Ablation and run-off on the White Glacier (38 km2), Axel Heiberg Island, N.W.T., were studied during the summers 1959-61 and some comparisons were made with nearby glaciers. Techniques of measuring ablation and run-off are described and assessed and the results of longterm, including full season, and short term measurements of both are presented. [...]

No description available.

The phenomenon of spallation, a process by which pieces of char "pop off" a heat shield, has been investigated as it occurs in carbon-reinforced phenolic ablators. Spallation is shown to be the result of a pressure buildup within the char. This pressure increase results from occlusion of the char and the consequent blockage of now. By selective fiber orientation, the spallation process can be alleviated. / Master of Science

A complex chemical system of teflon/air mixture over an axisymmetric decoy at hypersonic reentry flight conditions has been analyzed by using the nonequilibrium viscous shock-layer method. The equilibrium catalytic wall boundary condition was used to obtain the species concentration at the wall. The species conservation equation for binary mixture (air/teflon) was solved to obtain the concentration of freestream air at the wall. Two test cases were chosen to demonstrate the capability of the current code. Due to lack of experimental or theoretical data, the surface measurable quantities from the current code(VSLTEF) were compared with the equivalent air injection and no-mass injection data obtained from VSL7S code. The current code predicts a higher total heat-transfer rate than that predicted by the seven species nonequilibrium air code (VSL7S) with the same injection rate due to the high diffusional heat-transfer rate. The wall pressure was not affected by blowing, while the skin-friction coefficient was decreased (i.e., 43 % reduction for teflon ablation case ; 53 % for nonequilibrium air injection case at 125 kft) when compared with that of no-mass injection case. A shock-layer peak temperature drop ( 1512° R for 125 kft altitude and 848°R for 175 kft altitude) was observed at both cases. The temperature drops were chiefly due to endothermic reactions (dissociation) of the teflon ablation species. Due to large blowing of teflon, the average molecular weight increased substantially and resulted in a reduction of the specific heat ratio γ and an increase in the Prandtl number at the wall. The impurity of sodium was the major source of free electrons near the wall at the end of the vehicle at 125 kft altitude; however, at 175 kft altitude NO⁺ was the major source of free electrons over the entire body. The peak concentration of Na⁺ increased along the body, but that of NO⁺ decreased at both altitudes; While the chemical reaction rate data used is believed to be the best currently available, uncertainties in this data as were cited by Cresswell et al.(1967) may lead to quantitative changes in the above teflon ablation results. / Ph. D.

"One of the general processes of interest in Pulsed Plasma Thrusters is the ablation of the solid fuel. In general, ablation occurs when a short pulse of applied energy removes a portion of the fuel surface. Although this ablation process is relatively straight-forward in simple materials that sublimate, ablation in Pulsed Plasma Thrusters is significantly more complicated. This is caused by the transient conditions and the complex behavior of Teflon that does not sublimate but rather undergoes both physical and chemical changes prior to leaving the surface. These two effects combine to make Teflon ablation a highly nonlinear function of heat flux, material property variations, changing molecular weight, and phase transformation behavior. To gain greater insight into the ablation process, a one-dimensional ablation model is developed that addresses the more detailed thermal and thermodynamic behavior of Teflon during simulated operation of a Pulsed Plasma Thruster. The mathematical model is based on the work of Clark (1971), which focused on two-phase, one-dimensional Teflon ablation in the context of thermal protection systems. The model is modified for use in simulated PPT operations and implemented numerically using an adaptive non-uniform grid, explicit finite-difference techniques, and a volume fraction method to capture the interface between the crystalline and amorphous Teflon phases. The ablation model is validated against analytical heat transfer and ablation solutions and compared with previous experimental results. The Teflon ablation model is used to analyze several general ablation scenarios in addition to specific PPT conditions to gain greater insight into long-duration thruster firing, post-pulse material ablation, variable heat flux effects, variable material property effects, and the impact of surface re-crystallization on particulate emission. These simulations are considered in the context of prior experimental investigations of Pulsed Plasma Thrusters. The results of these simulations demonstrate the success of the numerical ablation model in predicting experimental trend and suggest potential paths of moderately improving thruster efficiency and operational repeatability in the future. "

Heatshield design and analysis has traditionally been a decoupled process, the designer creates the geometry generally without knowledge about how the design variables affect the thermostructural response or how the system will perform under off nominal conditions. Heatshield thermal and structural response analyses are generally performed as separate tasks where the analysts size their respective components and feedback their results to the designer who is left to interpret them. The analysts are generally unable to provide guidance in terms of how the design variables can be modified to meet geometric constraints and not exceed the thermal or structural design specifications. In general, the thermal response analysis of ablative thermal protection systems has traditionally been performed using a one-dimensional finite difference calculation. The structural analyses are generally one, two, or three-dimensional finite element calculations. In this dissertation, the governing differential equations for ablative thermal response are solved in three-dimensions using the finite element method. Darcy' Law is used to model the flow of pyrolysis gas through the ablative material. The three-dimensional governing differential equations for Darcy flow are solved using the finite element method as well. Additionally, the equations for linear elasticity are solved by the finite element method for the thermal stress using temperatures directly from the thermal response calculations. This dissertation also links the analysis of thermal protection systems to their design. The link to design comes from understanding the variation in the thermostructural response over the range of the design variables. Material property sensitivities are performed and an optimum design is determined based on a deterministic analysis minimizing the design specification of bondline temperature subject to appropriate constraints. A Monte Carlo simulation is performed on the optimum design to determine the probability of exceeding the design specifications. The design methodology is demonstrated on the Orion Crew Exploration Vehicle's compression pad design.

Mémoire (M.Sc.A.)--Université du Québec à Chicoutimi, 1987. / Document électronique également accessible en format PDF. CaQCU

In the McMurdo Dry Valleys, Victoria Land, East Antarctica, melting of glacial ice is the primary source of water to streams, lakes, and associated ecosystems. To better understand meltwater production, three hypotheses are tested: 1) that small changes in the surface energy balance on these glaciers will result in large changes in melt, 2) that subsurface melt does not contribute significantly to runoff, and 3) that melt from 25-m high terminal cliffs is the dominant source of baseflow during cold periods. These hypotheses were investigated using a surface energy balance model applied to the glaciers of Taylor Valley using 14 years of meteorological data and calibrated to ablation measurements. Inclusion of transmission of solar radiation into the ice through a source term in a one-dimensional heat transfer equation was necessary to accurately model summer ablation and ice temperatures. Results showed good correspondence between calculated and measured ablation and ice temperatures over the 14 years using both daily and hourly time steps, but an hourly time step allowed resolution of short duration melt events and melt within the upper 15 cm of the ice. Resolution of short duration melt events was not important for properly resolving seasonal ablation totals. Across the smooth surfaces of the glaciers, ablation was dominated by sublimation and melting was rare. Above freezing air temperatures did not necessarily result in melt, and low wind speed was important for melt initiation. According to the model, subsurface melt between 5 and 15 cm depth was extensive and lasted for up to six weeks in some summers. The model was better able to predict ablation if some subsurface melt was assumed to drain, lowering ice density, consistent with observations of a low density weathering crust that forms over the course of the summer on Dry Valley glaciers. In extreme summers, drainage of subsurface melt may have contributed up to half of the observed surface lowering through reduction of ice density and possibly through collapse of highly weathered ice. When applied spatially, the model successfully predicted proglacial streamflow at seasonal and daily time scales. This was despite omitting a routing scheme, and instead assuming that all melt generated exits the glacier on the same day, suggesting refreezing is not substantial. Including subsurface melt as runoff improved predictions of runoff volume and timing, particularly for the recession of large flood peaks. Because overland flow was rarely observed over much of these glaciers, these model results suggest that runoff may be predominantly transported beneath the surface in a partially melted permeable layer of weathered ice. According to the model, topographic basins, particularly the low albedo basin floors, played a prominent role in runoff production. Smooth glacier surfaces exhibited low melt rates, but were important during high melt conditions due to their large surface area. Estimated runoff contributions from cliffs and cryoconite holes was somewhat smaller than suggested in previous studies. Spatial and temporal variability in albedo due to snow and debris played a dominant role in flow variations between streams and seasons. In general, the model supported the existing assumption that snowmelt is insignificant, but in extreme melt years snowmelt in the accumulation area may contribute significantly to runoff in some locations.

Since the 1970's, NASA has relied on the use of rigid aeroshells and supersonic parachutes to enable robotic mission to Mars. These technologies are constrained by size and deployment condition limitations that limit the payload they can deliver to the surface of Mars. One candidate technology envisioned to replace the supersonic parachute is the supersonic inflatable aerodynamic decelerator (IAD). This dissertation presents an overview of work performed in maturing a particular type of IAD, the tension cone. The tension cone concept consists of a flexible shell of revolution that is shaped so as to remain under tension and resist deformation. Systems analyses that evaluated trajectory impacts of a supersonic IAD demonstrated several key advantages including increases in delivered payload capability of over 40%, significant gains in landing site surface elevation, and the ability to accommodate growth in the entry mass of a spacecraft. A series of supersonic wind tunnel tests conducted at the NASA Glenn and Langley Research Centers tested both rigid and flexible tension cone models. Testing of rigid force and moment models and pressure models demonstrated the new design to have favorable performance including drag coefficients between 1.4 and 1.5 and static stability at angles of attack from 0º to 20º. A separate round of tests conducted on flexible tension cone models showed the system to be free of aeroelastic instability. Deployment tests conducted on an inflatable model demonstrated rapid, stable inflation in a supersonic environment. Structural modifications incorporated on the models were seen to reduce inflation pressure requirements by a factor of nearly two. Through this test program, this new tension cone IAD design was shown to be a credible option for a future flight system. Validation of CFD analyses for predicting aerodynamic IAD performance was also completed and the results are presented. Inviscid CFD analyses are seen to provide drag predictions accurate to within 6%. Viscous analyses performed show excellent agreement with measured pressure distributions and flow field characteristics. Comparisons between laminar and turbulent solutions indicate the likelihood of a turbulent boundary layer at high supersonic Mach numbers and large angles of attack.

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No. of bitstreams: 1 CAMARGO_Janine_2017.pdf: 19878018 bytes, checksum: 517c1918bf6801b10fe87b0d806d8a50 (MD5) Previous issue date: 2017-03-14 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Cellulose is a biopolymer available in abundance in nature, which presents very interesting properties mainly for the textile, packaging and paper industry. However, due to its strongly hydrophilic character, resulting from the presence of a large number of free hydroxyl groups in its molecule, its use in certain areas becomes limited. In order to promote the modification of the wetting characteristic of the cellulose, and make it superhydrophobic, samples of this material were submitted to plasma processes performed in two steps: ablation and film deposition. Initially, the effect of variation of ablation time on the creation of adequate surface topography was studied. For this, the samples were exposed to the oxygen plasma at a pressure of 13 Pa and power of 150 W, varying the treatment time from 5 to 60 minutes. Then, the treated samples were submitted to the process called PECVD (Plasma Enhanced Chemical Vapor Deposition), in which a film was deposited on the surface from the precursor hexamethyldisiloxane (HMDSO) in the presence of argon, in a ratio of 70 e 30%, respectively. The deposition time was set to 30 minutes and the power applied was 150 W. In a second investigation, the effect of the thickness of the deposited film was studied. In this case, the cellulose samples were previously exposed to the oxygen ablation plasma for 60 minutes and then led to the PECVD process, with the deposition time varying from 5 to 30 minutes. In a third investigation, the duration of the ablation and deposition steps was reduced to 30 and 1 minute, respectively, in order to optimize the methodology. The samples were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), profilometry, static contact angle measurements and hysteresis and sliding angle measurements. The results demonstrated that longer exposure times for the ablation step provide a significant modification in the material, through the creation of nanoscale structures on the surface of the fibers. The deposition step promoted the formation of films with organosilicon character on the cellulose surface, with thicknesses varying between 144 and 910 nm. The combination of the topography created after 30 minutes of ablation with the thin film resulting from the deposition step for 1 minute provided a surface with low receptivity for both polar (? = 150°) and nonpolar (? ~ 120°) liquids, whose wettability property remained stable with treatment aging time. The low values obtained for hysteresis (9°) and sliding angle (7°) confirm the creation of a superhydrophobic roll-off surface. / A celulose é um biopolímero disponível em abundância na natureza e que possui propriedades bastante interessantes principalmente para a indústria têxtil, de embalagens e papel. No entanto, devido ao seu caráter fortemente hidrofílico, proveniente da presença de um grande número de grupos hidroxila livres em sua molécula, sua utilização em determinadas áreas se torna limitada. Com o objetivo de promover a modificação da característica de molhabilidade da celulose, de modo a torná-la superhidrofóbica, amostras deste material foram submetidas a processos a plasma realizados em duas etapas: ablação e deposição de filme. Inicialmente, estudou-se o efeito da variação do tempo de ablação na criação da topografia adequada da superfície. Para isto, as amostras foram expostas ao plasma de oxigênio a uma pressão de 13 Pa e 150 W de potência, variando-se o tempo de tratamento de 5 a 60 minutos. Em seguida, as amostras tratadas foram submetidas ao processo denominado PECVD (Plasma Enhanced Chemical Vapor Deposition), no qual foi depositado um filme sobre a superfície das mesmas, a partir do precursor hexametildisiloxano (HMDSO) na presença de argônio, numa proporção de 70 e 30%, respectivamente. O tempo de deposição foi fixado em 30 minutos e a potência aplicada foi de 150 W. Numa segunda investigação, foi estudado o efeito da espessura do filme depositado. Neste caso, as amostras de celulose foram previamente expostas ao plasma de ablação com oxigênio durante 60 minutos e posteriormente ao processo de PECVD, variando-se o tempo de deposição de 5 a 30 minutos. Numa terceira investigação, o tempo de duração das etapas de ablação e deposição foi reduzido para 30 e 1 minuto, respectivamente, com o intuito de otimizar a metodologia. As amostras foram caracterizadas por Microscopia Eletrônica de Varredura (MEV), Espectroscopia de Energia Dispersiva (EDS), Espectroscopia de Absorção no Infravermelho por Transformada de Fourier (FTIR), perfilometria, medição de ângulo de contato estático e medição de histerese e ângulo de deslizamento. Os resultados demonstraram que maiores tempos de exposição à etapa de ablação proporcionam uma modificação significativa no material, por meio da criação de estruturas em nanoescala na superfície das fibras. A etapa de deposição promoveu a formação de filmes de caráter organosilicone sobre a superfície da celulose, com espessuras variando entre 144 e 910 nm. A combinação entre a topografia criada após 30 minutos de ablação e o filme de menor espessura resultante da etapa de deposição durante 1 minuto, possibilitaram a obtenção de uma superfície de baixa receptividade tanto a líquidos polares (? =150°) quanto apolares (? ~120°), cuja propriedade de molhabilidade se manteve estável com o tempo de envelhecimento. Os baixos valores de histerese (9°) e ângulo de deslizamento (7°) obtidos confirmam a criação de uma superfície superhidrofóbica do tipo “roll-off”.