Showing papers by "Simón Bolívar University published in 1992"

Exact and Approximate Posterior Moments for a Normal Location Parameter

Abstract: The forms of first and second posterior moments for a normal location parameter are identified for a rather general class of prior distributions. Exact and approximate illustrations are given where the prior distribution is double exponential or Student t respectively

Effect of operating conditions on gas holdup in slurry bubble columns with a foaming liquid

Abstract: This work presents experimental data on gas holdup in slurry bubble columns with a foaming liquid. The effects of solids concentration, solid particle size, superficial phase velocities and column dimensions on the gas holdup are analyzed. At low superficial gas velocities (less than 4cm/s), for which the liquid does not foam, the presence of solids with small particle size does not affect the gas holdup whereas solids with large particle size induce foam formation and thus their presence increases the gas holdup. In the foaming regime, an increase of solids concentration decreases the gas holdup. The operating mode has a strong effect on the gas holdup: the semi-batch operating mode (stagnant liquid-solid suspension) increases the ability of the liquid to foam with respect to the continuous mode. Regarding the effect of column dimensions, the results presented show that the height of the bubble column does not affect at an appreciable extent the gas holdup in the range 6 < LID < 12. At high gas velocitie...

On foraging, recruitment systems and optimum number of scouts in eusocial colonies

Abstract: A numerical model of an eusocial colony foraging for food showed that, for each set of values of resource density, resource size and recruitment system employed, a given optimal proportion of scouts in the colony maximize the amount of resources retrieved by a colony during a fixed period. The model predicts that ants using mass recruitment systems should have larger colonies with small foragers, and should forage on large food sources. Retrieval of small food sources by small colonies is best achieved with large workers using individual foraging strategies. For mass foragers, several food sources are best retrieved using democratic decision-making systems in recruitment, whereas for very large food sources at very low mean food patch density, autocratic decision-making systems are optimal. Some of the experimental evidence available is discussed in the light of these findings, as they confirm the prediction that large colonies with small workers have mass recruitment systems, whereas workers of small colonies with large workers are generally lone foragers.

Approximate analytical expression for equation of ideal diode with series and shunt resistances

Abstract: A simple and continuous analytical approximation for the equation of an ideal diode with a series resistance and shunt conductance is proposed. When compared to the exact numerical solution, the approximate expression produces relative errors smaller than 3%, for any values of series resistance and shunt conductance.

Long-channel silicon-on-insulator MOSFET theory

Abstract: Based on the procedure of Pierret and Shields [1, Solid-St. Electron. 26 , 143, 1983] for the long-channel bulk MOSFET, a new single-integral expression is obtained to describe the current-voltage characteristics for the silicon-on-insulator (SOI) MOSFET. This expression is valid for: any degree of inversion, all back-gate bias conditions and any semiconductor film thickness. Our single-integral expression, applied to a given back-gate bias condition and using the appropriate approximations, can be simplified to the results of the previous models.

The Limits to Xylem Embolism Recovery in Pinus sylvestris L.

Abstract: In this study we test the hypothesis that, when water supply is under tension, reversal of cavitation can occur as long continuity is maintained in the vicinity of tracheids. The experiments were conducted on young branches. 7-8 mm diameter, of Pinus sylvestris L., freshly collected and allowed to lose water on the bench after being debarked. During dehydration, the volumetric fractions of water (V w ) and gas (V g ) changed steadily as relative water content (θ) declined

Thick aluminosilicate coatings on carbon steel via sol-gel

Abstract: Sol-gel aluminosilicate coatings were deposited on a carbon steel substrate Three application methods were compared: wet deposition techniques as dipping and spraying of the sol and xerogel thermal spraying with flame torch Characterization by X-ray diffraction and scanning electron microscopy shows that amorphous thick coatings (> 5 μm) were obtained Coatings present an acceptable oxidation resistance in air atmosphere at 600°C

Carnivory in pitcher plants of the genus Heliamphora (Sarraceniaceae)

Abstract: summary Field studies of Heliamphora nutans Benth., H. heterodoxa Steyerm., H. minor Gleason, H. ionasii maguire and H. tatei Gleason in the Guayana Highlands of Venezuela, show that the last named species is a true carnivore, possessing all of the following traits: attraction of prey through special visual and chemical signals, trapping and killing of prey, digestion of prey through secreted enzymes, presence of commensals, presence of wax scales aiding in the capture of prey, and absorption of nutrients. The other species possess all traits except they seem neither to secrete proteolytic enzymes nor produce wax scales. The pattern of the carnivorous syndrome among Heliamphora species suggests that the carnivorous habit has evolved in nutrient–poor habitats to improve the absorption of nutrients by capturing mainly ants and only H. tatei further evolved mechanisms for rapid assimilation of organic nutrients and for capturing a variety of flying insects. The carnivorous traits are lost in low light conditions, indicating that nutrient supply is limiting only under fast growth.

Hydroxamic acid glucosides in honeydew of aphids feeding on wheat

Abstract: DIMBOA glucoside (2-O-/gb-D-glucopyranosyl-4-hydroxy-7-meth-oxy-1,4-benzoxazin-3-one), the main hydroxamic acid (Hx) in intact wheat plants, was detected in the honey dew ofRhopalosiphum padi feeding on seedlings of six wheat cultivars that differed in their concentration of Hx, suggesting that the chemical circulates in the phloem. Neither the aglucone (DIMBOA) nor its main breakdown product were found in any of the honeydew samples. Honey dew production by aphids caged on seedlings of the wheat cultivars and DIMBOA glucoside concentrations in the honeydew followed biphasic curves when plotted against Hx concentration, suggesting passive ingestion of the chemical from the phloem at low Hx concentrations and limited ingestion due to feeding deterrency by Hx in mesophyll cells at high Hx concentrations. The presence of plant toxins such as Hx glucosides in the phloem sap, the main ingesta of aphids, and in the mesophyll cells, has major implications for plant defense, through a feeding deterrent effect during stylet penetration, and deterrency (antixenosis) along with antibiosis during feeding.

Aggregation‐Disaggregation Approach to multireservoir operation

Abstract: A group of optimization models for the real-time operation of a hydropower system of reservoirs is presented in this paper. The dimensionality problems usually found in dynamic programming formulations are solved by a space-time aggregation/disaggregation procedure that combines stochastic dynamic programming and linear programming techniques. The reservoirs in a hydropower system are aggregated in power units rather than in water units, and an optimal operating policy for the equivalent aggregated reservoir is found in the first part of this work. The objective function in this first part is to minimize the total cost or energy production for a hydrothermal system. The aggregated policy obtained is used in the real-time operation of the system to determine the recommended daily releases and power production from each reservoir of the system. The proposed methodology is applied to a case study, the Lower Caroni system in Venezuela, which is composed of four reservoirs in series and a total installed capacity of 17,000 MW, with satisfactory results.

Size of testes and scent glands in Capybaras, Hydrochaeris hydrochaeris (Rodentia : Caviomorpha)

Abstract: The volume of the morrillo (snout scent-gland) of adult capybara ( Hydrochaeris hydrochaeris ) males was found to correlate significantly with testes mass ( r = 0.359, d.f. = 110, P < 0.0001). This was independent of body size. Males in better condition did not have larger testes or morrillos, whereas age did correlate with size of the morrillo and testes. These results were related to the higher rates of scent marking and reproductive success of dominant males reported in previous studies.

Two-dimensional hydrogenlike atoms in the presence of a magnetic field: Quasifractional approximations.

Abstract: Two-point quasifractional approximations have been used to study the energy levels for a hydrogenic atom when a magnetic field is applied perpendicular to the x-y plane. Perturbation theory gives power-series expansions for weak magnetic fields and asymptotic expansions for very high magnetic fields. Using appropriate forms of the two-point quasifractional approximants, we recover both expansions and have found a better interpolation between the two limiting situations for the ground- and excited-state energies than those previously published.

Growth and dispersal of capybaras (Hydrochaeris* hydrochaeris) in the Llanos of Venezuela

Abstract: Growth rates and dispersal patterns of capybaras, Hydrochaeris* hydrochaeris, were studied in a ranch in the Venezuelan seasonally flooded savannas by first marking juvenile animals at specific locations in March–April 1989 and then, on two occasions the following year (February and May 1990), searching a wide area for recaptures. Animals grew a mean 2.8 kg per month in the 10 months between the first and second captures and apparently lost weight over the next three months, possibly due to water scarcity. From 22 recaptures, median dispersal distance was 3366 m, about six times the average home range width. Maximum dispersal distance was 5600 m and minimum was 0. There were no differences between males and females in dispersal distances. A group of three males and three females apparently dispersed together. Competition for resources—water and grass—under a social system involving group-living and territoriality is proposed as the cause of the patterns observed. No correlation was found between distance dispersed and initial and final weight or weight gain. Capybaras appeared to disperse predominantly north or south, possibly in order to reduce the chances of meeting established groups and to increase the probability of finding a stream, since all streams in the region run approximately west-east.

Microelectrode Techniques in Electrochemistry

Abstract: Microelectrode techniques have been increasingly utilized in electrochemistry since the distinct advantages of low-dimensioned electrodes began to be realized. Electrodes of micrometer dimensions were practically not used in electrochemical research or practice prior to 1980, but interest in them has increased exponentially during the past decade. Because many of the undesirable aspects of electrochemical techniques are reduced with microelectrodes and because they are easily implemented and involve relatively low costs, a growing number of laboratories are applying microelectrode techniques to investigate a wide variety of problems in diverse systems, some of them not accessible with conventional, larger electrodes. The purpose of the present chapter is to describe the most salient features of microelectrodes, reviewing the sound theoretical framework that has been woven around their use, as well as to discuss some practical aspects such as construction and handling techniques, instrumentation, and some of the problems to which they have been applied.

Thermal Resistance of Bacillus coagulans in Double Concentrated Tomato Paste

Abstract: Thermal resistance was determined on a strain of Bacillus coagulans in double concentrated tomato paste (aw = 0.95 at 23 degrees C, pH = 4.0, 30.3 degrees Brix, 70.1% moisture and acidity 1.30 g/100 g citric acid. A microsyringe method was used with an inoculum of 1.3 X 10(4) spores/mL. Values of D(90 degrees C)= 3.5 min and z = 9.5 degrees C were obtained.

Refractive index distribution measurements by means of spectrally-resolved white-light interferometry

Abstract: A new procedure to measure the spatial distribution of the refractive index in transparent media is presented. It is based on the spectral analysis of optical interferograms obtained from a wide, continuous-spectrum light source. The method yields fairly high precision (up to 10-8) in the measurements of local values of differential refractive index, Δn (Δn=n−nref), along a line in the sample. By means of a CCD TV-camera linked to a microcomputer, fast recording and automatic data processing are achieved. As an application, we present an experimental study of a thermal gradient in a liquid sample.

Leaf-cutting antAtta laevigata (Formicidae: Attini) marks its territory with colony-specific dufour gland secretion.

Abstract: The workers of the leaf-cutting antAtta laevigata were found to mark a territory around their nest and along trunk trails. Elsewhere, we reported that the territorial mark had orientation and agonistic functions. The presence of this mark induced an alarm behavior in intruder workers and stimulated the residents' aggressiveness. Of the parts of the body tested, only the extracts from gaster or Dufour gland were able to induce the same agonistic behavior. The compoundsn-heptadecane, (Z)-9-nonadecene, 8,11-nonadecadiene, and (Z)-9-tricosene, secreted by Dufour gland were identified on foraging trails in the field as components of the territorial odor.

Second quantization of the antisymmetric potential in the space of abelian surfaces

Abstract: We use the group of Abelian surfaces to develop a gauge-independent quantization for the two-index antisymmetric potential. An exact solution is found for the vacuum and the photon state, and a regularization scheme is proposed.