7,976 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
. 14 Año 5 (1998) septiembre-diciembre. Dimensión Antropológica
No full text- Efectos de la Reforma Agraria de 1992 sobre el patrimonio arqueológico por Daniela Rodríguez Herrera. - Políticas de control social para negros, mulatos y pardos libres durante el siglo XVII por Rina Cáceres. - Con veras de su corazón. Exvotos pintados del siglo XIX de enfermos que consiguieron su alivio por Guadalupe Zárate Miguel. - La religiosidad popular en el Santuario de San Miguel del milagro por María J. Rodríguez Shadow y Robert D. Shadow. - Reflexiones sobre las razas y el racismo; el problema de los negros, los indios, el nacionalismo y la modernidad por Catharine Good Eschelman. - Los villancicos de negro: breve introducción al género por Glenn Swiadon. - La significación del inframundo olmeca en el Altar 4 de la Venta, Tabasco por Marcia Castro-Leal. - Susan Kellogg, Law and the Transformation of Aztec Culture, 1500-1700, USA, University of Oklahoma Press, 1995, 285 pp. por Arturo Soberón Mora. - 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. por Martha C. Muntzel
Low threshold ion-implanted Nd:YAG channel waveguide laser
No full textThe first channel waveguide laser in Nd:YAG showing a threshold reduction of 20 times compared to a planar waveguide is described. With diode pumping this ion-implanted waveguide laser has been operated with absorbed power thresholds as low as ~500µW in good agreement with theoretical expectation. Output slope efficiencies of ~29% have also been demonstrated
A side-pumped Nd:YAG epitaxial waveguide laser
No full textWe report the first operation of a side-pumped crystal waveguide laser. Both diode array and dye laser side-pumping of an epitaxially grown Nd:YAG planar waveguide laser have been demonstrated, with thresholds as low as 8 mW observed for a simple plane mirror cavity. Output slope efficiencies of 19% have also been demonstrated. The threshold and slope efficiency of this system is found to be in good agreement with a simple analytical model
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