1,722,120 research outputs found
Conclusion: Levels of Creativity in Science and Mind
No full textthe aim was to discuss reductionism in many different sciences
Formal Grammars Generating Fractal Descriptions of Molecular Structures
No full textSimple rewriting rules are used to produce alphanumeric strings that embed fractal number sequences and are directly translatable into descriptions of hydrocarbon structures of considerable complexity, featuring hierarchical schemes. Rotations of the alphanumeric strings lead to radical rearrangements of the corresponding structures, which lose their initial schemes and become much less predictable, featuring different topologies of polygonal cycles. This shows that a complex and not necessarily ordered molecular structure may nevertheless have a rela-tively low algorithmic complexity. The variety and versatility of reorga-nization in chemical topology, due to the nonlocal representation of bonds in the coding string, may have played a role in prebiotic chemistry
Generative grammars for branched molecular structures
No full textThe application of formal languages to the generation of ideal chemical structures is demonstrated. The grammars are based on linear expressions of the molecular structure which are defined recursively using simple rules. The general concepts are applied to concrete systems: branched alkanes and branched polyethers. It is shown that the molecular dimension is necessarily limited for structures formulated by systematic replacements in linear expressions. In perspective, more general grammars can lead to an increasing application of formal languages to the proposal of new complex structures
Wide flow model for converging gravity currents and the effects of the flow resistance model on the propagation
No full textWe are investigating flows in the viscous-buoyancy balance regime in a converging channel with a cross section described by a power function y xkzr , where x and y are the streamwise and spanwise horizontal coordinates, respectively, and z is the vertical coordinate. We are interested in the different results depending on whether we use a simplified model of the flow resistance law, which varies depending on whether the height of the current is much greater/smaller than the channel width or a somewhat more general model described by the Darcy–Weisbach equation in which the flow resistance law depends on the shape of the cross section through the Fanning friction factor.
The simplified models, one of which developed here is original and new, allow a self-similar solution of the second kind, unlike the general model. The general model, to the best of our knowledge applied for the first time to a generic cross section described by a power function, requires numerical integration. However, a comparison of the front propagation of the gravity current according to the different models, performed by numerical integration of the differential problem, shows that the current assumes a self-similar arrangement as a good approximation for the general model. For some channel geometries, the three models give a very similar result, which results in a difficult attribution to a specific model based on experiments. The effects of anisotropy in the vertical direction of the channel cross section are also highlighted by both the numerical and self-similar solutions
Gravity currents of viscous fluids in a vertically widening and converging fracture
No full textWe are investigating flows in the viscous-buoyancy balance regime in a converging channel with an upward increase in the width, with the gap of the channel varying according to a x k z r power function, being x and z the horizontal and vertical coordinate, respectively, and with 0 < k < 1 and 0 < r < 1 in order to be consistent with the model. The fluid rheology is described according to the Ostwald-de Waele model, with a power-law relationship between shear stress and shear rate and with application for shear-thinning, shear-thickening and, as a special case, Newtonian fluids. While for the case of flow in the direction of widening of the horizontal channel, a self-similar solution of the first kind can be expected, for flow toward the origin, with channel narrowing horizontally, the solution is self-similar of the second kind, with the space and a reduced time coupled in a self-similar independent variable but with an unknown parameter of the transformation group that makes the differential problem invariant. The solution is found in the phase plane by numerical integration of the paths connecting pairs of singular points, separately for the pre-closure phase, in which the current front advances invading the channel, and for the post-closure or leveling phase, in which the fluid has reached the origin and the front no longer propagates, while the level progressively increases canceling the pressure gradient. Integration is performed with a trial and error procedure by modifying the unknown parameter, generally named eigenvalue and specifically critical eigenvalue when the path has been successfully integrated. The overall effect of an increasing permeability upward is that of an increase in the front speed, with the current profile also becoming locally steeper. The effect of an increase in the fluid-behavior index is mixed, as it reduces the speed of the front but still increases the steepness of the local current profile. In any case, the model implies that the eigenvalue tends to infinity for k → 1 even in the presence of an increase in the vertical permeability (r > 0)
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