Artículos de investigación PDI

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A convex optimization approach for solving large scale linear systems
(Bulletin of Computational Applied Mathematics, 2017-01) Cores, Débora; Figueroa, Johanna
The well-known Conjugate Gradient (CG) method minimizes a strictly convex quadratic function for solving large-scale linear system of equations when the co- efficient matrix is symmetric and positive definite. In this work we present and analyze a non-quadratic convex function for solving any large-scale linear system of equations regardless of the characteristics of the coefficient matrix. For finding the global minimizers, of this new convex function, any low-cost iterative opti- mization technique could be applied. In particular, we propose to use the low-cost globally convergent Spectral Projected Gradient (SPG) method, which allow us to extend this optimization approach for solving consistent square and rectangular linear system, as well as linear feasibility problem, with and without convex con- straints and with and without preconditioning strategies. Our numerical results indicate that the new scheme outperforms state-of-the-art iterative techniques for solving linear systems when the symmetric part of the coefficient matrix is indefi-nite, and also for solving linear feasibility problems.
A deterministic optimization approach for solving the rainfall disaggregation problem
(Bulletin of Computational Applied Mathematics, 2015-10-03) Cores, Débora; Guenni, Lelys; Torres, Lisbeth
One of the main problems in hydrology is the time scale of the historical rainfall data, available from many meteorological data bases. Most of the rainfall data is given at a time scale coarser than the one needed for many applications in hydrology and environmental sciences, as the estimation of spatially continuous rainfall at finer time scales, for drainage systems design and extreme rainfall analysis. A method to disaggregate monthly rainfall to daily or finer temporal scale is very important in many applications. Many authors have addressed this problem by using some stochastic methods including several stochastic rainfall models. The lowering resolution methods must be low-cost and low-storage since the amount of rainfall data is large. The purpose of this work is to formulate this problem as a constrained optimization problem and solve it with a low-cost and low-storage deterministic optimization method. We modify the objective function proposed by Guenni and Bárdossy for solving the disaggregation rainfall problem and we use the low-cost spectral projected gradient (SPG) method. In contrast with the stochastic method, a deterministic approach will take into account important information, as for example the gradient of the objective func- tion. The proposed method was applied to a data set from a rainfall network of the central plains of Venezuela, in which rainfall is highly seasonal and data avail- ability at a daily time scale or even higher temporal resolution is very limited. The numerical results show that the SPG method for solving the disaggregation rainfall problem avoids daily precipitations outliers that might occur as an artifact of the simulation procedure and accurately reproduces the probability distribution. Also, the proposed model and methodology outperforms the one proposed by Guenni and Bárdossy (2002) in the sense that it reduces the absolute error value for the statistical properties from the observed data.
A Generalized Two Point Ellipsoidal Anisotropic Ray Tracing for Converted Waves
(Optimization and Engineering, 2007-12) Cores, Débora; Loreto, Milagros C.
Rocks can be anisotropic due to a variety of reasons. When estimating rock velocities from seismic data, failure to introduce anisotropy into earth models could generate distortions in the final images that can have enormous economic impact. To estimate anisotropic earth velocities by tomographic methods, it is necessary to trace rays or to solve the wave equation in models where anisotropy has been properly considered. Thus, in this work we present a 3-D generalized ellipsoidal travel time formulation that allow us to trace rays in an anisotropic medium. We propose to trace rays in anisotropic media by solving a set of nonlinear optimization problems, where the group velocities for P and S wave propagation modes are 3-D ellipsoidal approximations that have been recently obtained. Moreover, we prove that this 3-D ellipsoidal anisotropic ray tracing formulation is a convex nonlinear optimization problem, and therefore any solution of the problem is a global minimum. Each optimization problem is solved by the global spectral gradient method, which requires first order information and has low computation and low storage requirements. Our approach for tracing rays in anisotropic media is a generalization in the sense that handles titled axis of symmetry and, close to the axis of symmetry, it is an accu-rate formulation for 2-D transversely isotropic media and 3-D orthorhombic media, depending on the input parameters. Moreover, this formulation gives the exact ray trajectories in 2-D and 3-D homogeneous isotropic media. The simplicity of the formulation and the low computational cost of the optimization method allow us to present a variety of numerical results that illustrate the behavior and computational advantages of the approach, and the difficulties when working in anisotropic media.
A low-cost optimization approach for solving minimum norm linear systems and linear least-squares problems
(Journal of Computational Mathematics, 2024) Cores, Débora; Figueroa, Johanna
Recently, the authors proposed a low-cost approach, named OPALS (Optimization Approach for Linear Systems) for solving any kind of a consistent linear system regarding the structure, characteristics, and dimension of the coe cient matrix A. The results obtained by this approach for matrices with no structure and with inde nite symmetric part were encouraging when compare with other recent and well-known techniques. In this work, we proposed to extend the OPALS approach for solving the Linear Least-Squares Problem (LLSP) and the Minimum Norm Linear System Problem (MNLSP) using any iterative low-cost gradient-type method, avoiding the construction of the matrices ATA or AAT , and taking full advantage of the structure and form of the gradient of the proposed nonlinear objective function in the gradient direction. The combination of those conditions together with the choice of the initial iterate allow us to produce a novel and e cient low-cost numerical scheme for solving both problems. Moreover, the scheme presented in this work can also be used and extended for the weighted minimum norm linear systems and minimum norm linear least-squares problems. We include encouraging numerical results to illustrate the practical behavior of the proposed schemes.
A new relationship on transport properties of nanofluids. Evidence with novel magnesium oxide based n-tetradecane nanodispersions
(Powder Technology, 397 (2022) 117082, 2022-02-03) Prado, Jose I.; Vallejo, Javier P.; Lugo, Luis
The worldwide increasing of thermal energy consumption fosters new technological solutions based on nanomaterials. The use of nanofluids enhances energy efficiency leading to eco-friendlier devices. Thus, researchers are encouraged to understand how modified thermophysical properties improve heat transfer capability. Magnesium oxide based n-tetradecane nanofluids are designed in terms of stability for cold storage application. Thermal conductivity, viscosity, density, and isobaric heat capacity were determined by transient hot wire, rotational rheometry, mechanical oscillation U-tube, and differential scanning calorimetry. Furthermore, a useful relationship on thermal conductivity and viscosity of nanofluids is proposed based on Andrade, Osida and Mohanty theories. Its reliability is checked with the here reported results and literature data of different nanofluids: titanium oxide within water, silver within poly(ethylene glycol), and aluminium oxide within (1-ethyl-3-methylimidazolium methanesulfonate + water). Similar trends have been found for all nanofluids excepting titanium oxide aqueous nanofluids, this differentiated behaviour being expected by the proposed relationship.
A rank-constrained coordinate ascent approach to hybrid precoding for the downlink of wideband massive MIMO systems
(2023-02-01) González-Coma, José P.; Fresnedo, Óscar; Castedo, Luis
An innovative approach to hybrid analog-digital precoding for the downlink of wideband massive MIMO systems is developed. The proposed solution, termed Rank-Constrained Coordinate Ascent (RCCA), starts seeking the full-digital pre coder that maximizes the achievable sum-rate over all the frequency subcarriers while constraining the rank of the overall transmit covariance matrix. The frequency-flat constraint on the analog part of the hybrid precoder and the non-convex nature of the rank constraint are circumvented by transforming the original problem into a more suitable one, where a convenient structure for the transmit covariance matrix is imposed. Such structure makes the resulting full-digital precoder particularly adequate for its posterior analog-digital factorization. An addi tional problem formulation to determine an appropriate power allocation policy according to the rank constraint is also pro vided. The numerical results show that the proposed method outperforms baseline solutions even for practical scenarios with high spatial diversity.
Adaptative Integral Sliding Mode Based Course Keeping Control of Unnamded Surface Vehicle
(Journal of Marine Science and Engineering, 2022, 10, 68, 2022) González-Prieto, José Antonio; Pérez-Collazo, Carlos; Sing, Yogangh
This paper investigates the course keeping control problem for an unmanned surface vehicle (USV) in the presence of unknown disturbances and system uncertainties. The simulation study combines two different types of sliding mode surface based control approaches due to its precise tracking and robustness against disturbances and uncertainty. Firstly, an adaptive linear sliding mode surface algorithm is applied, to keep the yaw error within the desired boundaries and then an adaptive integral non-linear sliding mode surface is explored to keep an account of the sliding mode condition. Additionally, a method to reconfigure the input parameters in order to keep settling time, yaw rate restriction and desired precision within boundary conditions is presented. The main strengths of proposed approach is simplicity, robustness with respect to external disturbances and high adaptability to static and dynamics reference courses without the need of parameter reconfiguration.
Addition of ferrocyanide-based compounds to repairing joint lime mortars as a protective method for porous building materials against sodium chloride damage
(Materials and structures, 2021) Feijoo, Jorge; Duygu, Ergenç; Fort, Rafael; Buergo, Mónica Álvarez de
Soluble salts are considered one of the main agents in weathering of porous materials used in building constructions. In this work, a comparison in terms of protection against the damage caused by salts, durability and harmful effects of the application of a joint lime repair mortar, with and without K4Fe(CN)6 in its composition, for bonding sandstone blocks contaminated with NaCl was evaluated. Results show that the introduction of ferrocyanide allowed: 1) to protect the mortar during the curing process, hindering the entry of salts into its pores during drying; 2) to improve the carbonation of the mortar and its adhesion to the porous materials´ surface and 3) to remove chlorides from the surrounding materials, reaching percentages of reduction close to 100%. All of these without causing aesthetic damage to the materials and maintaining a protective capacity against NaCl over time that allows considering this treatment not only as a shock treatment but also as a preventive measure.
Ajuste de modelo por optimización no lineal usados en la segmentación de estructuras vasculares en imágenes de tomografía computarizada
(Memorias CIMENICS 2010, 2010-10) Landaeta, Luis; La Cruz, Alexandra; Cores, Débora
Las imágenes de Tomografia Computarizada (TC) son las más utilizadas para el diagnóstico y evaluación de enfermedades vasculares de las extremidades inferiores. Una de las técnicas más usadas y ampliamente aceptadas por expertos radiólogos para la visualización de estas imágenes, es la técnica de Reformación de Curva Planar (Curve Planar Reformation-CPR), la cual utiliza las líneas centrales de las arterias. Dicha línea central debe describir el camino central de las arterias de manera correcta. Actualmente se utiliza un método de aproximación de líneas centrales bastante preciso, el cual trabaja tantos en segmentos arteriales sanos como en los no sanos (obstruidos, calcificados, etc). Sin embargo cuando se encuentra con una bifurcación falla en la determinación correcta de la línea central, por lo que el proceso de segmentación de líneas centrales en las bifurcaciones debe hacerse manualmente. Asumiendo que la imagen del corte transversal en una bifurcación se asemeja más a una spira de Perseus (spiric of Perseus) o sección spírica (spiric section), la cual es un caso particular de una sección tórica (toric section). Este trabajo consiste en la construcción de un modelo geométrico que permita estimar los parámetros de una sección espírica y los valores de la densidad media, tanto de las arterias como del fondo, que mejor se aproxime a la imagen del corte transversal de una bifurcación en una arteria, utilizando un método de optimización no lineal conocido como el método de la región de confianza de Newton. Esto nos da una aproximación más precisa del camino central de las arterias, incluyendo las bifurcaciones.
Alternating Minimization for Wideband Multiuser IRS-aided MIMO Systems under Imperfect CSI
(IEEE, 2023-11) Pérez-Adán, Darian; Joham, Michael; Fresnedo, Óscar; González-Coma, José P.; Castedo, Luis; Utschick, Wolfgang
This work focuses on wideband intelligent reflecting surface (IRS)-aided multiuser MIMO systems. One of the major challenges of this scenario is the joint design of the frequency dependent base station (BS) precoder and user filters, and the IRS phase-shift matrix which is frequency flat and common to all the users. In addition, we consider that the channel state information (CSI) is imperfect at both the transmitter and the receivers. A statistical model for the imperfect CSI is developed and exploited for the system design. A minimum mean square error (MMSE) approach is followed to determine the IRS phase-shift matrix, the transmit precoders, and the receiving filters. The broadcast (BC)-multiple access channel (MAC) duality is used to solve the optimization problem following an alternating minimization approach. Numerical results show that the proposed approach leads to substantial performance gains with respect to baseline strategies that neglect the inter-user interference and do not optimize the IRS phase-shift matrix. Further performance gains are obtained when incorporating into the system design the statistical information of the channel estimation errors.
Analysis of Heat Transfer Characteristics of a GnP Aqueous Nanofluid through a Double-Tube Heat Exchanger
(Nanomaterials 2021, 11(4), 844, 2021) Calviño, Uxía; Vallejo, Javier P.; Buschmann, Matthias H.; Fernández-Seara, José; Lugo, Luis
The thermal properties of graphene have proved to be exceptional and are partly maintained in its multi-layered form, graphene nanoplatelets (GnP). Since these carbon-based nanostructures are hydrophobic, functionalization is needed in order to assess their long-term stability in aqueous suspensions. In this study, the convective heat transfer performance of a polycarboxylate chemically modified GnP dispersion in water at 0.50 wt% is experimentally analyzed. After designing the nanofluid, dynamic viscosity, thermal conductivity, isobaric heat capacity and density are measured using rotational rheometry, the transient hot-wire technique, differential scanning calorimetry and vibrating U-tube methods, respectively, in a wide temperature range. The whole analysis of thermophysical and rheological properties is validated by two laboratories. Afterward, an experimental facility is used to evaluate the heat transfer performance in a turbulent regime. Convective heat transfer coefficients are obtained using the thermal resistances method, reaching enhancements for the nanofluid of up to 13%. The reported improvements are achieved without clear enhancements in the nanofluid thermal conductivity. Finally, dimensionless analyses are carried out by employing the Nusselt and Péclet numbers and Darcy friction factor.
Appraisal of non-destructive in situ techniques to determine moisture- and salt crystallyzation-induced damage in dolostones
(Journal of building engineering, 2022) Fort, R.; Feijoo, Jorge; Varas-Muriel, M.J.; Navacerrada, M.A.; Barbero-Barrera, M.M.; Prida, D. de la
The characterisation of both surface and subsurface pathologies (position, depth, width, …) that affects the porous materials used in building constructions, once in service, is important to establish the most suitable intervention strategy. In this sense, the use of non-destructive techniques allows the analysis of different properties without affecting the material. The present study shows the accuracy of different non-destructive in situ techniques, such as: electrical conductivity and capacitance, infrared thermography, ultrasonic pulse velocity, sound absorption, and electrical resistivity tomography, applied on dolostone ashlar stones outer façade of a sixteenthcentury belltower, affected by moisture and salt induced decay. The joint analysis of the results obtained with different techniques substantially improves the interpretation and characterisation of the detected pathologies, as they complement each other perfectly. Electrical resistivity tomography, which delivers resistivity cross-sections, yields very good results in detecting subsurface pathologies, and sound absorption is particularly useful for stone surfaces. In both cases, the frequency of the electric field and that of the acoustic emission to detect the extent of damage must be established in advance. The joint study of electrical conductivity and capacitance determines the degree of moisture/salts, both at the surface and subsurface, in the materials tested, one of the main causes of scaling and flaking in stony materials. However, the petrological characteristics of the materials used and the identification of the saline phases present must be known in advance to make a correct interpretation of the results.
Assessing the energy demand reduction in a surgical suite by optimizing the HVAC operation during off-use periods
(Applied Sciences (Switzerland), 2020-03-25) Cacabelos, Antón; López-González, José Luis; González-Gil, Arturo; Febrero-Garrido, Lara; Eguía-Oller, Pablo; Granada-Álvarez, Enrique
Hospital surgical suites are high consumers of energy due to the strict indoor air quality (IAQ) conditions. However, by varying the ventilation strategies, the potential for energy savings is great, particularly during periods without activity. In addition, there is no international consensus on the ventilation and hygrothermal requirements for surgical areas. In this work, a dynamic energy model of a surgical suite of a Spanish hospital is developed. This energy model is calibrated and validated with experimental data collected during real operation. The model is used to simulate the yearly energy performance of the surgical suite under different ventilation scenarios. The common issue in the studied ventilation strategies is that the hygrothermal conditions ranges are extended during off-use hours. The maximum savings obtained are around 70% of the energy demand without compromising the safety and health of patients and medical staff, as the study complies with current heating, ventilation and air conditioning (HVAC) regulations.
Assessment of Selected Alternative Fuels for Spanish Navy Ships According to Multi-Criteria Decision Analysis
(Journal of Marine Science and Engineering, 2023-12) Álvarez-Feijoo, Miguel A.; Alfonsín, Víctor; Álvarez-Feijoo, Miguel A.; Llopis, Lara
Climate change and environmental degradation are growing concerns in today’s society, which has led to greater awareness and responsibility regarding the need to adopt sustainable practices. The European Union has established the goal of achieving climate neutrality by 2050, which implies a significant reduction in greenhouse gas emissions in all sectors. To achieve this goal, renewable energies, the circular economy, and energy efficiency are being promoted. A major source of emissions is the use of fossil fuels in different types of ships (from transport ships to those used by national navies). Among these, it highlights the growing interest of the defense sector in trying to reduce these emissions. The Spanish Ministry of Defense is also involved in this effort and is taking steps to reduce the carbon footprint in military operations and improve sustainability in equipment acquisition and maintenance. The objective of this study is to identify the most promising alternative fuel among those under development for possible implementation on Spanish Navy ships in order to reduce greenhouse gas emissions and improve its capabilities. To achieve this, a multi-criteria decision-making method will be used to determine the most viable fuel option. The data provided by the officers of the Spanish Navy is of great importance, thanks to their long careers in front of the ships. The analysis revealed that hydrogen was the most suitable fuel with the highest priority, ahead of LNG, and scored the highest in most of the sections of the officials’ ratings. These fuels are less polluting and would allow a significant reduction in emissions during the navigation of ships. However, a further study would also have to be carried out on the costs of adapting to their use and the safety of their use.
Beamforming techniques for passive radar: an overview
(MDPI (Multidisciplinary Digital Publishing Institute), 2023-03-24) Núñez-Ortuño, José M.; González-Coma, José P.; Nocelo López, Rubén; Troncoso-Pastoriza, Francisco; Álvarez-Hernández, María
Passive radar is an interesting approach in the context of non-cooperative target detection. Because the signal source takes advantage of the so-called illuminator of opportunity (IoO), the deployed system is silent, allowing the operator cheap, portable, and practically undetectable deployments. These systems match perfectly with the use of antenna arrays to take advantage of the additional gains provided by the coherent combination of the signals received at each element. To obtain these benefits, linear processing methods are required to enhance the system’s performance. In this work, we summarize the main beamforming methods in the literature to provide a clear picture of the current state of the art. Next, we perform an analysis of the benefits and drawbacks and explore the chance of increasing the number of antenna elements. Finally, we identify the major challenges to be addressed by researchers in the future.
Biochar Amendments and Phytoremediation: A Combined Approach for Effective Lead Removal in Shooting Range Soils
(Toxics, 2024) Maceiras, Rocío; Pérez-Rial, Leticia; Alfonsín, Víctor; Feijoo, Jorge; López, Ignacio
The increasing contamination of soil with heavy metals poses a problem to environmental sustainability. Among these pollutants, lead is particularly concerning due to its persistence in the environment, with harmful effects on human health and ecosystems. Various strategies that combine phytoremediation techniques with soil amendments have emerged to mitigate lead contamination. In this context, biochar has gained significant attention for its potential to enhance soil quality and remediate metal-contaminated environments. This study aims to investigate the combined effect of biochar amendments on the phytoremediation of lead-contaminated shooting range soils. A series of experiments were conducted to determine the impact of the amount and distribution of biochar on lead removal from soil. Soil samples were incubated with biochar for one week, after which two types of seeds (Brassica rapa and Lolium perenne) were planted. Plant and root lengths, as well as the number of germinated seeds, were measured, and a statistical analysis was conducted to determine the influence of the amendments. After one month, the Pb concentration decreased by more than 70%. Our results demonstrate that seed germination and plant growth were significantly better in soil samples where biochar was mixed rather than applied superficially, with the optimal performance observed at a 10% wt. biochar amendment. Additionally, the combined use of biochar and phytoremediation proved highly effective in immobilizing lead and reducing its bioavailability. These findings suggest that the combination of biochar, particularly when mixed at appropriate concentrations, and Brassica rapa significantly improved lead removal efficiency.
Bioethanol production from industrial algae waste
(Waste management, 2019) Alfonsín, Víctor; Maceiras, Rocío; Gutiérrez, Carlos
This paper presents a study carried out to determine the feasibility to obtain bioethanol from an industrial algae waste. The influence of some parameters on acid hydrolysis was studied. The results indicated that the overall hydrolysis ability is a function of the combined action of acid concentration, temperature and reaction time. The hydrolysates with higher amount of reducing sugars were used for ethanolic fermentation using S. Cerevisiae. The highest bioethanol yield 11.6 gEtOH/galgae was obtained at 70 min, a sulphuric acid concentration of 9% wt. and acid/dried algae ratio of 7. The present study showed that the industrial waste of Euchema Spinosum contains carbohydrates that can be converted into bioethanol with an efficiency of 75%. Moreover, the potential of the residue after acid hydrolysis process as sustainable solid fuel regarding its properties was analysed. The results were compared with those obtained from the literature and discussed according to international standards.
C02 Capture via Adsorption Using Silica Gel
(European Journal of Sustainable Development, 2024-10-01) Pérez-Rial, Leticia; Alfonsín, Víctor; Maceiras, Rocío; Feijoo, Jorge; Vallejo, Javier P.
This paper investigates the potential of silica gel as an effective adsorbent for C02 capture. The study explores the adsorption mechanisms, the efficiency of C02 upta.ke, and sorne factors influencing the adsorption capacity of silica gel.The experimental results demonstrate the significant potential of silica gel for C02 adsorption under various conditions. Toe adsorption capacity was found to be highly dependent on parameters such as gas flow rate and the particle size of silica gel. The findings suggest that ,vith optimized conditions, silica gel could be a viable material for reducing atmospheric C02 levels. This research contributes to the development of sustainable and efficient technologies for mitigating climate change through carbon dioxide capture and storage.
Channel estimation and hybrid precoding for frequency selective multiuser mmWave MIMO systems
(IEEE, 2018-02) González-Coma, José P.; González-Prelcic, Nuria; Castedo, Luis; Heath, Robert W. Jr.
Configuring the hybrid precoders and combiners in a millimeter wave (mmWave) multiuser (MU) multiple-input multiple-output (MIMO) system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink (DL). In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations (MSs) on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates, the base station designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: 1) a factorization of the purely digital solution, and 2) an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.
CO2 Capture via Adsorption Using Silica Gel
(European Journal of Sustainable Development, 2024) Pérez-Rial, Leticia; Alfonsín, Víctor; Maceiras, Rocío; Feijoo, Jorge; Vallejo, Javier P.
This paper investigates the potential of silica gel as an effective adsorbent for CO2 capture. The study explores the adsorption mechanisms, the efficiency of CO2 uptake, and some factors influencing the adsorption capacity of silica gel. The experimental results demonstrate the significant potential of silica gel for CO2 adsorption under various conditions. The adsorption capacity was found to be highly dependent on parameters such as gas flow rate and the particle size of silica gel. The findings suggest that with optimized conditions, silica gel could be a viable material for reducing atmospheric CO2 levels. This research contributes to the development of sustainable and efficient technologies for mitigating climate change through carbon dioxide capture and storage.

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