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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.
On the use of the Spectral Projected Gradient method for Support Vector Machines
(Computational and Applied Mathematics, 2009) Cores, Débora; Escalante, René; González-Lima, María; Jiménez, Oswaldo
In this work we study how to solve the SVM optimization problem by using the Spectral Projected Gradient (SPG) method with three different strategies for computing the projection onto the constrained set. One of the strategies is based on Dykstra’s alternating projection algorithm since there is not a mathematical equation for the projection onto the whole constrained set but the projection on each restriction is easy to compute with exact formulations. We present another strategy based on the Karush-Kunh-Tucker optimality conditions, we call it the Projected-KKT algorithm. We compare these strategies with a third one proposed by Dai and Fletcher. The three schemes are low computational cost and their use within the SPG algorithm leads to a solution of the SVM problem. We study the computational performance of the three strategies when solving randomly generated as well as real life SVM problems. The numerical results show that Projected-KKT is competitive in general with the Dai and Fletcher algorithm, and it is more efficient for some specific problems. They both outperform Dykstra’s algorithm in all the tests.
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.
Development of an improved dynamic model of a Stirling engine and a performance analysis of a cogeneration plant
(Applied Thermal Engineering, 2014-08-13) Cacabelos, Antón; Eguía, Pablo; Míguez, José Luis; Rey, Guillermo; Arce, Elena
In this paper, the authors develop a dynamic model of a commercial micro-combined heat and power (mCHP) unit and analyse its dynamic behaviour when the engine is running at different mass flow inputs. The simulation predicts with a low root mean bias error (RMSE) the most important outputs from the cogeneration unit during the starting, steady-state and stopping periods. Furthermore, the presentedc transient model reproduces the behaviour of the cogeneration unit when the fuel and air mass flows are changing. The obtained results are discussed, and the different possibilities for the variation of the thermal to power ratio are analysed. These combinations include the variation of the flow distribution inside the machine and the position of the exhaust heat exchanger. The power to thermal ratio can be modified between 0.15 and 0.26 for these combinations. The performance of the engine and the variation of the heat source temperature are also analysed theoretically. The simulation results conclude that an important saving could be obtained when the electrical to thermal ratio (ETTR) is tracked for the power or thermal demands from a dwelling.
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.
Nonsmooth spectral gradient methods for unconstrained optimization
(Euro Journal on Computational Optimization, 2017) Loreto, Milagros C.; Aponte, Hugo; Cores, Débora; Raydan, Marcos
To solve nonsmooth unconstrained minimization problems, we combine the spectral choice of step length with two well-established subdifferential-type schemes: the gradient sampling method and the simplex gradient method. We focus on the interesting case in which the objective function is continuously differentiable almost everywhere, and it is of- ten not differentiable at minimizers. In the case of the gradient sampling method, we also present a simple differentiability test that allows us to use the exact gradient direction as frequently as possible, and to build a stochastic subdifferential direction only if the test fails. The proposed spectral gradient sampling method is combined with a monotone line search globalization strategy. On the other hand, the simplex gradient method is a direct search method that only requires function evaluations to build an approximation to the gradient direction. In this case, the proposed spectral simplex gradient method is combined with a suitable nonmonotone line search strategy. For both scenarios, we present preliminary nu- merical results on a set of nonsmooth test functions. These numerical results indicate that using a spectral step length can improve the practical performance of both methods.
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.
Design of Linear Precoders for Correlated Sources in MIMO Multiple Access Channels
(IEEE, 2018) Suárez-Pascal, Pedro; González-Coma, José P.; Fresnedo, Óscar; Castedo, Luis
This work focuses on distributed linear precod ing when users transmit correlated information over a fading Multiple-Input and Multiple-Output Multiple Access Channel. Precoders are optimized in order to minimize the sum-Mean Square Error (MSE) between the source and the estimated symbols. When sources are correlated, minimizing the sum-MSE results in a non-convex optimization problem. Precoders for an arbitrary number of users and transmit and receive antennas are thus obtained via a projected steepest-descent algorithm and a low-complexity heuristic approach. For the more restrictive case of two single-antenna users, a closed-form expression for the minimum sum-MSE precoders is derived. Moreover, for the scenario with a single receive antenna and any number of users, a solution is obtained by means of a semidefinite relaxation. Finally, we also consider precoding schemes where the precoders are decomposed into complex scalars and unit norm vectors. Simulation results show a significant improvement when source correlation is exploited at precoding, especially for low SNRs and when the number of receive antennas is lower than the number of transmitting nodes.
Hybrid Precoding with Time-Modulated Arrays for mmWave MIMO Systems
(IEEE, 2018) González-Coma, José P.; Maneiro-Catoira, Roberto; Castedo, Luis
We consider the utilization of Time-Modulated Arrays (TMAs) as a simple and cost-effective approach to hybrid digital-analog precoding in millimeter wave (mmWave) Multiple-Input Multiple-Output (MIMO) systems. Instead of conventional Variable Phase Shifters (VPSs), our proposed TMA hybrid precoders use Radio Frequency (RF) switches followed by 1-bit VPSs in the analog part. We study the insertion losses at mmWave of a TMA Analog Precoding Network (APN) in terms of Sideband Radiation (SR) and hardware efficiency. In addition, we present different algorithms for the design of both the baseband and RF parts of a TMA hybrid precoder. The proposed methods exhibit different trade-offs between performance, complexity, and power efficiency. Finally, TMA precoders are compared to those implemented with conventional VPSs in terms of insertion losses, chip area and cost, concluding that precoding with TMAs is a competitive solution for mmWave MIMO systems.
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.
Locally conformally flat weakly-Einstein manyfolds
(Archiv der Mathematik, 2018-11-21) García-Río, Eduardo; Haji-Badali, Ali; Mariño-Villar, Rodrigo; Vázquez-Abal, M. Elena
It is shown that locally conformally flat weakly-Einstein manifolds are either locally symmetric, and hence a product Nm1(c) × Nm2(−c), or other-wise they are locally some specific warped product metrics I ×f N(c). As an application we classify weakly-Einstein hypersurfaces in the Euclidean space.
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.
Hybrid LISA for Wideband Multiuser Millimeter Wave Communication Systems under Beam Squint
(IEEE, 2019) González-Coma, José P.; Utschick, Wolfgang; Castedo, Luis
This work jointly addresses user scheduling and precoder/combiner design in the downlink of a wideband mil limeter wave (mmWave) communications system. We consider Orthogonal frequency-division multiplexing (OFDM) modulation to overcome channel frequency selectivity and obtain a number of equivalent narrowband channels. Hence, the main challenge is that the analog preprocessing network is frequency flat and common to all the users at the transmitter side. Moreover, the effect of the signal bandwidth over the Uniform Linear Array (ULA) steering vectors has to be taken into account to design the hybrid precoders and combiners. The proposed algorithmic solution is based on Linear Successive Allocation (LISA), which greedily allocates streams to different users and computes the corresponding precoders and combiners. By taking into account the rank limitations imposed by the hardware at transmission and reception, the performance loss in terms of achievable sum rate for the hybrid approach is negligible. Numerical experiments show that the proposed method exhibits excellent performance with reasonable computational complexity.
Development of paraffinic phase change material nanoemulsions for thermal energy storage and transport in low–temperature applications
(Applied Thermal Engineering, 2019-05-27) Cabaleiro, David; Agresti, Filippo; Barison, Simona; Marcos, Marco A.; Prado, Jose I.; Rossi, Steffano; Bobbo, Sergio; Fedele, Laura
In this study, new phase change material nanoemulsions (PCMEs) were designed and characterized as possible storage and heat transfer media for low–temperature thermal uses. Water–and (ethylene glycol+water)–based emulsions with fine droplets of n–heptadecane and RT21HC commercial paraffin were produced by a solvent–assisted emulsification method. No phase separation or significant growth in PCM drops were observed for the prepared emulsions through storage, after freeze–thaw cycles and under mechanical shear. Phase change transitions were characterized and a significant sub–cooling was observed, with solidification temperatures up to 13 K below the melting point. One pure alkane and two commercial paraffin waxes with higher melting points were considered as nucleating agents to reduce sub–cooling effect. Although the emulsions exhibited diminutions in thermal conductivity up to 9% with respect to the carrier fluids used as base fluid, enhancements in energy storage capacity (considering an operational temperature interval equal to the sub–cooling) reached 26% in the case of RT21HC nanoemulsion based on the (ethylene glycol+water) mixture that contained 10% in mass of paraffin. In addition, the thermal reliability of the nanoemulsions was verified analyzing the changes in latent heat after storage and throughout 1000 thermal cycles.
Development of a calibrated simulation method for airborne particles to optimize energy consumption in operating rooms
(Energies, 2019-06-24) Febrero-Garrido, Lara; López-González, José Luis; Eguía-Oller, Pablo; Granada-Álvarez, Enrique
Operating rooms are stringent controlled environments. All influential factors, in particular, airborne particles, must be within the limits established by regulations. Therefore, energy efficiency stays in the background, prioritizing safety and comfort in surgical areas. However, the potential of improvement in energy savings without compromising this safety is broad. This work presents a new procedure, based on calibrated simulations, that allows the identification of potential energy savings in an operating room, complying with current airborne particle standards. Dynamic energy and airborne particle models are developed and then simulated in TRNSYS and calibrated with GenOpt. The methodology is validated through experimental contrast with a real operating room of a hospital in Spain. A calibrated model with around 2% of error is achieved. The procedure determines the variation in particle concentration according to the flow rate of ventilation supplied and the occupancy of the operating room. In conclusion, energy savings up to 51% are possible, reducing ventilation by 50% while complying with airborne particles standards.
Experimental study on thermophysical properties of alumina nanoparticle enhanced ionic liquids
(Journal of Molecular Liquids, 2019-07-07) Cherecheş, Elena Ionela; Prado, Jose I.; Cherecheş, Marius; Adriana Minea, Alina; Lugo, Luis
In this experimental study, several alumina Nanoparticle Enhanced Ionic Liquids were prepared and studied in regard to their stability, pH, density and thermal conductivity. These new fluids were manufactured by dispersing aluminium oxide nanoparticles in different mixtures based on water and 1-ethyl-3-methylimidazolium methanesulfonate ionic liquid. Furthermore, thermophysical properties (density, thermal conductivity) of pure and binary mixtures with water and 1-ethyl-3-methylimidazolium methanesulfonate were studied in order to select and propose base fluids to design new advanced heat transfer fluids. The pH of the dispersions was determined as around 8.0 - 8.5. In regard to density, the overall [C2mim][CH3SO3] density is higher by 25% than that of water and the influence of ionic liquid density over the mixtures was found to be much higher than that of water, while for the alumina Nanoparticle Enhanced Ionic Liquids the density respects classical equations. Evaluation of thermal conductivity revealed an increase of up to 13% in thermal conductivity when nanoparticles are added to the base fluids and new correlations based on mass fraction and temperature were proposed.
Wideband Hybrid Precoding using Time Modulated Arrays
(IEEE, 2020) González-Coma, José P.; Castedo, Luis
Hybrid digital-analog precoding is a cost effective solution for transmitting over the large bandwidthsand huge antenna arrays available in the millimeter wave frequency bands. In this work we focus on the transmission of wideband signals over hybrid precoders that utilize Time-Modulated Arrays in the analog domain. We consider analog precoders constructed with Single-Pole-Double-Throw switches which are both flexible and efficient. We pose two Orthogonal Frequency-Division Multiplexing symbol configurations to cope with the harmonic interference introduced by the Time-Modulated Array. One does not take advantage of the wireless channel frequency diversity and the other does. We optimize digital and analog precoders to maximize the achievable rate in both symbol configurations. Optimization takes into account the switching devices efficiency and the inherent losses of this antenna technique. Finally, we show the excellent performance obtained with the proposed approach, in terms of achievable rate, compared to that of conventional phased arrays.
Power Efficient Scheduling and Hybrid Precoding for Time Modulated Arrays
(IEEE, 2020) González-Coma, José P.; Castedo, Luis
We consider power efficient scheduling and precoding solutions for multiantenna hybrid digital-analog transmission systems that use Time-Modulated Arrays (TMAs) in the analog domain. TMAs perform beamforming with switches instead of conventional Phase Shifters (PSs). The extremely low insertion losses of switches, together with their reduced power consumption and cost make TMAs attractive in emerging technologies like massive Multiple-Input Multiple-Output (MIMO) and millimeter wave (mmWave) systems. We propose a novel analog processing network based on TMAs and provide an angular scheduling algorithm that overcomes the limitations of conventional approaches. Next, we pose a convex optimization problem to determine the analog precoder. This formulation allows us to account for the Sideband Radiation (SR) effect inherent to TMAs, and achieve remarkable power efficiencies with a very low impact on performance. Computer experiments results show that the proposed design, while presenting a significantly better power efficiency, achieves a throughput similar to that obtained with other strategies based on angular selection for conventional architectures.
Modeling of energy demand and savings associated with the use of epoxy-phase change material formulations
(Materials, 2020-02-01) Arce, Elena; Agrawal, Richa; Suárez, Andrés; Febrero-Garrido, Lara; Luhrs, Claudia C.
This manuscript integrates the experimental findings of recently developed epoxy-phase change material (PCM) formulations with modeling efforts aimed to determine the energy demands and savings derived from their use. The basic PCM system employed was composed of an epoxy resin, a thickening agent, and nonadecane, where the latter was the hydrocarbon undergoing the phase transformation. Carbon nanofibers (CNF) and boron nitride (BN) particulates were used as heat flow enhancers. The thermal conductivities, densities, and latent heat determined in laboratory settings were introduced in a model that calculated, using EnergyPlus software, the energy demands, savings and temperature profiles of the interior and the walls of a shelter for six different locations on Earth. A shipping container was utilized as exemplary dwelling. Results indicated that all the epoxy-PCM formulations had a positive impact on the total energy savings (between 16% and 23%) for the locations selected. The use of CNF and BN showed an increase in performance when compared with the formulation with no thermal filler additives. The formulations selected showed great potential to reduce the energy demands, increase savings, and result in more adequate temperatures for living and storage spaces applications.
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.

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