José Rincón

Leaf-litter breakdown in tropical streams: is variability the norm?

Many forested headwater streams are heterotrophic ecosystems in which allochthonous inputs of plant litter are a major source of energy. Leaves of riparian vegetation entering the stream are broken down by a combination of biotic and abiotic processes and, in most temperate and boreal streams, provide food and habitat for dense populations of detritivorous invertebrates. However, tropical streams in different parts of the world show substantial variability in the number and diversity of leaf-shredding detritivores (hereafter detritivores). We used data obtained with standardized methods from multiple streams in Africa, the Americas, Asia, and Australia to test the hypothesis that this variability would lead to differences in the relative role of detritivores and microorganisms in the breakdown process. We also tested the hypotheses that variability in litter breakdown rates changes with litter type (native litter mixtures vs nonnative alder [Alnus glutinosa]) and is higher across regions within than outside the tropics. We found that litter breakdown rates were highly variable across sites, with no consistent pattern within geographic areas, although litter consumption by detritivores was negligible at several sites, all in America. Geographic patterns of litter breakdown also varied between litter types, with higher breakdown rates for alder than for native litter in most but not all regions. When litter breakdown rates at the tropical sites were compared to previously reported values from temperate and boreal regions, we found that differences in variability between tropical and temperate sites were inconsistent, with great differences among studies. Further global-scale studies will be needed to assess the extent to which latitudinal changes in the diversity and composition of microbial and detritivore assemblages contribute to variability in litter breakdown rates.

Experimental and numerical evaluation of a solar passive cooling system under hot and humid climatic conditions

Abstract The thermal performance of a solar passive cooling system (SPCS) under a hot and humid climate is experimentally and numerically evaluated. The experimental data were obtained from two full scale cells, with identical walls, but different roof configurations. One cell has a highly-insulated roof and the other has an SPCS incorporated consisting of a thermal mass (water), which is cooled by evaporation and long wave nocturnal radiation. The study was conducted taking into account the local climatic conditions of Maracaibo, a topical city located in Venezuela. The numerical evaluation was accomplished using the computational code ‘EVITA’ which is based on the finite volume approach with high order bounded treatment of the convective terms. A PISO-like solution algorithm is used to solve the transient form of the continuity, momentum and energy equations. It has been demonstrated experimentally and numerically that under a hot and humid climate, it is possible to keep the indoor temperature below the outdoor temperature, using a passive cooling technique of a roof pond. The numerical results obtained using the model have demonstrated that the computational code used is a suitable cost-efficient alternative for the thermal performance evaluation of SPCS.

Biotic and abiotic variables influencing plant litter breakdown in streams: A global study

Plant litter breakdown is a key ecological process in terrestrial and freshwater ecosystems. Streams and rivers, in particular, contribute substantially to global carbon fluxes. However, there is little information available on the relative roles of different drivers of plant litter breakdown in fresh waters, particularly at large scales. We present a global-scale study of litter breakdown in streams to compare the roles of biotic, climatic and other environmental factors on breakdown rates. We conducted an experiment in 24 streams encompassing latitudes from 47.8° N to 42.8° S, using litter mixtures of local species differing in quality and phylogenetic diversity (PD), and alder (Alnus glutinosa) to control for variation in litter traits. Our models revealed that breakdown of alder was driven by climate, with some influence of pH, whereas variation in breakdown of litter mixtures was explained mainly by litter quality and PD. Effects of litter quality and PD and stream pH were more positive at higher temperatures, indicating that different mechanisms may operate at different latitudes. These results reflect global variability caused by multiple factors, but unexplained variance points to the need for expanded global-scale comparisons.

Numerical Simulation and Experiments of the Multiphase Flow in a Liquid-Liquid Cylindrical Cyclone Separator

A Liquid-Liquid Cylindrical Cyclone separator (LLCC) is a device used in the petroleum industry to separate the oil-water mixture obtained from the well. The use of this device has not been widespread due to the lack of tools for predicting its separation capability. This paper presents a numerical and experimental study of the fluid dynamic performance of this type of cylindrical cyclone separators. The use of numerical simulations would reduce the time and cost necessary to obtain information for predicting the behavior of the equipment. The objective of this study is to determine if CFD (Computational Fluid Dynamics) techniques are able to reproduce the behavior of a LLCC separator. The CFD software examined was ANSYS-CFX 5.6™ and numerical simulations were carried out using the dispersed model with oil as the dispersed phase. The oil and water mixture entering the separator is divided due to centrifugal and buoyancy forces in an upper (oil rich) exit and a bottom (water rich) exit. The separation capability is determined as the maximum amount of water removed from the mixture with the minimum amount of oil content in the water rich exit. The experiments were conducted in a transparent LLCC separator that allows the visualization of the mixture and the measurement of the oil content. Experiments were conducted for three variables: mixture velocity and water content at the entrance, and the split ratio. The split ratio is defined as the bottom exit flow rate divided by the water flow rate at the entrance. The results showed that CFD tools are able to reproduce the oil content obtained from the experiments for all analyzed conditions. Additionally, the mixture distribution images from numerical and experimental data showed good agreement. This study confirms the capacity of CFD tools for the multiphase flow analysis of LLCC separators.Copyright

Alternative designs for passive cooling of homes for Venezuela

A two-dimensional model has been applied to predict the indoor temperature fluctuations of a building using a finite difference technique based on the control volume approach and the SIMPLE algorithm. The model simulates thermal performance of a rectangular section where inside air motion by free convection and unsteady heat conduction through walls and roof, are allowed. Since it is a two-dimensional model and only the thermal load through external surfaces is considered, numerical results allow one first to study qualitative performance of a rectangular building section and, secondly, to make quantitative comparisons among alternative designs. In order to show the usefulness of the model, eight design cases were simulated under the transient climatic conditions of Maracaibo (latitude = 10 degrees,30 minutes; longitude = 71 degrees 36 minutes), a hot and humid region in Venezuela. Numerical results show how thermal load through external surfaces can be reduced 60 percent and over in relation to the highest thermal load case.

La descomposición de la hojarasca como herramienta para evaluar la integridad funcional de ríos altoandinos del sur del Ecuador

The ecological condition of tropical Andean rivers are threatened by many human activities including changes in land use and cover in watersheds. Current protocols diagnose the structural condition of streams but not their function. In this study we proposed an assessment tool using the leaf-litter breakdown as a measure of the functional condition and health of high Andean streams in Southern Ecuador, including methodology and threshold values adapted to local conditions. We selected 15 streams in three micro-basins (Mazán, Llaviuco and Matadero) within or in the vicinity of Cajas National Park where we deployed litterbags for 15, 28 and 64 days. We used two types of mesh (coarse and fine) for the litterbags in order to separate the contribution of macro-invertebrates and microorganisms in the decomposition process and also tested two different leaf substrates: Alder (Alnus acuminata) and Miconia (Miconia bracteolata). In each collection period we determined physical and chemical parameters, applied the riparian quality index (QBR-And) and river habitat index (IHF). Decomposition rates (-k) were determined from the time period at which approximately 50 % ash-free mass would have been lost. We used Ktotal and the ratio of the rate of decomposition between coarse and fine mesh (Kg/Kf) to define the functional condition thresholds. To test the metrics sensitivity we used graphical methods, one-way ANOVA and discriminative efficiency (ED). The analysis of watershed land-use and cover showed a gradient of intervention: Matadero ≥ Llaviuco > Mazán. The composition of the environmental variables of Mazán streams were different with respect to Llaviuco and Matadero, whose values were similar to each other. The QBR-And index showed a significant gradient from very low levels in Matadero (27.5), moderately high values in Llaviuco (66.5) to high values in Mazán (87). Alder leaf-litter breakdown rates were significantly faster in Mazán than in Matadero and Llaviuco streams, while breakdown rates of Miconia were not significantly different among watersheds. The Kg/Kf ratio was significantly higher in Mazán compared to Llaviuco and Matadero, which did not differ. We consider the Kg/Kf metric sensitive enough to discriminate impacts in the studied areas. Our results support the use of the leaf-litter breakdown as an appropriate tool to assess functional ecological condition, complementing the structural assessments of these Andean rivers.

Ley Orgánica de Hidrocarburos y Ley Orgánica de Ciencia, Tecnología e Innovación. Un Análisis de su Relación con el Proceso Político Venezolano.

The aim of this paper is to present an analysis of the relationship between the Organic Law on Science, Technology and Innovation (2010) and the Organic Law of Hydrocarbons in Venezuela (2006) as instruments of public policy of the Venezuelan political process. We present a conceptual framework that allows us to place ourselves in the context of public policy analysis. The method used is both interpretative laws in question as the theoretical framework for the analysis of public policies to establish the relationship between the above-mentioned laws. Arises as a result the development of both legal instruments are part of the political process in Venezuela, aimed at generating public value, to meet the needs of stakeholders and to the development of the country, emphasizing the sovereignty, both energy and science and technology.

A Comparison of Metamodel-Assisted Pre-Screening Criteria for Multi-Objective Genetic Algorithms

Over the last few years, research activity in approximation (e.g. metamodels) and optimization (e.g. genetic algorithms) methods has improved upon current practices in engineering design and optimization of complex systems with respect to multiple performance metrics, by reducing the number of evaluations of the system’s model that are needed to obtain the set of non-dominated solutions to a given multi-objetive optimal design problem. To this end, several authors have proposed to enhance Multi-Objective Genetic Algorithms (MOGAs) with metamodel-based pre-screening criteria (PSC), so that only those solutions that have the most potential to improve the current approximation of the Pareto Front are evaluated with the (costly) system model. The main goals of this work are to compare the performance of several PSC with an array of test functions taken from the literature, and to study the potential effect on their effectiveness and efficiency of using multi-response metamodels, instead of building independent, individual metamodels for each objective function, as has been done in previous work. Our preliminary results show that no single PSC is observed to be superior overall, though the Minimum of Minimum Distances and Expected Improvement criteria outperformed other PSC in most cases. Results also show that the use of multi-response metamodels improved both the effectiveness and efficiency of PSC and the quality of solution at the end of the optimization in 50% to 60% of test cases.Copyright