Showing papers by "Simón Bolívar University published in 1988"

Two Applications of Urn Processes The Fringe Analysis of Search Trees and The Simulation of Quasi-Stationary Distributions of Markov Chains

Abstract: From an urn containing colored balls, one ball is drawn and replaced by a random number of differently colored balls, with the distribution of the added balls depending only on the color of the ball drawn. Under mild regularity conditions, the proportions of different colors will converge to deterministic limits. Two applications of this standard result are described.

Phractocephalus hemiliopterus (Pimelodidae, Siluriformes) from the upper Miocene Urumaco Formation, Venezuela: a further case of evolutionary stasis and local extinction among South American fishes

Abstract: A fossil catfish skull from the upper Miocene Urumaco Formation of Falcon State, Venezuela is identified as Phractocephalus hemiliopterus (Pimelodidae), a living species of the large, lowland rivers east of the Andes. This fossil provides another example, in a growing list of cases, of a long and conservative evolutionary history for an advanced South American riverine fish. The phylogenetic position of Phractocephalus implies that other higher level lineages of Pimelodidae had originated before six Ma. This discovery corroborates geological and other biogeographic evidence for a connection of the ancient Orinoco system to the Caribbean coast river systems of Venezuela. The fossil catfish and other large aquatic vertebrates from the same area suggest former, freshwater communities on the north coast that were much richer than modern faunas. Increasing aridity and loss of large permanent rivers in post-Miocene times has presumably caused local extinction of these communities.

The tubulin genes of Trypanosoma cruzi.

Abstract: The organization of the alpha- and beta-tubulin genes in the genome of Trypanosoma cruzi have been analysed by Southern blotting using tubulin probes derived from Trypanosoma brucei. The tubulin array appears to be more complex in this organism than in other members of the same family. Some tubulin genes are tightly clustered in an alternating (alpha-beta)n array with a basic repeat unit length of 4.3 kb. However, other pairs of alternating alpha- and beta-tubulin sequences appear to be physically separated from the basic group. This finding indicates that the tubulin gene cluster present in T. cruzi is less perfectly conserved than in T. brucei. T. (Herpetosoma) rangeli is similar to T. (Schizotrypanum) cruzi in its tubulin gene organization whereas most of these genes are tandemly clustered in the genome of T. (Trypanozoon) evansi, with a basic repeat unit length of 3.6 kb as previously described for T. (Trypanozoon) brucei. Two overlapping recombinant clones containing T. cruzi tubulin sequences have been isolated from a genomic cosmid library of T. cruzi epimastigotes using the T. brucei tubulin probes. Partial sequencing of the T. cruzi beta-tubulin gene has confirmed its identity and shows more than 70% homology with the sea urchin, chicken and T. b. rhodesiense beta-tubulin reported gene sequences. Analysis of tubulin gene organization through the parasite life cycle does not show evidence of major rearrangements within the repeat unit. Several T. cruzi strains and cloned lines whilst sharing the 4.3-kb tubulin repeat unit, exhibited very variable tubulin gene organization with tubulin probes. These striking differences in the organization of this structural gene among T. cruzi strains and cloned lines suggest that the heterogeneity previously reported in parasite populations may be related to a very dynamic, diploid genome.

Thermal radiation and laminar forced convection in a gas pipe flow

Abstract: The interaction of forced convection and thermal radiation in laminar pipe flow is numerically studied in this paper. An absorbing and emitting gas is considered for the analysis and treated to be a gray medium. The method of moments is used to describe approximately the radiative heat flux in the energy equation. The conservation equations that govern the problem are reduced to a coupled system consisting of a partial and an ordinary differential equations. The resulting system was reformulated into an initial value problem with the help of the method of lines in conjunction with the control volume approach. Temperature distributions in the thermal entrance region of the gas flow are computed by the Runge-Kutta integration scheme. Results for the mean bulk temperatures and total Nusselt numbers showed a good quantitative agreement with those utilizing an exact description for the radiative heat flux. Moreover, this simple methodology provided great savings in CPU time. Therefore, the use of the method of moments in internal gas flows with combined modes of heat transfer seems justifiable.

The foundation of a charge-sheet model for the thin-film MOSFET

Abstract: Based on charge-sheet ideas, a physical model for the thin-film n -channel inversion-mode single-crystal MOSFET has been developed. This model, which is valid for moderate and strong inversion, describes analytically the front-channel charge density and accounts for the charge coupling between the front and the back gates. The utility of this model, which has a maximum relative error of less than 5%, follows from the elimination of the need to numerically evaluate the integral for the (front) channel charge density. This is the first step toward the development of a complete charge sheet model for the thin-film transistor.

Dielectric properties of fluid-saturated bone: a comparison between diaphysis and epiphysis.

Abstract: OJEMAN, R. G. and BLACK, P. M. (1982) Hydrocephalus in adults. In Neurological surgery. YOUMANS, J. R. (Ed.), W. B. Saunders Co., Vol. E, Chap. 37, 1423-1435. SORV.K, S., BEAR, J. and KAgNI, Z. (1988) A non-steady cornpartmental flow model of the cerebrovascular system. J. Biomech., (in press). SYMON, L. and HINZPETER, T. (1976) The enigma of normal pressure hydrocephalus: tests to select patients for surgery and to predict shunt function. Clin. Neurosurg., 24, 285-315.

Phenomenological approach to the coexistence of planar antiferromagnetism with high- t c type II superconductivity

Abstract: A phenomenological study in d-spatial dimensions is presented for the coexistence of planar antiferromagnetism and type II superconductivity. In this approach critical temperatures in the range 30–100 K arise due to the proposed coupling of magnons and conduction electron pairs. We discuss some remarkable features of oxide ceramics, including a longer London penetration depth, shorter coherence length, lower H c 1, higher H c 2, large current densities, and large pinning force.

Morphological differences between Venezuelan and African microfilariae of Onchocerca volvulus

Abstract: Comparative morphological and biometric characteristics of microfilariae of Onchocerca gutturosa and O. volvulus from different geographical areas (Upper Orinoco, Venezuela; Togo; Liberia) were assessed. "Stepwise" discriminant analysis and Mahalanobis estimators were applied to measure distance between populations. The results indicate a strong similarity between the two strains from the Upper Orinoco (Venezuela) and the Togo strain, as well as a clear separation between these strains and that of O. gutturosa. The Liberian strain was easily distinguishable from microfilariae from Togo and Venezuela. Discriminant analysis showed the Liberian deme to be as different from the Venezuelan and Togo demes as these demes were from microfilariae of the reference species, O. gutturosa. Although it is necessary to confirm these data using formalin-fixed specimens obtained from the skin, the present findings suggest the existence of geographically-different strains of O. volvulus in America and Africa.

On a solution of the Graetz-Nusselt problem and its extension with uniform wall heating

Abstract: This paper deals with an analytical and numerical procedure to solve the classical Graetz-Nusselt problem and its natural extension accounting for a uniform heat flux at the wall. The derivation of a new formulation coupling the technique of separation of variables and the method of lines is outlined in detail. Unlike conventional finite difference approaches, the procedure developed here is neither explicit nor implicit. It has special characteristics and its main advantage is that the temperature profile can be calculated directly at any axial station in the pipe by solving a single ordinary differential equation which incorporates the size of the station. Results for the entire thermal entry region of both problems analyzed in this paper agree well with existing benchmark solutions.

Improving flow patterns in a distillation tray by modifying downcomer apron shape

Abstract: A simple modification of the downcomer apron shape has been proposed and experimentally studied. Experiments were carried out in a 40 [cm] diameter tray with water and with no vapor flow. Weir length to diameter ratios were varied from 0.6 to 0.775 and water flow rates between 150 and 600 [cm3/s]. Curved, triangular, trapezoidal and toothed apron shapes were tested. Velocity distributions on the plate were measured using the hydrogen bubble technique up to a flow rate of 200 [cm3/s]. Stagnation point loci were determined for all operating conditions, and the fractional forward-flow area (FFA) was then calculated. The FFA's were considered a measure of the efficiency of the tray, and hence used to compare the performance of the various apron shapes. Three flow patterns were experimentally observed: regular, inverted and transition. Flow pattern maps were obtained, which permit the selection of an optimum shape when an appropriate scale up criterion is used. To this end, it was found that the best criterion...

a Functional Light-Cone Gauge Construction of a Bosonic String Compactified on a Torus

Abstract: The closed bosonic string compactified on a torus is decomposed into two open strings corresponding to left and right movers respectively Multiloop amplitudes are expressed as a product of a holomorphic and an anti-holomorphic function of the moduli of the world-sheet Riemann surface Such a product is obtained by analytically continuing left and right open string amplitudes such that one is the complex conjugate of the other A justification for this analytic continuation is provided using a second quantized field theory of strings The extra parameters needed for complexification are shown to arise from the constraint expressing invariance under choice of origin for string parametrization The chiral string is discussed

Numerical prediction of combined laminar convection in isothermal vertical tubes via the method of lines

Abstract: Simultaneous hydrodynamic and thermal development of a laminar flow due to superimposed natural and forced convection in an isothermal vertical pipe is investigated numerically. The distinct feature of the computational procedure is that it uses the method of lines, wherein the Navier-Stokes equations accounting for buoyancy and constant properties are reduced to a system of first order ordinary differential equations. The latter is readily solved by a standard fourth-order Runge-Kutta technique. The paper includes a detailed discussion of the proposed methodology, and in addition to this, it provides the distributions of velocities, pressure and temperature. All calculations are based on a coarse grid with ten lines uniformly distributed in the cross-stream direction of the pipe when the velocity profile at the entrance is assumed parabolic. Computed results accounting for both upflow and downflow situations are in good agreement with other more elaborate numerical investigations and also with available experimental data employing air and water as working fluids.