3,685 research outputs found
Thermal stress cycling of GaAs solar cells
A thermal cycling experiment was performed on GaAs solar cells to establish the electrical and structural integrity of these cells under the temperature conditions of a simulated low-Earth orbit of 3-year duration. Thirty single junction GaAs cells were obtained and tests were performed to establish the beginning-of-life characteristics of these cells. The tests consisted of cell I-V power output curves, from which were obtained short-circuit current, open circuit voltage, fill factor, and cell efficiency, and optical micrographs, spectral response, and ion microprobe mass analysis (IMMA) depth profiles on both the front surfaces and the front metallic contacts of the cells. Following 5,000 thermal cycles, the performance of the cells was reexamined in addition to any factors which might contribute to performance degradation. It is established that, after 5,000 thermal cycles, the cells retain their power output with no loss of structural integrity or change in physical appearance
Integración de Ecuaciones Diferenciales rígidas de valor de contorno en problemas de combustión con cinética de reacción detallada
Modelos detallados de la llama de difusión plana, laminar de jets opuestos requieren
la solución de las ecuaciones de conservación de cantidad de movimiento, energía y especies químicas en conjunto con una cinética de reacción química stiff. El problema tiene soluciones autosimilares y se puede resolver mediante integración numérica de un conjunto de ecuaciones diferenciales ordinarias stiff de segundo orden, con condiciones de contorno, cada una de las
cuales tiene un punto de torno de primer orden debido a la ecuación de conservación de cantidad de movimiento. El uso de discretización en diferencias finitas standard (en el dominio espacial) y la
expansión en serie de Taylor alrededor de la iteración anterior (en el espacio temporal) del término cinético de las reacciones químicas lleva a una ecuación de matrices en la cual la matriz de coeficientes es del tipo tridiagonal en bloques y muy grande de tamaño. También representa una matriz de banda y se puede obtener una solución al problema mediante una descomposición
LU. Inestabilidades numéricas debidas a la malla finita de espaciamiento irregular y a la difusión de errores numéricos hacia los bordes forzaron el uso de una malla regular con un gran número de puntos. Esto restringió el estudio a la solución de problemas con esquemas cinéticos relativamente simples, debido a la alta necesidad de memoria de computador. Este dilema fue resuelto por el desarrollo de un esquema de diferenciación modificado, que asume que la solución puntual del problema está dada por una suma de exponenciales. A Usando esta nueva técnica fue posible hallar la solución de una llama con 150 reacciones
envolviendo las ecuaciones de ccnservación de 70 especies químicas. En este artículo formulamos el problema y describimos resultados de los varios esquemas numéricos mediante los que se intentó resolver el mismo. El foco principal, sin embargo, está
u dirigido hacia la modificación del esquema de diferencias finitas, acoplado con la expansión en serie de Taylor de los términos de reacción química.Detailed models of the flat, laminar, opposed jet diffusion flame involve the solution of the momentu, energy and species conservation equations coupled with stiff chemical kinetics. The problem has self similar solutions and can be solved through numerical integration of a set of second order, stiff, boundary valued, ordinary differential equations, each with a regular first order turning point arising from convection. Use of standard finite difference discretization (in the spatial domain) and expansion of the reaction rate source terms in a Taylor series abaut the backward iteration (in the temporal
domain), lead to a matrix equation in which the coefficient matrix is a very large block
tridiagonal matrix. It is also a band matrix and solution is obtained through LU descomposition. Instabiiities originating from the unevenly spaced grid and from diffusion of numerical errors towards the boundaries forced the use of a large number of equally spaced grid points which contrained the program to solution of relatively small kinetic problems (due to core storage limitations). This dilemma was resolved by developing a modified central difference discretization which assumes that the solution at a mesh point is given by the sum of exponentials. Using the new technique it was possible to obtain the solution of the opposed jet problem with 150 reactions and 70 species on the available CYBER 175 computer.Peer Reviewe
Integración de Ecuaciones Diferenciales rígidas de valor de contorno en problemas de combustión con cinética de reacción detallada
Modelos detallados de la llama de difusión plana, laminar de jets opuestos requieren
la solución de las ecuaciones de conservación de cantidad de movimiento, energía y especies químicas en conjunto con una cinética de reacción química stiff. El problema tiene soluciones autosimilares y se puede resolver mediante integración numérica de un conjunto de ecuaciones diferenciales ordinarias stiff de segundo orden, con condiciones de contorno, cada una de las
cuales tiene un punto de torno de primer orden debido a la ecuación de conservación de cantidad de movimiento. El uso de discretización en diferencias finitas standard (en el dominio espacial) y la
expansión en serie de Taylor alrededor de la iteración anterior (en el espacio temporal) del término cinético de las reacciones químicas lleva a una ecuación de matrices en la cual la matriz de coeficientes es del tipo tridiagonal en bloques y muy grande de tamaño. También representa una matriz de banda y se puede obtener una solución al problema mediante una descomposición
LU. Inestabilidades numéricas debidas a la malla finita de espaciamiento irregular y a la difusión de errores numéricos hacia los bordes forzaron el uso de una malla regular con un gran número de puntos. Esto restringió el estudio a la solución de problemas con esquemas cinéticos relativamente simples, debido a la alta necesidad de memoria de computador. Este dilema fue resuelto por el desarrollo de un esquema de diferenciación modificado, que asume que la solución puntual del problema está dada por una suma de exponenciales. A Usando esta nueva técnica fue posible hallar la solución de una llama con 150 reacciones
envolviendo las ecuaciones de ccnservación de 70 especies químicas. En este artículo formulamos el problema y describimos resultados de los varios esquemas numéricos mediante los que se intentó resolver el mismo. El foco principal, sin embargo, está
u dirigido hacia la modificación del esquema de diferencias finitas, acoplado con la expansión en serie de Taylor de los términos de reacción química.Detailed models of the flat, laminar, opposed jet diffusion flame involve the solution of the momentu, energy and species conservation equations coupled with stiff chemical kinetics. The problem has self similar solutions and can be solved through numerical integration of a set of second order, stiff, boundary valued, ordinary differential equations, each with a regular first order turning point arising from convection. Use of standard finite difference discretization (in the spatial domain) and expansion of the reaction rate source terms in a Taylor series abaut the backward iteration (in the temporal
domain), lead to a matrix equation in which the coefficient matrix is a very large block
tridiagonal matrix. It is also a band matrix and solution is obtained through LU descomposition. Instabiiities originating from the unevenly spaced grid and from diffusion of numerical errors towards the boundaries forced the use of a large number of equally spaced grid points which contrained the program to solution of relatively small kinetic problems (due to core storage limitations). This dilemma was resolved by developing a modified central difference discretization which assumes that the solution at a mesh point is given by the sum of exponentials. Using the new technique it was possible to obtain the solution of the opposed jet problem with 150 reactions and 70 species on the available CYBER 175 computer.Peer Reviewe
Measurements and modeling of flow structure in the wake of a low profile wishbone vortex generator
The results of an experimental examination of the vortex structures shed from a low profile 'wishbone' generator are presented. The vortex generator height relative to the turbulent boundary layer was varied by testing two differently sized models. Measurements of the mean three-dimensional velocity field were conducted in cross-stream planes downstream of the vortex generators. In all cases, a counter-rotating vortex pair was observed. Individual vortices were characterized by three descriptors derived from the velocity data; circulation, peak vorticity, and cross-stream location of peak vorticity. Measurements in the cross plane at two axial locations behind the smaller wishbone characterize the downstream development of the vortex pairs. A single region of stream wise velocity deficit is shared by both vortex cores. This is in contrast to conventional generators, where each core coincides with a region of velocity deficit. The measured cross-stream velocities for each case are compared to an Oseen model with matching descriptors. The best comparison occurs with the data from the larger wishbone
Giant microwave photoresistance of two-dimensional electron gas
We measure microwave frequency (4-40 GHz) photoresistance at low magnetic
field B, in high mobility 2D electron gas samples, excited by signals applied
to a transmission line fabricated on the sample surface. Oscillatory
photoresistance vs B is observed. For excitation at the cyclotron resonance
frequency, we find an unprecedented, giant relative photoresistance (\Delta
R)/R of up to 250 percent. The photoresistance is apparently proportional to
the square root of applied power, and disappears as the temperature is
increased.Comment: 4 pages, 3 figure
Hemostatic activation during cardiopulmonary bypass with different aprotinin dosages in pediatric patients having cardiac operations
The Magnetic Sensitivity of the Ba II D1 and D2 Lines of the Fraunhofer Spectrum
The physical interpretation of the spectral line polarization produced by the
joint action of the Hanle and Zeeman effects offers a unique opportunity to
obtain empirical information about hidden aspects of solar and stellar
magnetism. To this end, it is important to achieve a complete understanding of
the sensitivity of the emergent spectral line polarization to the presence of a
magnetic field. Here we present a detailed theoretical investigation on the
role of resonance scattering and magnetic fields on the polarization signals of
the Ba II D1 and D2 lines of the Fraunhofer spectrum, respectively at 4934 \AA\
and 4554 \AA. We adopt a three-level model of Ba II, and we take into account
the hyperfine structure that is shown by the Ba and Ba
isotopes. Despite of their relatively small abundance (18%), the contribution
coming from these two isotopes is indeed fundamental for the interpretation of
the polarization signals observed in these lines. We consider an optically thin
slab model, through which we can investigate in a rigorous way the essential
physical mechanisms involved (resonance polarization, Zeeman, Paschen-Back and
Hanle effects), avoiding complications due to radiative transfer effects. We
assume the slab to be illuminated from below by the photospheric solar
continuum radiation field, and we investigate the radiation scattered at 90
degrees, both in the absence and in the presence of magnetic fields,
deterministic and microturbulent. We show in particular the existence of a
differential magnetic sensitivity of the three-peak Q/I profile that is
observed in the D2 line in quiet regions close to the solar limb, which is of
great interest for magnetic field diagnostics.Comment: 40 pages, 1 table and 19 figures. Accepted for publication in The
Astrophysical Journal (ApJ
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