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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Experimentelle Untersuchung der Vorgänge in engen Spalten zwischen den Unterkanälen von Stabbündeln bei turbulenter Strömung

Moller, Sergio Vicosa January 1988 (has links)
"Experimental study of the phenomena of turbulent flow in the narrow gaps between subchannels of rod bundles". lt was observed that the turbulent intensities in the narrow gaps between the subchannels of rod bundles are strongly anisotropic and higher than in pipes. In rod bundles, both the axial and azimuthal components of the fluctuating velocity have a quasi-periodic behaviour. The intensities increase with decreasing distance between the rods or between rod and channel wall, respectively. To determine the origin of this phenomenon, experiments were performed in rod bundles with different pitch-to-diameter (P/D) and wall-to-diameter (W/D) ratios. In these experiments, two components of the fluctuating velocity were measured with hot wires simultaneously at two different locations of a wall subchannel, together with the pressure fluctuations at the wall measured by microphones. The output signals were registered with an analog tape recorder. Afterwards they were digitized and evaluated to obtain spectra as well as auto and cross correlations. The results were analysed to determine the interdependence between pressure and velocity fluctuations. Attention was devoted to the analysis of turbulence spectra and the identification of their specific ranges. The dominant frequency of the turbulent motion, taken from the spectra, was found to be a function of the gap width and of the flow velocity. The corresponding Strouhal Number is a geometrical parameter which can be expressed in terms of P/D and W/D. Based on the observation of transit time between the probes, measured with help of cross correlations, on the form and the presence of peaks on spectra, a phenomenological model was developed, to explain the studied phenomenon. The model describes the formation of large eddies near the gaps and their effect on the fluid motion through rod bundles. The relationship between the mixing process and the studied phenomenon was determined.
2

Experimentelle Untersuchung der Vorgänge in engen Spalten zwischen den Unterkanälen von Stabbündeln bei turbulenter Strömung

Moller, Sergio Vicosa January 1988 (has links)
"Experimental study of the phenomena of turbulent flow in the narrow gaps between subchannels of rod bundles". lt was observed that the turbulent intensities in the narrow gaps between the subchannels of rod bundles are strongly anisotropic and higher than in pipes. In rod bundles, both the axial and azimuthal components of the fluctuating velocity have a quasi-periodic behaviour. The intensities increase with decreasing distance between the rods or between rod and channel wall, respectively. To determine the origin of this phenomenon, experiments were performed in rod bundles with different pitch-to-diameter (P/D) and wall-to-diameter (W/D) ratios. In these experiments, two components of the fluctuating velocity were measured with hot wires simultaneously at two different locations of a wall subchannel, together with the pressure fluctuations at the wall measured by microphones. The output signals were registered with an analog tape recorder. Afterwards they were digitized and evaluated to obtain spectra as well as auto and cross correlations. The results were analysed to determine the interdependence between pressure and velocity fluctuations. Attention was devoted to the analysis of turbulence spectra and the identification of their specific ranges. The dominant frequency of the turbulent motion, taken from the spectra, was found to be a function of the gap width and of the flow velocity. The corresponding Strouhal Number is a geometrical parameter which can be expressed in terms of P/D and W/D. Based on the observation of transit time between the probes, measured with help of cross correlations, on the form and the presence of peaks on spectra, a phenomenological model was developed, to explain the studied phenomenon. The model describes the formation of large eddies near the gaps and their effect on the fluid motion through rod bundles. The relationship between the mixing process and the studied phenomenon was determined.
3

Experimentelle Untersuchung der Vorgänge in engen Spalten zwischen den Unterkanälen von Stabbündeln bei turbulenter Strömung

Moller, Sergio Vicosa January 1988 (has links)
"Experimental study of the phenomena of turbulent flow in the narrow gaps between subchannels of rod bundles". lt was observed that the turbulent intensities in the narrow gaps between the subchannels of rod bundles are strongly anisotropic and higher than in pipes. In rod bundles, both the axial and azimuthal components of the fluctuating velocity have a quasi-periodic behaviour. The intensities increase with decreasing distance between the rods or between rod and channel wall, respectively. To determine the origin of this phenomenon, experiments were performed in rod bundles with different pitch-to-diameter (P/D) and wall-to-diameter (W/D) ratios. In these experiments, two components of the fluctuating velocity were measured with hot wires simultaneously at two different locations of a wall subchannel, together with the pressure fluctuations at the wall measured by microphones. The output signals were registered with an analog tape recorder. Afterwards they were digitized and evaluated to obtain spectra as well as auto and cross correlations. The results were analysed to determine the interdependence between pressure and velocity fluctuations. Attention was devoted to the analysis of turbulence spectra and the identification of their specific ranges. The dominant frequency of the turbulent motion, taken from the spectra, was found to be a function of the gap width and of the flow velocity. The corresponding Strouhal Number is a geometrical parameter which can be expressed in terms of P/D and W/D. Based on the observation of transit time between the probes, measured with help of cross correlations, on the form and the presence of peaks on spectra, a phenomenological model was developed, to explain the studied phenomenon. The model describes the formation of large eddies near the gaps and their effect on the fluid motion through rod bundles. The relationship between the mixing process and the studied phenomenon was determined.
4

Contribuição ao estudo da densidade superficial de carga na estabilidade de colóides magnéticos : uma abordagem eletroquímica

Campos, Alex Fabiano Cortez January 2005 (has links)
Tese (doutorado)-Universidade de Brasília, Instituto de Química, 2005. / Submitted by Allan Wanick Motta (allan_wanick@hotmail.com) on 2010-10-19T16:18:37Z No. of bitstreams: 1 2005_AlexFabianoCortezCampos.pdf: 4071639 bytes, checksum: 44a39a3da72f2aadc8b237360a7afc22 (MD5) / Approved for entry into archive by Daniel Ribeiro(daniel@bce.unb.br) on 2010-10-19T22:35:51Z (GMT) No. of bitstreams: 1 2005_AlexFabianoCortezCampos.pdf: 4071639 bytes, checksum: 44a39a3da72f2aadc8b237360a7afc22 (MD5) / Made available in DSpace on 2010-10-19T22:35:51Z (GMT). No. of bitstreams: 1 2005_AlexFabianoCortezCampos.pdf: 4071639 bytes, checksum: 44a39a3da72f2aadc8b237360a7afc22 (MD5) / As variadas e interessantes aplicações a que os colóides magnéticos se destinam estão fortemente relacionadas às propriedades específicas desses materiais, que, por sua vez, decorrem diretamente de um balanço entre o comportamento intrínseco das partículas e as interações entre elas. Dessa forma, evocam-se algumas questões fundamentais no que tange principalmente à discussão dos métodos de elaboração das nanopartículas, o controle da estabilidade coloidal do sistema e o comportamento magnético e magneto-ótico desses materiais. Dentro desse contexto, objetivamos neste trabalho a investigação de fenômenos associados à interface superfície / dispersão no sistema coloidal magnético, que estão relacionados à caracterização de alterações químicas e estruturais no limite da partícula bem como ao processo de geração de carga superficial às nanopartículas. Nesse sentido, propomos o desenvolvimento de um método original de determinação da densidade superficial de carga de EDL-MF a partir de medidas eletroquímicas efetuadas em amostras de fluidos magnéticos baseados em nanopartículas de natureza química diferente e tamanhos variados. Nossos resultados permitiram estabelecermos a especiação dos sítios superficiais em função do pH da dispersão, em que foi possível se caracterizar a pH-dependência da densidade superficial de carga. Ainda, o estudo do comportamento do valor de saturação da densidade superficial de carga com o tamanho das nanopartículas possibilitou se evidenciar efeitos de interface e de tamanho finito em nossas nanopartículas. No que concerne à estabilidade coloidal, destacamos que, como conseqüência das propriedades ácido-base de Brönsted da superfície das partículas, a densidade superficial de carga se trata de um parâmetro de ajuste fino do potencial interpartícula em EDL-MF. Dessa forma, monitorando-se o pH da dispersão caracterizamos três fases distintas: sol, gel tixotrópico e coagulada. Finalmente, utilizamos um potencial DLVO estendido (X-DLVO) para simular o balanço das interações atrativas de van der Waals e dipolar magnética com a repulsiva do tipo eletrostática no sistema, para a compreensão das transições de fase pH-dependentes evidenciadas.
5

Micromecânica e microhidrodinâmica de suspensões magnéticas / Micromechanics and microhydrodynamics of magnetic suspensions

Gontijo, Rafael Gabler 08 October 2013 (has links)
Tese (doutorado)—Universidade de Brasília, Departamento de Engenharia Mecânica, 2013. / Submitted by Alaíde Gonçalves dos Santos (alaide@unb.br) on 2014-02-19T12:58:55Z No. of bitstreams: 1 2013_RafaelGablerGontijo.pdf: 66372737 bytes, checksum: baff89898bb728627de14170ccba18fc (MD5) / Approved for entry into archive by Guimaraes Jacqueline(jacqueline.guimaraes@bce.unb.br) on 2014-02-19T13:26:33Z (GMT) No. of bitstreams: 1 2013_RafaelGablerGontijo.pdf: 66372737 bytes, checksum: baff89898bb728627de14170ccba18fc (MD5) / Made available in DSpace on 2014-02-19T13:26:33Z (GMT). No. of bitstreams: 1 2013_RafaelGablerGontijo.pdf: 66372737 bytes, checksum: baff89898bb728627de14170ccba18fc (MD5) / A presente tese realiza uma investigação da hidrodinâmica de fluidos magnéticos, tanto do ponto de vista discreto, através de investigações numéricas acerca da mecânica de suspensões magnéticas, quanto do ponto de vista macroscópico, através de um estudo completo sobre as equações de balanço que modelam o comportamento de escoamentos de fluidos magnéticos. No estudo relativo às equações de balanço da ferrohidrodinâmica um novo modelo matemático para a representação de escoamentos de fluidos magnéticos com tensor de tensões assimétrico é proposto. Um acoplamento completo entre o magnetismo e a hidrodinâmica para fluidos polares assimétricos é discutido em detalhes e um modelo fisicamente mais consistente é proposto. É ainda discutido como este novo modelo está relacionado à reologia de suspensões magnéticas. A análise microestrutural de suspensões magnéticas traz uma proposta de um modelo matemático capaz de representar o movimento de partículas de um ferrofluido sujeitas à diversas forças, como brownianas, magnéticas devido `a interações entre momentos de dipolo magnéticos de partícula se de um campo externo, forças de contato e forças repulsivas de lubrificação entre partículas e forças repulsivas associadas à representação de paredes físicas. Esta análise consiste inicialmente em um estudo baseado em um modelo de duas partículas com interações hidrodinâmicas para compreensão do comportamento de suspensões magnéticas diluídas, baseado em análises de trajetórias e posteriormente é feito um estudo envolvendo o problema de muitos corpos interagindo magneticamente, no qual é realizada uma análise numérica da estabilidade do código e posteriormente algumas propriedades de suspensões magnéticas são analisadas. Por fim o efeito de interações hidrodinâmicas convergentes é considerado e é verificada a influência destas interações na física da suspensão. Novos modelos são propostos para a determinação da magnetização de equilíbrio da suspensão em função também de parâmetros associados à hidrodinâmica. _______________________________________________________________________________________ ABSTRACT / This Thesis makes an investigation on the hydrodynamics of magnetic fluids. The study aims to analyse the microscopic and macroscopic behaviour of magnetic suspensions. The micromechanics analysis is based on direct numerical simulations on the particles scale. The macroscopic analysis consists in a complete study on the formulation of the ferrohydrodynamics balance equations. In the study on the balance equations of the ferrohydrodynamics a new mathematical model for the representation of magnetic fluid flows with asymmetric stress tensor is proposed. It is shown that the complete coupling between magnetism and hydrodynamics for asymmetric polar fluids requires the use of an evolutive equation for the internal angular momentum of the particles that make up the magnetic suspension. The microscopic analysis of magnetic suspensions consists in the proposal of a mathematical model capable of representing the motion of particles of a ferrofluid subjected to Brownian forces, forces due to magnetic interactions between magnetic dipole moments of particles (considering interactions particle-particle and particle-external magnetic field), contact forces between particles (for particles with higher inertia), repulsive forces between particles (lubrication), net gravitational forces and repulsive forces associated with the representation of the physical walls. This microscopic analysis is initially based on a study with a two-particle model considering hydrodynamic interactions to understand the behavior of dilute magnetic suspensions, based on a trajectory analysis. Later it is studied the problem involving many body interacting magnetically, which is performed a numerical analysis stability of the code and then some properties of magnetic suspensions are analyzed, as the magnetization of a ferrofluid and analysis of variance of particle velocities as a function of volume fraction and the magnetic interaction parameter. Finally hydrodynamic interactions are added to the many bodies problem and their effects on the physics of the suspension is discussed in details.
6

Simulação térmica e estrutural de um trocador de calor cerâmico para operação em altas temperaturas/

Villanueva, H. H. S. January 2015 (has links) (PDF)
Dissertação (Mestrado em Engenharia Mecânica) - Centro Universitário FEI, São Bernardo do Campo, 2015.
7

Um modelo matemático tridimensional em regime transiente para estudo de circulação em águas rasas

Freitas, Christiane Brisolara de January 1985 (has links)
O objetivo deste trabalho e apresentar um modelo matemático para o estudo da circulação transiente em sistemas de aguas rasas e a sua implementação no sistema HYDRO que e uma linguagem orientada para Hidrodinâmica, desenvolvida pelo Curso de Pós-graduação em Engenharia Civil da UFRGS. O modelo visa descrever o fluxo em sistemas hídricos (lagos, estuários, rios) devido ao efeito de mares, ventos e rotação terrestre. A solução das equações que governam o problema se obtém empregando o método de elementos finitos. A particularidade deste modelo é a de tentar simular as características tridimensionais do movimento do fluido, podendo-se então determinar o perfil de velocidades . / The objetive of this work is to present a mathematical model for the study of transient circulation in "Shallow" water systems and its implementation in the HYDRO system, which is an oriented language for computational hydrodinamic developed by the Postgraduation Course in Civil Engineering at UFRGS. This mathematical model aims to describe the flow in hydric systems (lakes, estuaries, rivers) induced by tidal effects, wind actions and earth movements. The solution of the equations that govern the problem is obtained through the use of the finite element method. One of the most important features of this model is its possibility to simulate the threedimensional characteristic of the flow, so that the velocity profile can be. determinated.
8

Modulação matemática de águas térmicas em meio hídrico

Sousa, José Manuel Matias Vieira de January 2006 (has links)
Tese de mestrado. Fundamentos e Aplicações da Mecânica dos Fluídos. 2006. Faculdade de Engenharia. Universidade do Porto
9

Interacção entre um jacto circular turbulento e um diedro rectangular

Mendes, João Carlos Antunes Ferreira January 1988 (has links)
Dissertação apresentada para obtenção do grau de Mestre em Engenharia Térmica na Faculdade de Engenharia da Universidade do Porto, sob orientação do Prof. Doutor Xavier Viegas
10

Formation and dynamics of oil ganglia in porous media

Dias, Madalena Maria Gomes de Queiroz January 1984 (has links)
Tese de doutoramento. Engenharia Química. Departamento de Engenharia Química. Universidade de Houston, Faculdade de Engenharia. Universidade do Porto. 1984

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