Facultad de Ciencias Exactas y Naturales

URI permanente para esta comunidadhttp://10.0.96.45:4000/handle/11056/14053

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A combined experimental and theoretical study of gas sorption on nanoporous silver triazolato metal-organic frameworks
(2014-01-01) Yang, Guang; Santana, Juan A; Rivera Ramos, Milton E.; García - Ricard, Omar; Saavedra-Arias, José; Ishikawa, Yasuyuki; Hernandez-Maldonado, Arturo; Raptis, Raphael G.
A new silver-triazolate metal-organic framework (MOF), {Ag 3[Ag5(μ3-3,5-tBu 2tz)6](BF4)2}n (2), has been prepared and structurally characterized. The CO2 sorption properties of this new MOF and previously reported isostructural analog {Ag 3[Ag5(μ3-3,5-Ph2tz) 6](NO3)2}n (1), were determined experimentally and probed theoretically by a density functional (DF) method. The structures of 1 and 2 are based on flexible Ag5tz 6-structural building units (SBUs), which allow the expansion/contraction of the structure, depending on either the steric requirements of the pendant groups - tBu, or Ph - of the SBUs, or the pressure of the gaseous sorbate. Both silver-triazolate MOFs sorb CO 2 preferentially to N2, O2, CH4 and H2. DF calculations show CO2 to be bound by van der Waals forces to the organic moieties of the porous framework. This work shows that van der Waals interactions of CO2 and the aromatic and aliphatic hydrocarbons of silver-triazolate MOFs are sufficient to achieve preferential sorption of CO2.
A methane sink in the Central American high elevation páramo: Topographic, soil moisture and vegetation effects
(Elsevier B.V., 2020-03-15) Chai, Leanne L.; Hernández Ramirez, Guillermo; Hik, David S; Barrio, Isabel C.; Frost, Carol M.; Chinchilla Soto, Isabel Cristina; Esquivel-Hernández, Germain
Methane (CH4) is a strong greenhouse gas with a global warming potential 23 times larger than that of carbon dioxide. Characterizing ecosystems as either sources or sinks for methane and their magnitudes informs on biosphere contributions to the global CH4 budget and to warming of the atmosphere. We quantified methane fluxes for the first time in a neotropical alpine páramo (Valle de Los Conejos, Chirripó Massif, Costa Rica) and examined the relationships of these fluxes with topography, soil moisture and vegetation, during the transition from dry to rainy season. Using closed chambers and laser spectroscopy, we measured soil CH4 and CO2 fluxes across a field site encompassing: a grassy plain as well as a plain, a gentle slope and a plateau dominated by a dwarf bamboo (Chusquea subtessellata Hitchcock). We found that the páramo landscape acts as a sink for CH4 [−53.1 ± 29.6 (mean ± SE) µg C m−2 hr−1]. Of the four field areas, the grassy plain was on average the strongest CH4 sink, likely because this soil profile had no drainage restrictions and was well aerated. By contrast, in the slope and plateau, a heavily-consolidated subsurface layer was shown to perch water, increasing surface soil moisture and limiting CH4 uptake. Conversely, in certain parts of the plain, where Chusquea grew vigorously in discrete, tall patches, we found intense CH4 uptake beneath these patches. Within the Chusquea plain, these hot spots of CH4 uptake localized under the tall Chusquea had double the uptake rates than outside these patches, with even greater uptake than the average in the grassy plain. Our results show that CH4 uptake in the páramo is driven by moisture interacting with impeding soil layers, vegetation and topography.
A mixed virtual element method for a pseudostress-based formulation of linear elasticity
(Elsevier B.V., 2019) Cáceres, Ernesto; Gatica, Gabriel N.; Sequeira, Filander
In this paper we introduce and analyze a mixed virtual element method (mixed-VEM) for a pseudostress-displacement formulation of the linear elasticity problem with non homogeneous Dirichlet boundary conditions. We follow a previous work by some of the authors, and employ a mixed formulation that does not require symmetric tensor spaces in the finite element discretization. More precisely, the main unknowns here are given by the pseudostress and the displacement, whereas other physical quantities such as the stress, the strain tensor of small deformations, and the rotation, are computed through simple postprocessing formulae in terms of the pseudostress variable. We first recall the corresponding variational formulation, and then summarize the main mixed-VEM ingredients that are required for our discrete analysis. In particular, we utilize a well known local projector onto a suitable polynomial subspace to define a calculable version of our discrete bilinear form, whose continuous version requires information of the variables on the interior of each element. Next, we show that the global discrete bilinear form satisfies the hypotheses required by the Babuška–Brezzi theory. In this way, we conclude the well-posedness of our mixed-VEM scheme and derive the associated a priori error estimates for the virtual solutions as well as for the fully computable projections of them. Furthermore, we also introduce a second element-by-element postprocessing formula for the pseudostress, which yields an optimally convergent approximation of this unknown with respect to the broken H(div)-norm. In addition, this postprocessing formula can also be applied to the postprocessed stress tensor. Finally, several numerical results illustrating the good performance of the method and confirming the theoretical rates of convergence are presented.
A mixed virtual element method for quasi-Newtonian stokes flows
(SIAM, 2018) CACERES, ERNESTO; Gatica, Gabriel; Sequeira, Filander
In this paper we introduce and analyze a virtual element method (VEM) for an augmented mixed variational formulation of a class of nonlinear Stokes models arising in quasi-Newtonian fluids. While the original unknowns are given by the pseudostress, the velocity, and the pressure, the latter is eliminated by using the incompressibility condition, and in order to handle the nonlinearity involved, the velocity gradient is set as an auxiliary one. In this way, and adding a redundant term arising from the constitutive equation relating the psdeudostress and the velocity, an augmented formulation showing a saddle point structure is obtained, whose well-posedness has been established previously by using known results from nonlinear functional analysis. Then, following the basic principles and ideas of the mixed- VEM approach, we introduce a Galerkin scheme employing generic virtual element subspaces and projectors satisfying suitable abstract conditions and derive the corresponding solvability analysis, along with the associated a priori error estimates for the virtual element solution as well as for the fully computable projection of it. Next, we provide two specific choices of subspaces and local projectors verifying the required hypotheses, one of them yielding an optimally convergent mixed- VEM for the fully nonlinear problem studied here, and the other one providing a new approach for the linear version of it, that is, for the Stokes problem. In addition, we are able to apply a second element-by-element postprocessing formula for the pseudostress, which yields an optimally convergent approximation of it with respect to the broken H(div)-norm. Finally, several numerical results illustrating the good performance of the method and confirming the theoretical rates of convergence are reported. © 2018 Society for Industrial and Applied Mathematics.
A mixed virtual element method for the boussinesq problem on polygonal meshes
(Global Science Press, 2021) Gatica, Gabriel; Munar Benitez, Edgar Mauricio; Sequeira, Filander
In this work we introduce and analyze a mixed virtual element method (mixed-VEM) for the two-dimensional stationary Boussinesq problem. The continuous formulation is based on the introduction of a pseudostress tensor depending nonlinearly on the velocity, which allows to obtain an equivalent model in which the main unknowns are given by the aforementioned pseudostress tensor, the velocity and the temperature, whereas the pressure is computed via a postprocessing formula. In addition, an augmented approach together with a fixed point strategy is used to analyze the well-posedness of the resulting continuous formulation. Regarding the discrete problem, we follow the approach employed in a previous work dealing with the Navier-Stokes equations, and couple it with a VEM for the convection-diffiusion equation modelling the temperature. More precisely, we use a mixed-VEM for the scheme associated with the uid equations in such a way that the pseudostress and the velocity are approximated on virtual element subspaces of H(div) and H1, respectively, whereas a VEM is proposed to approximate the temperature on a virtual element subspace of H1. In this way, we make use of the L2-orthogonal projectors onto suitable polynomial spaces, which allows the explicit integration of the terms that appear in the bilinear and trilinear forms involved in the scheme for the uid equations. On the other hand, in order to manipulate the bilinear form associated to the heat equations, we define a suitable projector onto a space of polynomials to deal with the fact that the diffiusion tensor, which represents the thermal conductivity, is variable. Next, the corresponding solvability analysis is performed using again appropriate fixed-point arguments. Further, Strang-type estimates are applied to derive the a priori error estimates for the components of the virtual element solution as well as for the fully computable projections of them and the postprocessed pressure. The corresponding rates of convergence are also established. Finally, several numerical examples illustrating the performance of the mixed-VEM scheme and confirming these theoretical rates are presented.
A New Hybrid of Serpocaulon (Polypodiaceae) from Costa Rica
(Bio One Research Evolved, 2013) Rojas Alvarado, Alexander; Chaves-Fallas, José Miguel
A new hybrid Serpocaulon 3 sessilipinnum A. Rojas & J.M. Chaves is described. The new taxon has characters intermediate between S. fraxinifolium (Jacq.) A.R. Sm. and S. ptilorhizon (Christ) A.R. Sm., which are related mainly with leaf size, rhizome scales, shape and division of blade, shape and width of pinnae, number of areolae and sori series, and shape and color of spores
A posteriori error analysis of a mixed virtual element method for a nonlinear Brinkman model of porous media flow
(Elsevier Ltd, 2020-06-25) Munar, Mauricio; Sequeira, Filander
In this paper we present an a posteriori error analysis of a mixed-VEM discretization for a nonlinear Brinkman model of porous media flow, which has been proposed by the authors in a previous work. Therein, the system is formulated in terms of a pseudostress tensor and the velocity gradient, whereas the velocity and the pressure of the fluid are computed via postprocessing formulas. Furthermore, the well-posedness of the associated augmented formulation along with a priori error bounds for the discrete scheme also were established. We now propose reliable and efficient residualbased a posteriori error estimates for a computable approximation of the virtual solution associated to the aforementioned problem. The resulting error estimator is fully computable from the degrees of freedom of the solutions and applies on very general polygonal meshes. For the analysis we make use of a global inf–sup condition, Helmholtz decomposition, local approximation properties of interpolation operators and inverse inequalities together with localization arguments based on bubble functions. Finally, we provide some numerical results confirming the properties of our estimator and illustrating the good performance of the associated adaptive algorithm
A preliminary isotope-based evapotranspiration partitioningapproach for tropical Costa Rica
(John Wiley and Sons Ltd, 2021-07) Iraheta, Alberto; Birkel, Christian; Benegas, Laura; Ríos, Ney; Sánchez-Murillo, Ricardo; Beyer, Matthias
Spatially and seasonally distributed information on transpiration (T) fluxes is limited in the tropics. Here, we applied a coupled isotope mass balance model to separate water fluxes of T and evapotranspiration (ET) from precipitation (P). The mean annual T was estimated at a resolution of 100 m for Costa Rica (51,100 km2) and a partitioning of monthly T and evaporation (E) for the 2370-km2 San Carlos catchment. The dominant flux in the forest ecosystems was T with a mean annual T of 1086 mm that ranged from 700 mm in Tropical Montane Very Humid Forest to 1400 mm in Subtropical and Tropical Low Montane Rainforests. We estimated an average 85% of ET was T, which is concurrent with expectations for forested tropical regions, but varied according to model parameterization and data sources. A model comparison exercise showed a range of mean annual T estimates from 988 to 1465 mm and a range of T/P from 0.35 to 0.5 with temperature and relative humidity exhibiting the highest impact on the model results. Across Costa Rica, we estimated an average loss of precipitation by T of 38% (1085 mm), whereas interception (I) constitutes 10% (230 mm) and direct evaporation (E) only 7% (192 mm). Similarly, the results at the catchment scale indicated that monthly T contributes 76% (85-mm monthly average) to total ET and E corresponds to 24% (24-mm monthly average). The T rates exhibited an opposite seasonality to rainfall with highest T over the drier months from December to April with a peak in March (101–144 mm) and the minimum T in September (53–71 mm). Around 17% (79–130 mm) of precipitation over the catchment area is lost to T, both E (10–35 mm) and I (15–38 mm) correspond to 5%. Despite the inherent uncertainties of the data assumptions and simplifications, including data interpolation errors, the coupled isotope mass balance model showed in comparison to other global products reasonable water partitioning for different ecosystems in Costa Rica and the San Carlos catchment area. These results can help to evaluate the impact of land cover conversion on the hydrological cycle in Costa Rica, and the simple isotope-based model could be transferred to different biomes of the tropics.
A Priori and a Posteriori Error Analyses of an Augmented HDG Method for a Class of Quasi-Newtonian Stokes Flows
(Springer, 2016-12) Gatica, Gabriel; Sequeira, Filander
In a recent work we developed a new hybridizable discontinuous Galerkin (HDG) method for a class of nonlinear Stokes models arising in quasi-Newtonian fluids. The approach there uses the incompressibility condition to eliminate the pressure, sets the gradient of the velocity as an auxiliary unknown, and enriches the original formulation with convenient redundant equations, thus giving rise to an augmented scheme. However, the corresponding analysis, which makes use of a fixed point strategy that depends on a suitably chosen parameter, yields optimal rates of convergence for only two of the six resulting unknowns, whereas the reported numerical results, showing higher orders than predicted, support the conjecture that the a priori error estimates are not sharp. In the present paper, the main features of the aforementioned augmented formulation are maintained, but after introducing slight modifications of the finite element subspaces for the pseudostress and velocity, we are able to significantly improve our previous analyses and results. More precisely, on one hand we realize here that it suffices to choose the stabilization tensor as the identity times the meshsize, and hence neither fixed-point arguments nor related parameters are needed anymore to establish the well-posedness of the discrete scheme, and on the other hand we now prove optimally convergent approximations for all the unknowns. Furthermore, we develop a reliable and efficient residual-based a posteriori error estimator, and propose the associated adaptive algorithm for our HDG approximation of the nonlinear model problem. Finally, several numerical results illustrating the performance of the method, confirming the theoretical properties of the estimator, and showing the expected behaviour of the adaptive refinements, are presented. © 2016, Springer Science+Business Media New York
A priori and a posteriori error analyses of an HDG method for the Brinkman problem
(Elsevier, 2018-01-15) Gatica, Luis; Sequeira, Filánder
In this paper we introduce and analyze a hybridizable discontinuous Galerkin (HDG) method for the linear Brinkman model of porous media flow in two and three dimensions and with non-homogeneous Dirichlet boundary conditions. We consider a fully-mixed formulation in which the main unknowns are given by the pseudostress, the velocity and the trace of the velocity, whereas the pressure is easily recovered through a simple postprocessing. We show that the corresponding continuous and discrete schemes are well-posed. In particular, we use the projection-based error analysis in order to derive a priori error estimates. Furthermore, we develop a reliable and efficient residual-based a posteriori error estimator, and propose the associated adaptive algorithm for our HDG approximation. Finally, several numerical results illustrating the performance of the method, confirming the theoretical properties of the estimator and showing the expected behavior of the adaptive refinements are presented. © 2017 Elsevier Ltd
A RTk - P-k approximation for linear elasticity yielding a broken H(div) convergent postprocessed stress
(Universidad Nacional, Costa Rica, 2015) Gatica, Gabriel N.; Gatica, Luis F.; Sequeira, Filander
We present a non-standard mixed finite element method for the linear elasticity problem in R-n with non-homogeneous Dirichlet boundary conditions. More precisely, our approach is based on a simplified interpretation of the pseudostress displacement formulation originally proposed in Arnold and Falk (1988), which does not require symmetric tensor spaces in the finite element discretization. We apply the classical Babuska-Brezzi theory to prove that the corresponding continuous and discrete schemes are well-posed. In particular, Raviart-Thomas spaces of order k >= 0 for the pseudostress and piecewise polynomials of degree <= k for the displacement can be utilized. In addition, complementing the results in the aforementioned reference, we introduce a new postprocessing formula for the stress recovering the optimally convergent approximation of the broken H(div)-norm. Numerical results confirm our theoretical findings. (C) 2015 Elsevier Ltd. All rights reserved.
A RTk− Pk approximation for linear elasticity yielding a broken H (div) convergent postprocessed stress
(Elsevier, España, 2015) Gatica, Gabriel N.; Gatica, Luis F.; Sequeira, Filander
We present a non-standard mixed finite element method for the linear elasticity problem in Rn with non-homogeneous Dirichlet boundary conditions. More precisely, our approach is based on a simplified interpretation of the pseudostress–displacement formulation originally proposed in Arnold and Falk (1988), which does not require symmetric tensor spaces in the finite element discretization. We apply the classical Babuˇska–Brezzi theory to prove that the corresponding continuous and discrete schemes are well-posed. In particular, Raviart–Thomas spaces of order k ≥ 0 for the pseudostress and piecewise polynomials of degree ≤k for the displacement can be utilized. In addition, complementing the results in the aforementioned reference, we introduce a new postprocessing formula for the stress recovering the optimally convergent approximation of the broken H(div)-norm. Numerical results confirm our theoretical findings.
A study of the social perception of technology in public spaces in Alajuela Central Cantón, Costa Rica
(Institute of Electrical and Electronics Engineers Inc., 2019-08-20) Víquez Víquez, Andrés; Hernández Ruiz, Irene; Pérez Gutiérrez, Fabián
The present work makes an analysis of the results of a study of the social perception of technology applied in the public spaces of the central canton of Alajuela, Costa Rica. This study is of a non-probabilistic type, commonly used in exploratory research, which counted with the participation of 122 visitors, 55 women and 67 men, aged between 15 and 79 years, in 6 parks of the central canton of Alajuela, using QGIS tool for the spatial analysis of the data. © 2019 IEEE.
ABP desde las trincheras: un caso de estudio en la enseñanza de la Ingeniería de Sistemas
(Alborg University Denmark, 2017) Sandoval-Carvajal, Maria-Marta; Cortés Chavarría, Rita; Porras Piedra, Elena; Lizano Madriz, Fulvio
Este trabajo describe la implementación de la filosofía ABP (Aprendizaje basado en problemas), desde una perspectiva bottom-up, en la Escuela de Informática de la Universidad Nacional (UNA), Costa Rica. La implementación inició de manera intuitiva en los cursos de Ingeniería de Sistemas y luego de un contacto con expertos en el tema, se formalizó su uso, que continúa evolucionando hasta la fecha. Las semejanzas de los principios del POPBL (Lehmann et al. 2008), por sus siglas en inglés, aprendizaje basado en problemas orientado a proyectos y las acciones semi-estructuradas que se implementaron desde el 2006, propiciaron el desarrollo de esta filosofía, para una propuesta y posterior implementación formal del ABP en la UNA, a partir del 2009. Durante la implementación fueron integradas, tanto las habilidades técnicas como las blandas, logrando con ello una buena práctica y un fuerte vínculo de colaboración con la industria. En este contexto, los estudiantes lideran técnicamente un proyecto de desarrollo de software en una empresa real con un problema real. Entre otras cosas, influyó la experiencia, la intuición y el compromiso de los docentes involucrados, así como la utilización de diferentes técnicas tales como; procesos de sensibilización, auto capacitación, análisis comparativos de la teoría y la práctica, junto con asesorías formales e informales. Para medir resultados y su impacto, como parte del proceso, se realizó una investigación con la participación de docentes, estudiantes, egresados y representantes de la industria. Los resultados del estudio indicaban que una iniciativa de implementación del ABP desde el nivel jerárquico más bajo, era percibida como una experiencia beneficiosa para los involucrados. Los principales planes y acciones futuras se relacionan con el fortalecimiento de elementos ágiles, como son el marco de trabajo Scrum, más formación para docentes y una incorporación más activa de los estudiantes en su propio proceso de aprendizaje. Existen oportunidades de mejora y retos que desde las trincheras, en el nivel jerárquico más bajo de la organización, se deben enfrentar, pero también se ha demostrado que no solo fue posible la implementación bottom-up del ABP en la UNA, sino que se ha mejorado evolucionado y mantenido durante más de 10 años.
Academic Emotions in Programming Learning: Women’s Impact on the Software Sector
(Springer Verlag, 2019) Grass, Beatriz; Coto, Mayela; Collazos, Cesar
This document presents an analysis based on a systematic review carried out on the most recognized topics related to academic emotions in the different mechanisms of research and emotional evaluation, trying to focus attention on the initial programming courses, based on the basic programming concepts, independently of the tool in which these concepts are applied, taking into account that the programming courses are considered relevant courses for the training of computer engineers; mechanisms for the evaluation of emotions are also identified. The main emotions in academic contexts are identified and the aim is to identify elements and analysis through the gender variable. Analyses are done focusing on academic emotions. Subsequently, the factors by which women are not linked to the area of software engineering are analyzed, from the perspective of the high drop-out rates due to the programming courses and the impact of the low participation of women in the software sector in the world order, taking into account the roles in which women perform satisfactorily in the software industry.
Actitud del docente de educación secundaria ante la historia de las matemáticas y su implementación como recurso didáctico en las regiones educativas Heredia y San José Central
(Universidad Nacional (Costa Rica), 2020) Álvarez Carranza, Adriana de los Ángeles; Navarro Castillo, Leonel David; Alfaro Arce, Ana Lucía
Describe la actitud, la formación y el uso de la historia de las matemáticas como recurso didáctico que manifiestan los docentes en educación secundaria de las regiones educativas Heredia y San José Central. La investigación sigue un enfoque cuantitativo de tipo descriptivo.
Adaptación del método de descarga de arco voltaico para la síntesis de nanotubos de carbono de pared simple
(Universidad Nacional (Costa Rica), 2009-11) Ramírez Amador, Sergio Alberto; Vega Baudrit, José
Los nanotubos de carbono fueron descubiertos por el físico japonés Sumio lijima en 1991. Sus propiedades físicas y químicas catalogadas como extraordinarias, siguen demostrando gran potencial en multitud de campos. Los nanotubos de carbono son tubos diminutos, basados únicamente en átomos de carbono. Poseen un diámetro de apenas unos pocos nanómetros, sin embargo, su longitud puede ser de hasta un milímetro, (dispone de una relación longitud: diámetro hasta ahora sin precedentes). Están formados a partir de una o varias láminas de grafeno enrolladas sobre sí mismas en forma cilíndrica. Algunos nanotubos están cerrados por media esfera de fulereno y no están cerrados. Existen nanotubos monocapa (NTCPS, un sólo tubo) y multicapa (varios tubos concéntricos). Los nanotubos de una sola capa se llaman nanotubos de carbono de pared simple (Single-Wall Nanotube, SWNT, inglés) y los de varias capas, nanotubos de carbono de multipared, NTCPM, (Multi-Walled Nanotubes, MWNT, en inglés). Debido a la forma en que los átomos de carbono se enlazan en las estructuras de los nanotubos, éstos son muy fuertes. Incluso más fuertes que los diamantes, que poseen también una estructura tridimensional basada principalmente de átomos de carbono. Estos nanotubos son las fibras más fuertes conocidas, un sólo nanotubo perfecto es cerca de 10 a 100 veces más fuerte que el acero por unidad de peso. La investigación sobre nanotubos de carbono es interesante por sus múltiples aplicaciones y posibilidades, pero a la vez compleja por la variedad de sus propiedades electrónicas, térmicas y estructurales que cambian según el diámetro, la longitud, la forma de enrollar y otros. Los nanotubos de carbono, además de ser muy resistentes, poseen propiedades eléctricas interesantes, pues pueden comportarse como conductores o semiconductores de electricidad según el acomodamiento de sus átomos. En el presente proyecto, se buscó la optimización de la metodología encontrada en la literatura para la síntesis de nanotubos de carbono de pared simple por descarga de arco voltaico. Para alcanzar esta optimización, se sintetizaron distintas muestras de NTCPS bajo la modificación de las variables del proceso. Las variables estudiadas fueron la corriente eléctrica de la fuente de poder, presión y tipo de gas. Las sintetizadas fueron caracterizadas por análisis termogravimétrico (TGA), microscopia de fuerza atómica (AFM), microscopia de transmisión electrónica (TEM), microscopia de barrido electrónico (SEM) y por análisis de infrarrojo con transformada de Fourier (FTIR). El fin fue el de corroborar la existencia de NTCPS y su grado de pureza.

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