Nuestros investigadores

Alejandro Rivas Nieto

Publicaciones científicas más recientes (desde 2010)

Autores: Aramburu, Jorge, (Autor de correspondencia); Antón, R; Rivas, Alejandro; et al.
ISSN 1025-5842  Vol. 22  Nº 5  2019  págs. 518 - 532
Balloon-occluded transarterial chemoembolisation (B-TACE) is an intraarterial transcatheter treatment for liver cancer. In B-TACE, an artery-occluding microballoon catheter occludes an artery and promotes collateral circulation for drug delivery to tumours. This paper presents a methodology for analysing the haemodynamics during B-TACE, by combining zero-dimensional and three-dimensional modelling tools. As a proof of concept, we apply the methodology to a patient-specific hepatic artery geometry and analyse two catheter locations. Results show that the blood flow redistribution can be predicted in this proof-of-concept study, suggesting that this approach could potentially be used to optimise catheter location.
Autores: Aramburu, Jorge, (Autor de correspondencia); Antón, R; Rivas, Alejandro; et al.
ISSN 2040-7939  Vol. 34  Nº e2983  2018 
Autores: Goikoetxea, E.; Rivas, Alejandro; Murgia, X. ; et al.
ISSN 0278-6826  Vol. 51  Nº 2  2017  págs. 168 - 177
Surfactant aerosol delivery in conjunction with a noninvasive respiratory support holds the potential to treat neonatal respiratory distress syndrome in a safe manner. The objective of the present study was to gain knowledge in order to optimize the geometry of an intracorporeal inhalation catheter and improve surfactant aerosol delivery effectiveness in neonates. Initially, a mathematical model capable of predicting the aerosol flow generated by this inhalation catheter within a physical model of the neonatal trachea was implemented and validated. Subsequently, a numerical study was performed to analyze the effect of the aerosol liquid droplet size and mass flow rate on surfactant delivery and on the required aerosolization time period. Experimental validation of the mathematical model showed a close prediction of the air axial velocity at the distal end of the physical model, with an absolute error between 0.01 and 0.15 m/s. Furthermore, an admissible absolute error between 0.2 and 2 mm was attained in the prediction of the aerosol mean aerodynamic diameter and mass median aerodynamic diameter in this region. The numerical study highlighted the beneficial effects of generating an intracorporeal aerosol with a mass median aerodynamic diameter higher than 4 mm and a surfactant mass flow rate above 8.93 mg/s in order to obtain effective surfactant delivery in neonates with minimal airway manipulation.
Autores: Aramburu, Jorge; Antón, R; Rivas, Alejandro; et al.
ISSN 2040-7939  2017  págs. e02895
Liver radioembolization is a promising treatment option for combating liver tumors. It is performed by placing a microcatheter in the hepatic artery and administering radiation-emitting microspheres through the arterial bloodstream so that they get lodged in the tumoral bed. In avoiding nontarget radiation, the standard practice is to conduct a pretreatment, in which the microcatheter location and injection velocity are decided. However, between pretreatment and actual treatment some of the parameters that influence the particle distribution in the liver can vary, resulting in radiation-induced complications. The present study aims to analyze the influence of a commercially available microcatheter with an angled tip and particle injection velocity in terms of segment-to-segment particle distribution. Specifically, four tip orientations and two injection velocities are combined to yield a set of eight numerical simulations of the particle-hemodynamics in a patient-specific truncated hepatic artery. For each simulation, four cardiac pulses are simulated. Particles are injected during the first cycle, and the remaining pulses enable the majority of the injected particles to exit the computational domain. Results indicate that, in terms of injection velocity, particles are more spread out in the cross-sectional lumen areas as the injection velocity increases. The tip's orientation also plays a role because it influences the near-tip hemodynamics, therefore altering the particle travel through the hepatic artery. However, results suggest that particle distribution tries to match the blood flow split, therefore particle injection velocity and microcatheter tip orientation playing a minor role in segment-to-segment particle distribution.
Autores: Aramburu, Jorge; Antón, R; Rivas, Alejandro; et al.
ISSN 2040-7939  Vol. 33  Nº 2  2017 
Liver radioembolization (RE) is a treatment option for patients with unresectable and chemorefractory primary and metastatic liver tumours. RE consists of intra-arterially administering via catheter radioactive microspheres that locally attack the tumours, sparing healthy tissue. Prior to RE, the standard practice is to conduct a treatment-mimicking pretreatment assessment via the infusion of Tc-99m-labelled macroaggregated albumin microparticles. The usefulness of this pretreatment has been debated in the literature, and thus, the aim of the present study is to shed light on this issue by numerically simulating the liver RE pretreatment and actual treatment particle-haemodynamics in a patient-specific hepatic artery under two different literature-based cancer scenarios and two different placements of a realistic end-hole microcatheter in the proper hepatic artery. The parameters that are analysed are the following: microagent quantity and size (accounting for RE pretreatment and treatment), catheter-tip position (near the proper hepatic artery bifurcation and away from it), and cancer burden (10% and 30% liver involvement). The conclusion that can be reached from the simulations is that when it comes to mimicking RE in terms of delivering particles to tumour-bearing segments, the catheter-tip position is much more important (because of the importance of local haemodynamic pattern alteration) than the infused microagents (i.e. quantity and size). Cancer burden is another important feature because the increase in blood flow rate to tumour-bearing segments increases the power to drag particles. These numerical simulation-based conclusions are in agreement with clinically observed events reported in the literature. Copyright (c) 2016 John Wiley & Sons, Ltd.
Autores: Aramburu, Jorge; Antón, R; Borro, Diego; et al.
ISSN 2057-1976  Vol. 2  Nº 1  2016  págs. 015001
The analysis of the progression of cardiovascular diseases is an active area of ongoing research. This paper develops an image registration-based methodology to quantify the patient-specific local blood vessel shape variations that occur in the radial direction (i.e. expansion or shrinkage) over an imaging follow-up period, and an example is presented as proof of principle. The methodology can be used for complex vessels with bifurcations, and it is able to identify and address vessel deformations if changes in tortuosity or longitudinal direction are small. The methodology consists of (a) overlapping the baseline and follow-up vessel surfaces by matching the lumen centerline, (b) dividing the region of interest into slices perpendicular to the centerline and centering each slice, and (c) dividing each centered slice into sectors. The local approach consists of analyzing a representative point in each sector of each slice (i.e. each patch). In this paper the algorithm is applied to a patient-specific abdominal aortic aneurysm (AAA) as a proof of principle of the method. Six patient-specific image reconstructions from a single subject followed for 28 months are analyzed in pairs, yielding five time spans to which the algorithm was applied. The algorithm was able to quantify the AAA radial growth. The average AAA radial growths for the five case studies are ¿2.13 mm, 3.43 mm, ¿0.25 mm, 1.41 mm, and 0.84 mm, whereas the maximum local growths are 4.76 ± 0.15 mm, 9.30 ± 1.13 mm, 2.08 ± 0.05 mm, 4.10 ± 0.14 mm, and 4.16 ± 0.45 mm. The tolerance of the geometric local measurements is related to the matching processes (i.e. overlapping the geometries and centering each slice) because of the vessel deformation that took place over time. Thus, this methodology has been used to quantify the average AAA growth and the maximum local AAA growth (± the tolerance) as metrics of the vessel's radial growth.
Autores: Martín, José Manuel, (Autor de correspondencia); Rivas, Alejandro; et al.
ISSN 0032-5899  Vol. 59  Nº 2  2016  págs. 128 - 141
Present empirical correlations to predict the median particle size of water atomised powders have a validity restricted to a particular atomiser and alloy family. This work proposes a mathematical function that takes into account the influence of the heat transfer coefficient and, therefore, of the solidification time on the median particle size. This equation is applied in combination with previously proposed empirical correlations to extend their validity to a broader range of alloys. Experiments were conducted with alloys of different melting point (Fe base, Cu base and Sn). Quantitative measurements of the median particle size, tap density and several shape factors, and qualitative observations of the particle shape confirmed the importance of the heat transfer rate. It is shown that the inclusion of the solidification time effect results in a better agreement between calculated and experimental data when both low and high melting temperature alloys are taken together.
Autores: Martin, L.; Nieto, N.; et al.
ISSN 1359-4311  Vol. 102  2016  págs. 1081 - 1094
Environmental sustainability, more efficient use of energy, and active safety concepts are becoming important requirements for the actual elevation sector. In this context IK4-IKERLAN and ORONA have designed an auxiliary energy storage system (ESS) for a residential elevation application based on lithium-ion cells. Safety and specially lifetime are two of the main concerns surrounding this new technology, which is closely related to the cells operating behavior and temperature asymmetries in the complete ESS. Therefore, the temperature of the cells in battery packs (BPs) needs to be controlled in an efficient way. This paper describes the development of the thermal management system (TMS) designed for this application based on various Computational Fluid Dynamics (CFD) mathematical models. The accuracy of Transient model is validated by using a single module to compare the simulation temperature results with experimental measurements, with a maximum time-averaged temperature prediction error of 1.5 degrees C. The proposed design is validated as it fulfills the requirements for a wide operating window, with a maximum cell temperature of 39 degrees C and a thermal dispersion at system level below 3 degrees C for the worst tested case. A more realistic current profile is checked numerically in the worst ambient and operative conditions for different virtual design variants to propose improvements. (C) 2016 Elsevier Ltd. All rights reserved.
Autores: Aramburu, Jorge; Antón, R; Rivas, Alejandro; et al.
ISSN 0021-9290  Vol. 49   Nº 15   2016  págs. 3705 - 3713
Radioembolization, which consist of the implantation of radioactive microspheres via intra-arterially placed microcatheter, is a safe and effective treatment for liver cancer. Nevertheless, radioembolization-related complications and side effects may arise, which are an active area of ongoing research. The catheter design has been claimed as an option in reducing these complications. In this paper, the influence of catheter type and location are investigated. The study was undertaken by numerically simulating the particle¿hemodynamics in a patient-specific hepatic artery during liver radioembolization. The parameters modified were cancer scenario (30% liver involvement in the right lobe, `scenario A¿, and in both lobes, `scenario B¿), catheter type (standard end-hole microcatheter, SMC, and antireflux catheter, ARC), and the location of the tip in the proper hepatic artery (in the straight part, `inlet¿, and near the bifurcation, `bifurcation¿). Comparing ARC with SMC, the maximum and average (over segments) absolute difference in the percentage of particles that reached each segment were 19.62% and 9.06% when injecting near the inlet for scenario A; 3.54% and 1.07% injecting near the bifurcation for scenario A; and 18.31% and 11.85% injecting near the inlet for scenario B. It seems, therefore, that the location of the catheter tip in the artery is crucial in terms of particle distribution. Moreover, even though the near-tip blood flow was altered due to the presence of a catheter, the particle distribution matched the flow split if the distance between the injection point and the first bifurcation encountered enabled the alignment of particles with blood flow.
Autores: Aramburu, Jorge; Antón, R; Rivas, Alejandro; et al.
ISSN 2040-7939  Vol. 32  Nº 11  2016  págs. e02764
Some of the latest treatments for unresectable liver malignancies (primary or metastatic tumours), which include bland embolisation, chemoembolisation, and radioembolisation, among others, take advantage of the increased arterial blood supply to the tumours to locally attack them. A better understanding of the factors that influence this transport may help improve the therapeutic procedures by taking advantage of flow patterns or by designing catheters and infusion systems that result in the injected beads having increased access to the tumour vasculature. Computational analyses may help understand the haemodynamic patterns and embolic-microsphere transport through the hepatic arteries. In addition, physiological inflow and outflow boundary conditions are essential in order to reliably represent the blood flow through arteries. This study presents a liver cancer arterial perfusion model based on a literature review and derives boundary conditions for tumour-bearing liver-feeding hepatic arteries based on the arterial perfusion characteristics of normal and tumorous liver segment tissue masses and the hepatic artery branching configuration. Literature-based healthy and tumour-bearing realistic scenarios are created and haemodynamically analysed for the same patient-specific hepatic artery. As a result, this study provides boundary conditions for computational fluid dynamics simulations that will allow researchers to numerically study, for example, various intravascular devices
Autores: Aramburu, Jorge; Antón, R; Rivas, Alejandro; et al.
ISSN 0021-9290  Vol. 49  Nº 15  2016  págs. 3714 - 3721
Liver radioembolization is a treatment option for patients with primary and secondary liver cancer. The procedure consists of injecting radiation-emitting microspheres via an intra-arterially placed microcatheter, enabling the deposition of the microspheres in the tumoral bed. The microcatheter location and the particle injection rate are determined during a pretreatment work-up. The purpose of this study was to numerically study the effects of the injection characteristics during the first stage of microsphere travel through the bloodstream in a patient-specific hepatic artery (i.e., the near-tip particle¿hemodynamics and the segment-to-segment particle distribution). Specifically, the influence of the distal direction of an end-hole microcatheter and particle injection point and velocity were analyzed. Results showed that the procedure targeted the right lobe when injecting from two of the three injection points under study and the remaining injection point primarily targeted the left lobe. Changes in microcatheter direction and injection velocity resulted in an absolute difference in exiting particle percentage for a given liver segment of up to 20% and 30%, respectively. It can be concluded that even though microcatheter placement is presumably reproduced in the treatment session relative to the pretreatment angiography, the treatment may result in undesired segment-to-segment particle distribution and therefore undesired treatment outcomes due to modifications of any of the parameters studied, i.e., microcatheter direction and particle injection point and velocity.
Autores: Martín-Gómez, César; Ibáñez, María; Bermejo-Busto, J.; et al.
ISSN 0143-6244  Vol. 37  Nº 4  2016  págs. 431 - 449
The article describes from an architectonical point of view the design, assembly, and energy behavior of a prototype for air-conditioning in residential buildings using Peltier cells, which means the application in the field of construction of a technology used in very specific areas. The new system has been designed as an independent, prefabricated, modular construction element that must fit perfectly between the structural floors and is easily adapted to the demands of different buildings. The thermoelectric cooling heating unit is designed to offer a high level of comfort to those living in the building. The only mechanical elements are the dissipation heat fans placed on the outside of the prototype, and heat sinks to transfer the heat from the power elements, reducing the possibilities of failure. The result of all these ideas is the construction of a prefabricated module, consisting of a simplified inhabited housing unit with a thermoelectric installation serving the module, which has obtained a national patent. The results of the thermal and electric behavior demonstrate that the system does not work as well as had been expected; nevertheless, the system has a high potential for its use in buildings associated with photovoltaic.
Autores: Larraona. Gorka S.; Viñolas, Jordi; et al.
ISSN 0954-4097  Vol. 229  Nº 2  2015  págs. 186 - 200
In a crosswind scenario, the risk of high-speed trains overturning increases when they run on viaducts since the aerodynamic loads are higher than on the ground. In order to increase safety, vehicles are sheltered by fences that are installed on the viaduct to reduce the loads experienced by the train. Windbreaks can be designed to have different heights, and with or without eaves on the top. In this paper, a parametric study with a total of 12 fence designs was carried out using a two-dimensional model of a train standing on a viaduct. To asses the relative effectiveness of sheltering devices, tests were done in a wind tunnel with a scaled model at a Reynolds number of 1x10(5), and the train's aerodynamic coefficients were measured. Experimental results were compared with those predicted by Unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of flow, showing that a computational model is able to satisfactorily predict the trend of the aerodynamic coefficients. In a second set of tests, the Reynolds number was increased to 12x10(6) (at a free flow air velocity of 30m/s) in order to simulate strong wind conditions. The aerodynamic coefficients showed a similar trend for both Reynolds numbers; however, their numerical value changed enough to indicate that simulations at the lower Reynolds number do not provide all required information. Furthermore, the variation of coefficients in the simulations allowed an explanation of how fences modified the flow around the vehicle to be proposed. This made it clear why increasing fence height reduced all the coefficients but adding an eave had an effect mainly on the lift force coefficient. Finally, by analysing the time signals it was possible to clarify the influence of the Reynolds number on the peak-to-peak amplitude, the time period and the Strouhal number.
Autores: Aramburu, Jorge; Antón, R; Bernal, N.; et al.
ISSN 0954-4119  Vol. 229  Nº 4  2015  págs. 291 - 306
Physiological outflow boundary conditions are necessary to carry out computational fluid dynamics simulations that reliably represent the blood flow through arteries. When dealing with complex three-dimensional trees of small arteries, and therefore with multiple outlets, the robustness and speed of convergence are also important. This study derives physiological outflow boundary conditions for cases in which the physiological values at those outlets are not known (neither in vivo measurements nor literature-based values are available) and in which the tree exhibits symmetry to some extent. The inputs of the methodology are the three-dimensional domain and the flow rate waveform and the systolic and diastolic pressures at the inlet. The derived physiological outflow boundary conditions, which are a physiological pressure waveform for each outlet, are based on the results of a zero-dimensional model simulation. The methodology assumes symmetrical branching and is able to tackle the flow distribution problem when the domain outlets are at branches with a different number of upstream bifurcations. The methodology is applied to a group of patient-specific arteries in the liver. The methodology is considered to be valid because the pulsatile computational fluid dynamics simulation with the inflow flow rate waveform (input of the methodology) and the derived outflow boundary conditions lead to physiological results, that is, the resulting systolic and diastolic pressures at the inlet match the inputs of the methodology, and the flow split is also physiological.
Autores: Goikoetxea, E.; Murgia, X.; Serna-Grande, P.; et al.
Revista: PLOS ONE
ISSN 1932-6203  Vol. 9  Nº 9  2014 
Objective: Aerosol delivery holds potential to release surfactant or perfluorocarbon (PFC) to the lungs of neonates with respiratory distress syndrome with minimal airway manipulation. Nevertheless, lung deposition in neonates tends to be very low due to extremely low lung volumes, narrow airways and high respiratory rates. In the present study, the feasibility of enhancing lung deposition by intracorporeal delivery of aerosols was investigated using a physical model of neonatal conducting airways. Methods: The main characteristics of the surfactant and PFC aerosols produced by a nebulization system, including the distal air pressure and air flow rate, liquid flow rate and mass median aerodynamic diameter (MMAD), were measured at different driving pressures (4-7 bar). Then, a three-dimensional model of the upper conducting airways of a neonate was manufactured by rapid prototyping and a deposition study was conducted. Results: The nebulization system produced relatively large amounts of aerosol ranging between 0.3 +/- 0.0 ml/min for surfactant at a driving pressure of 4 bar, and 2.0 +/- 0.1 ml/min for distilled water (H(2)Od) at 6 bar, with MMADs between 2.61 +/- 0.1 mu m for PFD at 7 bar and 10.18 +/- 0.4 mu m for FC-75 at 6 bar. The deposition study showed that for surfactant and H(2)Od aerosols, the highest percentage of the aerosolized mass (similar to 65%) was collected beyond the third generation of branching in the airway model. The use of this delivery system in combination with continuous positive airway pressure set at 5 cmH(2)O only increased total airway pressure by 1.59 cmH(2)O at the highest driving pressure (7 bar). Conclusion: This aerosol generating system has the potential to deliver relatively large amounts of surfactant and PFC beyond the third generation of branching in a neonatal airway model with minimal alteration of pre-set respiratory support.
Autores: Antón, R; Larraona. Gorka S.; et al.
ISSN 0142-727X  Vol. 46  2014  págs. 43 - 60
A perforated plate placed behind an axial fan (push cooling) is a common assembly in electronic systems. The flow approaching the screen will have a swirling component, and therefore, there is uncertainty in the prediction of the flow pattern at the outlet of the screen and the pressure drop through the screen. Correctly predicting the flow field is important in order to properly place the electronic components. This work tries to give some insight into these issues. A wind tunnel was manufactured in order to produce the typical flow field at the outlet of an axial fan and to measure the field at the inlet and at the outlet of the perforated plate using the Particle Image Velocimety (PIV) technique; the pressure drop through the screen was also measured. The velocity contours measured at the screen inlet were used as boundary conditions for computational fluid dynamics (CFD) simulations. Several turbulence models (k-epsilon, k-omega and RSTM) and their variations were used for the simulations and the results at the outlet of the perforated plate are compared with the Particle Image Velocimetry results. Two screens with very different geometrical characteristics were used. Results show that if k-e models are used a significant error is made in the prediction of the velocity field and in the pressure drop. Although the k-omega models predicted better than the k-e models, the RSTM were shown to be the most reliable. (C) 2014 Elsevier Inc. All rights reserved.
Autores: Antón, R; Larraona. Gorka S.; et al.
ISSN 1994-2060  Vol. 8  Nº 4  2014  págs. 623 - 638
A perforated plate placed downstream of an axial fan in order to avoid electromagnetic interferences (push cooling) is a common assembly in electronic systems. Because of the swirling component that the flow approaching the screen has, there is no accuracy in knowing how the screen affects the flow pattern downstream of the screen and the pressure drop through the screen. Since cooling capacity is related to velocity, the placement of the components downstream of the screen will be related to the velocity magnitude. Thus, properly predicting the flow pattern is highly important, and the results of this work may serve a good guideline for thermal designers to surmount this challenge. In order to establish how the screen affects the flow pattern, a parametric study is carried out. This study is performed by a central composite face-centered (CCF) Design of Experiment (DoE), which demanded 81 Computational Fluid Dynamics (CFD) simulations and for which the Reynolds Stress Transport Model was used as a turbulence model. Thanks to the numerical results, the influence that different operational and geometrical parameters have on the flow pattern downstream of a screen and on the total pressure drop is analyzed. The swirl that the flow has at the inlet is found to be related to the screen's capacity to homogenize the flow downstream of the screen, as its thickness plays an important role in the flow's tangential component destruction. The main effects of the parameters and the interactions between them are shown. At the same time, through DoE techniques, different reduced models that predict how the flow pattern changes because of the screen are presented as useful tools for thermal designers.
Autores: Nieto, N.; Diaz, L.; et al.
ISSN 0378-7753  Vol. 272  2014  págs. 291 - 302
Battery packs conformed by large format lithium-ion cells are increasingly being adopted in hybrid and pure electric vehicles in order to use the energy more efficiently and for a better environmental performance. Safety and cycle life are two of the main concerns regarding this technology, which are closely related to the cell's operating behavior and temperature asymmetries in the system. Therefore, the temperature of the cells in battery packs needs to be controlled by thermal management systems (TMSs). In the present paper an improved design methodology for developing TMSs is proposed. This methodology involves the development of different mathematical models for heat generation, transmission, and dissipation and their coupling and integration in the battery pack product design methodology in order to improve the overall safety and performance. The methodology is validated by comparing simulation results with laboratory measurements on a single module of the battery pack designed at IK4-IKERLAN for a traction application. The maximum difference between model predictions and experimental temperature data is 2 degrees C. The models developed have shown potential for use in battery thermal management studies for EV/HEV applications since they allow for scalability with accuracy and reasonable simulation time. (C) 2014 Elsevier B.V. All rights reserved.
Autores: Ramos, Juan Carlos; Rivas, Alejandro; et al.
ISSN 1359-4311  Vol. 62  Nº 1  2014  págs. 215 - 228
An algebraic thermal zonal model of the ventilation of underground transformer substations during a standardised temperature rise test is presented in this paper. The development and adjustment of the proposed model rely on the analysis of the air flow pattern and temperature distributions obtained by a more complex model numerically solved by means of CFD techniques. The flow domain of the model represents a section of the substations divided into several interrelated zones where the mass and the energy conservation equations are formulated and the generated system of nonlinear algebraic equations is solved. The model is validated by comparing its results with the ones obtained by the CFD model and with the experimental results of eight temperature rise tests under different conditions. A parametric analysis was carried out on the model to prove its utility as an efficient tool to improve and optimise the thermal performance of transformer substations during the design process. From the parametric study it has been inferred that the main parameters affecting the ventilation of the substations are the pass area between the LV-MV zone and the transformer zone, the surface area of the ventilation grilles in the substation with horizontal ventilation, and the perimeter of the protruding ventilation vents in the substation with vertical ventilation. (c) 2013 Elsevier Ltd. All rights reserved.
Autores: Pradera, Ainara; Jacobsen, F.; Svendsen, C.; et al.
ISSN 1610-1928  Vol. 99  Nº 1  2013  págs. 130 - 138
The purpose of this paper is to demonstrate that a recently published methodology for predicting flow generated noise by compact surfaces under free-field conditions [1] can be extended to a different and more complex configuration of industrial interest. In the previous paper, the methodology was applied to low Mach number flow past a circular cylinder in free-field, where the Green's function and its derivative were obtained analytically. In this paper, the method will be applied to the case of low Mach number flow past a complex confined scattering geometry where both compact and non-compact surfaces are involved. Here the generation of noise is dominated by the interaction of the flow with a surface whose maximum dimension is shorter than the wavelength of interest. The analysis is based on the surface-source term of the Ffowcs Williams-Hawkings equation. The acoustic source data of the flow are generated by use of a Computational Fluid Dynamics (CFD) simulation. Due to the complexity of the scattering surfaces, the derivative of the Green's function must be obtained numerically through a Computational Acoustics (CA) simulation. The results have been validated through comparison with sound power measurements.
Autores: Satrustegui, Marco; Rivas, Alejandro; et al.
ISSN 1974-9821  Vol. 6  Nº 5  2013  págs. 1446 - 1451
This paper describes an algebraic model for predicting the air flow-rate in the cooling system of an open self-ventilated machine. The method is easily applicable to any type of air-cooled electrical machine. The air flow-rates in different areas of the machine as well as the head losses of the hydraulic system were obtained, and the results were compared with a more detailed model developed with Computational Fluid Dynamic (CFD) techniques. Moreover, a sensitivity analysis was carried out in order to compare the different topologies of the machine studied
Autores: Nieto, N.; Diaz, L.; et al.
ISSN 0013-4651  Vol. 160  Nº 2  2013  págs. A212 - A217
This paper describes a thermal model that represents the heat generation behavior of a large format (10.5 Ah) Li-ion pouch cell. The thermal model is based on the calculation of the heat generation from experimental measurements of internal resistance and the entropic heat coefficient. Predictions from the thermal model are compared with experimental adiabatic calorimetry data. Higher discharge rates and larger temperature operation ranges than the ones reported in prior studies are considered. Results from the thermal model simulations have a prediction error of 21% in comparison with the experimental ones for discharge processes carried out at moderate rates. For discharge processes carried out at high discharge rates a maximum prediction error of 15% has been determined. The advantages and disadvantages of the model are further discussed, taking into account aspects such as accuracy, model development and implementation in different thermal management system designs. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.042302jes] All rights reserved.
Autores: Larraona. Gorka S.; Rivas, Alejandro; Antón, R; et al.
ISSN 1359-4311  Vol. 52  Nº 2  2013  págs. 428 - 438
A parametric study based on design of experiments (DoE) techniques was carried out by computational simulation in order to evaluate the effect that design parameters have on heat transfer and pressure loss of an impinging jet in a cross-flow configuration. The main effects of each parameter and the interactions between parameters were analyzed in detail through the Response Surface Methodology (RSM). Additionally, the potential of the impinging jet in a cross-flow configuration was assessed by calculating the optimal values of the parameters and comparing the cooling efficiency of the resulting configuration with the efficiency of the conventional cross-flow configuration. It was found that the degree to which the average heat transfer coefficient is enhanced as the result of adding an impinging jet depends on the height of the cooled component. Specifically, it was found that the higher the component, the more significant the enhancement. (C) 2013 Elsevier Ltd. All rights reserved.
Autores: Ramos, Juan Carlos; et al.
ISSN 1359-4311  Vol. 51  Nº 1-2  2013  págs. 852 - 863
Ventilation by natural convection of two underground transformer substations has been numerically modelled. The model has been verified in terms of discretization errors and it has been validated with the experimental results of eight temperature rise tests carried out under different conditions of ventilation and transformer power losses. The results of the simulations serve to analyse the air flow pattern and the air temperature distributions inside the substation. A correlation for the air mass flow rate as a function of the ventilation conditions (discharge coefficient and area of the grilles) and the heat dissipated by the transformer has been fitted. The heat transfer coefficients on the surfaces of the transformer and the walls of the enclosure can also be obtained from the simulations of the model. All this information will be used in a future paper to develop a zonal thermal model of the ventilation of the substations that can be employed as a design and optimisation tool. (C) 2012 Elsevier Ltd. All rights reserved.
Autores: Goikoetxea, E.; Rivas, Alejandro; Antón, R; et al.
ISSN 0903-1936  Vol. 42  Nº 57  2013  págs. 2062
Background: Aerosolized surfactant is a promising technique to treat neonatal Respiratory Distress Syndrome (RDS). However, aerosol delivery to neonates is complex and few studies have addressed its feasibility in vitro. Methods: A computer-design of an infant airway model was drawn in CAD, and 3D pieces were printed by means of rapid prototyping. Natural surfactant aerosols were produced by a pneumatically-driven intratracheal inhalation catheter (driving pressure range 4-6bar). Characterization of surfactant aerosols (particle size and distribution) was performed using Time of Flight technology. Further, deposition of surfactant aerosols within the thermoplastic model of the upper airways was measured. Results: The printed neonatal tracheobronchial airway model successfully passed the quality control. Maximum surfactant aerosol production rate was achieved at 6bar (0.39±0.01ml/min; 31.54±0.52mg/min). Although a high percentage of deposition of the aerosolized mass (between 23.75±6.45% 4bar and 26.48±11.43% 6bar) was deposited within the printed model, the highest percentage of mass (between 64.95±7.40% 4bar and 66.43±11.46% 6bar) was measured beyond the distal exit of the model. The Mass Median Aerodynamic Diameter (MMAD) ranged between 8.52±0.16µm (6bar) and 9.36±0.35µm (4bar); higher MMAD values (13.26±3.41µm) where measured at the exit of the printed model. Conclusion: Surfactant aerosolization seems to be feasible and holds potential as a treatment for RDS; however, further research is needed to adapt current technology to the requirements of the neonatal population.
Autores: Rivas, Alejandro; Ramos, Juan Carlos; et al.
ISSN 1044-5110  Vol. 22  Nº 9  2012  págs. 733 - 755
This paper presents a combined experimental and theoretical investigation of the disintegration of fan-shaped liquid sheets produced by industrial fan-spray atomizers. The disintegration regimes observed for different geometries and operating conditions are described, proving the paramount role of nozzle flow on the final characteristics of the spray produced. The concept of breakup length is redefined to account for the stochastic nature of liquid stream disintegration. An analogy is established between the breakup of a liquid sheet dominated by the wave mode and a radial sheet, obtaining good agreement with the experiments. However, in those cases where several disintegration regimes coexist, the breakup length cannot be given by an analytical expression. Finally, the influence of the disintegration regime on both the droplet size and the spatial distribution of the droplets is investigated, confirming the strong influence of rim breakup.
Autores: Antón, R; Rivas, Alejandro; et al.
ISSN 1043-7398  Vol. 134  Nº 1  2012  págs. 011004 - 011004-8.
The performance of axial fans in close proximity to the electromagnetic compatibility (EMC) screens was analyzed by means of an experimental parametric study. The following geometrical parameters were studied: the hub-to-tip ratio, the ratio between fan thickness and fan diameter, the porosity and thickness of the perforated plate, and finally, the distance between the perforated plate and the inlet and the outlet of the fan. Screen porosity was found to be the most important parameter. Fan performance degradation is expressed by means of two correlations: one for the deterioration in the fan pressure at the no-flow point and the other for the flow rate reduction at the free delivery point. Both correlations were formulated as functions of screen porosity and the distance between the fan and the screen. We believe that the correlations can serve as a good guide for correct fan placement in a telecommunications cabinet. [DOI: 10.1115/1.4005913]
Autores: Rivas, Alejandro; Larraona. Gorka S.; et al.
ISSN 0142-727X  Vol. 38  2012  págs. 50 - 71
The air flow around a cubic obstacle mounted on one wall of a rectangular channel was studied experimentally. The obstacle represents an electronic component and the channel the space between two parallel printed circuit boards (PCBs). The flow was produced by the combination of a channel stream and a jet which issued from a circular nozzle placed at the wall opposite from where the component is mounted. With this aim, a test rig was designed and built to carry out experiments with both the above mentioned configurations and other cooling arrangements. Planar Particle Image Velocimetry (PIV) was employed to measure the instantaneous flow velocity on several planes covering the space around the component. The mean velocity and the Reynolds stresses were obtained from averaging the instantaneous velocity, and the mean flow showed a complex pattern with different features such as recirculation bubbles, vortices, detachment and reattachment zones. The influence of two parameters, namely the channel Reynolds number and the jet-to-channel Reynolds number ratio, on these flow features was studied considering nine cases that combined three values of the channel Reynolds number (3410, 5752 and 8880) and three values of the ratio (0.5, 1.0 and 1.5). The results show that the Reynolds number ratio determines the drag produced on the jet and the deflection from its geometric axis due to the channel stream. In all the cases corresponding to the lowest value of the ratio, the jet was dragged and did not impact the component. This fact accounts for the non-existence of the Upper Horseshoe Vortex and changes in the flow characteristics at the region over the component. (C) 2012 Elsevier Inc. All rights reserved.
Autores: Ramos, Juan Carlos; Larraona. Gorka S.; et al.
ISSN 1359-4311  Vol. 31  Nº 4  2011  págs. 493 - 505
The thermal behaviour of several ONAN (Oil Natural Air Natural) distribution transformers has been numerically modelled. A simplified differential model has been developed with the aim of reducing the computational cost that would require a model with the complete geometrical description. This model has been capable of reproducing the expected oil flow and the thermal distribution inside the transformer. The influence of turbulence modelling in the obtained results has been evaluated and the model has been verified in terms of discretization errors. The thermal boundary conditions have been thoroughly analysed searching for the most appropriate expressions for this particular case instead of using inadequate mean values obtained from bibliography. The devised model has been validated by comparing the numerical results with the experimental ones obtained for different transformers and power losses. This mathematical tool can be used to study the natural convection of the oil inside the transformers and allows the manufacturers to optimise their designs from a thermal point of view. (C) 2010 Elsevier Ltd. All rights reserved.
Autores: Antón, R; Ramos, Juan Carlos; et al.
ISSN 0949-149X  Vol. 27  Nº 4  2011  págs. 805 - 812
A Multiple Approach Competing Practical Exercise (MACPE), is an open exercise that confronts a group of students with a practical engineering problem using their own resources and choosing among multiple approaches in a competing environment. A case study with a teacher's assessment and a student's assessment has shown that this sort of exercise is able to motivate the students' learning process. It has also been found that it is a fun way to foster team work and constitutes an open door to innovation.
Autores: Pradera, Ainara; Keith, G.; Jacobsen, F.; et al.
ISSN 1610-1928  Vol. 97  Nº 1  2011  págs. 14 - 23
Sound generation has been widely studied using numerical hybrid methods. The aim of this paper is to introduce a flexible procedure where the acoustic source data may be synthesized and stored from commercially available Computational Fluid Dynamics (CFD) codes and later used to predict radiated noise. Different applications will require either analytical or numerical methods for the radiation calculations. Attention is restricted to low Mach number flows where the noise generation is dominated by the interaction of the flow with a surface with at least one characteristic dimension short compared to the wavelength of interest. This makes it possible to focus on the surface source term of the Ffowcs Williams-Hawkings equation. In this paper, in order to illustrate the basic method for storing and utilizing data from the CFD analysis, the flow past a circular cylinder at a Reynolds number of Re = 1.4 . 10(5) will be studied, where the cylinder is compact and therefore the analytical free-space Green's function may be used.
Autores: Ramos, Juan Carlos; Rivas, Alejandro; et al.
ISSN 1359-4311  Vol. 31  Nº 17-18  2011  págs. 4024 - 4035
The present paper proposes an algebraic zonal model describing the cooling process, via internal oil and external air natural convection, of distribution transformers during a standardised heating test. The conception and adjustment of the presented algebraic zonal model rely on the oil flow and thermal results obtained by a more complex numerical differential model that has been verified and validated by the authors. The domain of the original differential model is divided into several interrelated control volumes or zones where mass and energy conservation laws are applied in conjunction with the necessary boundary conditions. The generated nonlinear algebraic equation system is solved, obtaining in the process the top oil temperature, the internal and external surface temperatures and the heat dissipation distribution. The results of the zonal model have been validated using the previous results from the differential model as well as experimental measurements from heating tests with three different transformers under various power loads. The model developed has proven to be an efficient tool in improving and optimising the thermal design of these devices. (C) 2011 Elsevier Ltd. All rights reserved.
Autores: Rivas, Alejandro; Ramos, Juan Carlos; et al.
ISSN 1070-6631  Vol. 22  Nº 7  2010  págs. 074103-01 - 074103-11
The present paper focuses on the linear spatial instability of a viscous two-dimensional liquid sheet bounded by two identical viscous gas streams. The Orr-Sommerfeld differential equations and the boundary conditions of the flow configuration are numerically solved using Chebyshev series expansions and the collocation method. The strong dependence of the instability parameters on the velocity profiles is proven by using both quadratic and error functions to define the base flow in the liquid sheet and the gas shear layer. The sensitivity of the spatial instability growth rate to changes in the dimensionless parameters of the problem is assessed. Regarding the liquid sheet Reynolds number, it has been observed that, when this parameter increases, both the most unstable growth rate and the corresponding wavenumber decrease, whereas the cutoff wavenumber increases. The results of this analysis are compared with temporal theory through Caster transformation. The effects liquid and gas viscosity have on instability are studied by comparing the instability curves given by the presented model with those from an inviscid liquid sheet and a viscous liquid sheet in an inviscid gaseous medium. The model presented in this paper features a variation in the cutoff wavenumber with all the governing parameters of the problem, whereas that provided by cases that account for an inviscid surrounding gas depends only on the liquid sheet Weber number and the ratio of gas to liquid densities. Results provided by the presented model have been experimentally validated and show that quadratic profiles have a greater capacity to predict the disturbance wavelength. (C) 2010 American Institute of Physics. [doi:10.1063/1.3460348]
Autores: Rivas, Alejandro; et al.
Libro:  Advances in Fluid Mechanics VIII
2010  págs. 569 - 584
Autores: Martín-Gómez, César; Eguaras, María; et al.
Libro:  37th IAHS World Congress on Housing Science
2010  págs. 26 - 29