full textThesis (M.S.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics. This thesis was digitized by the Institute of Optics in 2014 and was determined to have lapsed into the public domain. If you are the author and have questions about the digitization of your work, please contact Kari Brick, Graduate Program Coordinator for the Institute of Optics, at [email protected]. Other contact information for the Institute is available at http://www.optics.rochester.eduThe effects of the dispersion in homogeneous glass on the quality of lenses are known. In many cases these color related effects limit the performance of an optical lens. Gradient index materials offer a means to improve the performance of a lens in both monochromatic and polychromatic applications. For these materials to be incorporated into a lens design, accurate knowledge of the index of refraction profile and the dispersion of the gradient are necessary.
Multiple wavelength a.c. interferometry has been investigated as a method of measurement to obtain both the index of refraction and the dispersion as a function of spatial coordinates. The theory for multiple wavelength a.c. interferometry has been developed. Computer programs have been written which predict the amplitude of the a.c. signal (a measure of the dispersion) and calculate the dispersion of the gradient from the amplitude of the a.c. signal.
An a.c. Mach Zehnder interferometer has been built to measure the index of refraction profile and the chromatic dispersion of the gradient. Optical signals for single wavelength and multiple wavelength irradiance patterns are obtained simultaneously to eliminate registration errors of the spatial coordinates. The optical signals are processed electronically to yield the index of refraction profile and the amplitude of the a.c. signal. The system is interfaced to a computer to eliminate problems associated with handling of data. Programs have been written which yield the index of refraction at one wavelength and the dispersion of the gradient as a function of spatial coordinates.
The measurements confirm the validity of the theory of multiple wavelength a.c. interferometry for measuring the dispersion of gradient index materials
Velma B. Pickett, Cheryl Black y Vicente Marcial Cerqueda, Gramática Popular del Zapoteco del Istmo, México, Centro de Investigación y Desarrollo Binnizá, A.C./Instituto Lingüístico de Verano, A.C., 1998, 1998, 123 pp.. Dimensión Antropológica Vol. 14 Año 5 (1998) septiembre-diciembre
No full textEsta obra –que fue preparada por Velma B. Pickett y Cheryl Black, del Instituto Lingüístico de Verano, y Vicente Marcial Cerqueda, del Centro de Investigación y Desarrollo Binnizá A.C. de Juchitán, Oaxaca- se publica por primera vez en español. La intención de esta Gramática popular es conocer la estructura del zapoteco del Istmo, conocido como diidxazá, en forma sencilla, clara y correcta para que facilite a los zapotecos, o binnizá, bilingües a comprender la estructura de su propia lengua, la cual es como cualquier otra del mundo
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