560 research outputs found
Análisis criptopaleontológico del lapidario de Teofrastro (s. III a.C.)
La obra"Perì lÃthon" o"Tratado de Piedras" de Teofrasto, es considerado el texto más antiguo conservado sobre minerales y rocas, aunque haya llegado hasta nosotros incompleto. En este trabajo se analiza su contenido comparándolo con el de otros lapidarios antiguos y se concluye que once de las piedras que refiere y que vienen indicadas con nombres geográficos o crÃpticos son fósiles de plantas, vertebrados o quÃmicos. Por eso, esta obra debe considerarse también como el tratado más antiguo que conocemos sobre paleontologÃa, pues además de los fósiles que menciona, en ella se alude expresamente al proceso de fosilización. Palabras clave: CriptopaleontologÃa
The role of unequal diffusivities in ignition and extinction fronts in strained mixing layers
We have studied flame propagation in a strained mixing layer formed between a fuel stream and an oxidizer stream, which can have different initial temperatures. Allowing
the Lewis numbers to deviate from unity, the problem is first formulated within the framework of a thermo-diffusive model and a single irreversible reaction. A compact formulation is then derived in the limit of large activation energy, and solved analytically for high values of the Damkohler number. Simple expressions describing the flame shape and its propagation velocity are obtained. In particular, it is found that the Lewis numbers affect the propagation of the triple flame in a way similar to that obtained in the studies of stretched premixed flames. For example, the flame curvature determined by the transverse enthalpy gradients in the frozen mixing layer leads to flame-front velocities which grow with decreasing values of the Lewis numbers.
The analytical results are complemented by a numerical study which focuses on preferentialdiffusion effects on triple flames. The results cover, for different values of the fuel Lewis number, a wide range of values of the Damkohler number leading to propagation speeds which vary from positive values down to large negative values
Triple flames in mixing layers with nonunity Lewis numbers
The present paper is devoted to the study of the effects of nonunity Lewis numbers on triple-flame propagation in nonuniform mixtures. For definiteness, the case of a strained reactive mixing layer is considered. The fuel and oxidizer that are fed to the mixing layer are allowed to have different initial temperatures. Specifically, we examine how the triple flames encountered in this context are influenced by (a) the transverse gradients in the temperature and composition of the fresh reactive mixture and (b) by differential-diffusion effects. The analysis is carried for a single irreversible reaction with a large activation energy and using the thermo-diffusive model. Analytical expressions describing the flame shape, the local burning speed, and the propagation velocity of the triple flame are obtained. In particular, it is found that the Lewis numbers affect the propagation of the triple flame in a way similar to that obtained in the studies of stretched premixed flames. For example, the flame curvature determined by the transverse gradients in the frozen mixing layer leads to flame-front velocities that grow with decreasing values of the Lewis numbers
Methodological approach to the quality of the regional productive structure: The case of Andalusia
Cultural values in the study of the society’s entrepreneurial potential
This paper attempts to measure the importance of a region’s cultural values in
determining the entrepreneurial intention of each of its members separately. Schwartz’s
(2004) approach will be followed in measuring cultural values, together with Ajzen’s
(1991) theory of planned behaviour with regard to entrepreneurial intentions. The
empirical study has been conducted on a sample of 2974 university graduates as part of
VIE project. Results show that the region’s culture indirectly influences the
entrepreneurial intention of its member. People in some regions do feel a more positive
attitude, subjective norm or perceived behavioural control due to its cultural
characteristics.Junta de AndalucÃa. ConsejerÃa de Innovación, Ciencia y Empresas P08-SEJ-0354
Global controllability tests for geometric hybrid control systems
Hybrid systems are characterized by having an interaction between continuous
dynamics and discrete events. The contribution of this paper is to provide
hybrid systems with a novel geometric formulation so that controls can be
added. Using this framework we describe some new global controllability tests
for hybrid control systems exploiting the geometry and the topology of the set
of jump points, where the instantaneous change of dynamics take place.
Controllability is understood as the existence of a feasible trajectory for the
system joining any two given points. As a result we describe examples where
none of the continuous control systems are controllable, but the associated
hybrid system is controllable because of the characteristics of the jump set.Comment: 27 pages, 5 figure
Capillary rise of a liquid between two vertical plates making a small angle.
The penetration of a wetting liquid in the narrow gap between two vertical plates making a small angle is analyzed in the framework of the lubrication approximation. At the beginning of the process, the liquid rises independently at different distances from the line of intersection of the plates except in a small region around this line where the effect of the gravity is negligible. The maximum height of the liquid initially increases as the cubic root of time and is attained at a point that reaches the line of intersection only after a certain time. At later times, the motion of the liquid is confined to a thin layer around the line of intersection whose height increases as the cubic root of time and whose thickness decreases as the inverse of the cubic root of time. The evolution of the liquid surface is computed numerically and compared with the results of a simple experiment
Source terms for calculations of vaporizing and burning fuel sprays with non-unity Lewis numbers in gases with temperature-dependent thermal conductivities
Liquid-fueled burners are used in a number of propulsion devices ranging from internal
combustion engines to gas turbines. The structure of spray flames is quite complex and
involves a wide range of time and spatial scales in both premixed and non-premixed
modes (Williams 1965; Luo et al. 2011). A number of spray-combustion regimes can be
observed experimentally in canonical scenarios of practical relevance such as counterflow
diffusion flames (Li 1997), as sketched in figure 1, and for which different microscalemodelling strategies are needed. In this study, source terms for the conservation equations are calculated for heating, vaporizing and burning sprays in the single-droplet combustion regime. The present analysis provides extended formulation for source terms, which include non-unity Lewis numbers and variable thermal conductivities
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