The development, testing, and use of liquid propellant and hybrid rocket propulsion systems for spacecraft and their launch vehicles routinely involves the use of cryogenic propellants. These propellants provide high energy densities that enable high propulsive efficiency and high engine thrust to vehicle weight ratios. However, use of cryogenic propellants also introduces technical problems not associated with other types of propellants. One of the major technical problems is the phenomenon of propellant tank pressurant and ullage gas collapse. This collapse is mainly caused by heat transfer from most of the ullage gas to tank walls and interfacing propellant, which are both at temperatures well below those of this gas. Pressurant gas is supplied into cryogenic propellant tanks in order to initially pressurize these tanks and then to maintain required pressures as propellant is expelled from these tanks. The cryogenic propellants expelled from the tanks feed rocket engine assemblies, subassemblies, and components at required interface pressures and mass flow rates. The net effect of pressurant and ullage gas collapse is increased total mass and mass flow rate requirements of pressurant gases. For flight vehicles this leads to significant and undesirable weight penalties. For rocket engine component and subassembly ground test facilities this results in high construction and operational cost impacts. Accurate predictions of pressurant gas mass transfer and flow rate requirements are essential to the proper design of systems used to supply these gases to cryogenic propellant tanks. While much work has been done in the past for predicting these gas requirements at low subcritical tank pressures, very little has been done at supercritical tank pressure conditions and there are selected cases where errors of analytical predictions are high. The objectives of this study are to develop a new generalized and improved computer program to determine pressurant gas requirements at both subcritical and supercritical tank pressure conditions, and then evaluate and validate the consistent accuracy of this program over a wide range of conditions by comparison of program results to empirical data.

[pt] O presente trabalho tem como objetivo estudar o comportamento de estruturas geotécnicas mediante o uso de Análise Limite Numérica. Para isto foi desenvolvido o programa GEOLIMA (GEOtechnical LIMit Analysis) com base na teoria de Análise Limite Numérica utilizando o Método de Elementos Finitos (MEF), considerando problemas bidimensionais e tridimensionais. Devido ao fato das propriedades do solo serem variáveis aleatórias, a Análise Não Determinística também foi considerada mediante o uso do Método Estatístico Linear e do Método de Monte Carlo. Inicialmente, são apresentados os fundamentos da teoria de Análise Limite Determinística e sua formulação mista pelo Método de Elementos Finitos. A seguir são apresentados os fundamentos de Análise Não Determinística, onde os métodos Estatístico Linear e Monte Carlo são descritos. As fases de desenvolvimento do GEOLIMA são descritas de forma resumida e a validação é feita mediante a comparação de resultados obtidos com soluções analíticas ou outras soluções. A seguir, uma aplicação em 2D é apresentada com a finalidade de ilustrar a Análise Limite Determinística e Não Determinística mediante o método Estatístico Linear e o método de Monte Carlo. Finalmente, duas aplicações em 3D são apresentadas: um problema relativo à frente de escavação de um túnel e um estudo de painéis de mineração. Os resultados deste trabalho indicam a viabilidade de usar Análise Limite Determinística e Não Determinística no estudo de problemas geotécnicos. / [en] The present work has the purpose of studying the behavior of geotechnical structures by means of numerical analysis. For this, program GEOLIMA (GEOtechnical LIMit Analysis) was developed based on the theory of Numerical Limit Analysis using the Finite Element Method (FEM), considering bidimensional and three-dimensional problems. Due to the fact that the properties of the ground are generally random variables, Non Deterministic Analysis was also considered by means of the Linear Statistical and the Monte Carlo Methods. Initially, the fundamentals of Deterministic Limit Analysis and its mixed formulation are presented. Then, the fundamentals of Non Deterministic Theory are presented, and the Linear Statistic and the Monte Carlo Methods are described. The development phases of GEOLIMA are briefly described. Its validation is made by comparing the results obtained with analytical solutions or other solutions. Following, a 2D application is made with the purpose of illustrating Deterministic and Non Deterministic Limit Analysis. Finally, two 3D applications are presented: a problem related to the excavation of a tunnel front and a problem related to mining panels. The results of this work indicate the viability of using Deterministic and Non Deterministic Limit Analysis in the study of geotechnical problems.