597 research outputs found
Thermal and structural assessments of a ceramic wafer seal in hypersonic engines
The thermal and structural performances of a ceramic wafer seal in a simulated hypersonic engine environment are numerically assessed. The effects of aerodynamic heating, surface contact conductance between the seal and its adjacent surfaces, flow of purge coolant gases, and leakage of hot engine flow path gases on the seal temperature were investigated from the engine inlet back to the entrance region of the combustion chamber. Finite element structural analyses, coupled with Weibull failure analyses, were performed to determine the structural reliability of the wafer seal
Holographic Dual of the Lowest Landau Level
We describe the lowest Landau level of a quantum electron star in AdS4. In
the presence of a suitably strong magnetic field, the dynamics of fermions in
the bulk is effectively reduced from four to two dimensions. These
two-dimensional fermions can subsequently be treated using the techniques of
bosonization and the difficult many-body problem of building a gravitating,
charged quantum star is reduced to solving the sine-Gordon model coupled to a
gauge field and a metric. The kinks of the sine-Gordon model provide the
holographic dual of the lowest Landau levels of the strongly-coupled d=2+1
dimensional boundary field theory. The system exhibits order one oscillations
in the magnetic susceptibility, now arising as a classical effect in the bulk.
Moreover, as the chemical potential is varied, we find jumps in the charge
density, oscillations in the fractionalised charge density and plateaux in the
cohesive charge densityComment: 39 pages; 8 Figure
High temperature NASP engine seals: A technology review
Progress in developing advanced high temperature engine seal concepts and related sealing technologies for advanced hypersonic engines are reviewed. Design attributes and issues requiring further development for both the ceramic wafer seal and the braided ceramic rope seal are examined. Leakage data are presented for these seals for engine simulated pressure and temperature conditions and compared to a target leakage limit. Basic elements of leakage flow models to predict leakage rates for each of these seals over the wide range of pressure and temperature conditions anticipated in the engine are also presented
Experimental and analytical analysis of stress-strain behavior in a (90/0 deg)2s, SiC/Ti-15-3 laminate
The nonlinear stress strain behavior of 90 degree/0 degree sub 2s, SiC/Ti-15-3 composite laminate was numerically investigated with a finite element, unit cell approach. Tensile stress-strain curves from room temperature experiments depicted three distinct regions of deformation, and these regions were predicted by finite element analysis. The first region of behavior, which was linear elastic, occurred at low applied stresses. As applied stresses increased, fiber/matrix debonding in the 90 degree plies caused a break in the stress-strain curve and initiated a second linear region. In this second region, matrix plasticity in the 90 degree plies developed. The third region, which was typified by nonlinear, stress-strain behavior occr red at high stresses. In this region, the onset of matrix plasticity in the 0 degree plies stiffened the laminate in the direction transverse to the applied load. Metallographic sections confirmed the existence of matrix plasticity in specific areas of the structure. Finite element analysis also predicted these locations of matrix slip
Thermal/structural analyses of several hydrogen-cooled leading-edge concepts for hypersonic flight vehicles
The aerodynamic heating at high flight Mach numbers, when shock interference heating is included, can be extremely high and can exceed the capability of most conventional metallic and potential ceramic materials available. Numerical analyses of the heat transfer and thermal stresses are performed on three actively cooled leading-edge geometries (models) made of three different materials to address the issue of survivability in a hostile environment. These analyses show a mixture of results from one configuration to the next. Results for each configuration are presented and discussed. Combinations of enhanced internal film coefficients and high material thermal conductivity of copper and tungsten are predicted to maintain the maximum wall temperature for each concept within acceptable operating limits. The exception is the TD nickel material which is predicted to melt for most cases. The wide range of internal impingement film coefficients (based on correlations) for these conditions can lead to a significant uncertainty in expected leading-edge wall temperatures. The equivalent plastic strain, inherent in each configuration which results from the high thermal gradients, indicates a need for further cyclic analysis to determine component life
A network visualisation approach and global stock market integration
This paper applies a visualized network approach to explain the tendency of
integration or co-movement among global equity markets. We utilize daily prices of stock market indices of 57 countries from January 1997 to August 2012 to establish both the Minimum Spanning Tree Network (MSTN) and Graphic Network (GN). We study network features including connectivity and centrality through robust indicators.
Our results clearly show that there has been a tendency over time for markets to
become more integrated globally even during periods of market stress. The centrality results suggest the US and Hong Kong markets have been the most dominant markets in their geographic region. For Europe, we find three dominant centres, the UK, France and Germany in contrast to previous literature suggesting that the UK was the dominant country. We further identify that Japan and Australia, instead of acting as dominant countries in their region, serve as bridging countries between the region and the rest of the world. Finally, we find that Africa does not form a cluster and individual African countries tend to connect to other developed markets in a scattered manner
Development of a Classical Force Field for the Oxidised Si Surface: Application to Hydrophilic Wafer Bonding
We have developed a classical two- and three-body interaction potential to
simulate the hydroxylated, natively oxidised Si surface in contact with water
solutions, based on the combination and extension of the Stillinger-Weber
potential and of a potential originally developed to simulate SiO2 polymorphs.
The potential parameters are chosen to reproduce the structure, charge
distribution, tensile surface stress and interactions with single water
molecules of a natively oxidised Si surface model previously obtained by means
of accurate density functional theory simulations. We have applied the
potential to the case of hydrophilic silicon wafer bonding at room temperature,
revealing maximum room temperature work of adhesion values for natively
oxidised and amorphous silica surfaces of 97 mJ/m2 and 90mJ/m2, respectively,
at a water adsorption coverage of approximately 1 monolayer. The difference
arises from the stronger interaction of the natively oxidised surface with
liquid water, resulting in a higher heat of immersion (203 mJ/m2 vs. 166
mJ/m2), and may be explained in terms of the more pronounced water structuring
close to the surface in alternating layers of larger and smaller density with
respect to the liquid bulk. The computed force-displacement bonding curves may
be a useful input for cohesive zone models where both the topographic details
of the surfaces and the dependence of the attractive force on the initial
surface separation and wetting can be taken into account
Influenza delle dimensioni della densità e della concentrazione delle particelle sull'usura erosiva nel trasporto pneumatico
La concentrazione di particelle è uno dei fattori principali che influenzano la velocità di erosione delle superfici in caso di urto delle particelle, come accade per esempio nelle curve dei trasportatori pneumatici. È noto che una riduzione della velocità di erosione si verifica per alte concentrazioni di particelle, un fenomeno denominato "effetto schermo ". La causa della schermatura si crede risiedere nella maggiore probabilità di collisioni inter-particella quando la concentrazione aumenta. In questo studio sono stati indagati gli effetti della concentrazione sull’erosione di una superficie curva in acciaio utilizzando tre diversi materiali in un impianto di trasporto pneumatico. I materiali sono stati scelti in modo che due avessero la stessa densità , ma granulometria molto diversa, e allo stesso tempo due avessero granulometria molto simile ma la densità diversa. Risultati sperimentali confermano l'effetto schermante al variare della concentrazione di particelle e mostrano che la densità ha un effetto molto più significativo rispetto alla dimensione delle particelle sull’effetto di schermatura.
È stato stabilito un nuovo metodo di correzione per tener conto del degradamento del materiale dovuto a passaggi multipli nella stessa linea di trasporto. Inoltre, è stato formulato un nuovo modello empirico del tasso di erosione specifico basandosi sulla definizione della resistenza all’usura. Con il modello è stato possibile trovare il tasso specifico di erosione quando la concentrazione di particelle tende a zero, e valutare il comportamento dei diversi materiali alla variazione di concentrazione delle particelle
An Experimental and Analytical Investigation of Stirling Space Power Converter Heater Head
NASA has identified the Stirling power converter as a prime candidate for the next generation power system for space applications requiring 60000 hr of operation. To meet this long-term goal, several critical components of the power converter have been analyzed using advanced structural assessment methods. Perhaps the most critical component, because of its geometric complexity and operating environment, is the power converter's heater head. This report describes the life assessment of the heater head which includes the characterization of a viscoplastic material model, the thermal and structural analyses of the heater head, and the interpolation of fatigue and creep test results of a nickel-base superalloy, Udimet 720 LI (Low Inclusions), at several elevated temperatures for life prediction purposes
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