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Solar co-generation, i.e., the generation of electricity and heat in a single device by concentrating the sunbeams, has the potential to significantly increase the overall system performance. The main challenge is related to the cooling of solar cells. In order to do so, it is essential to reduce the thermal resistance between the cell and heat transfer fluid. This paper features the optimization procedure of a low-cost custom concentrated photovoltaic thermal (CPV-T) receiver for a parabolic trough collector using silicon solar cells. A finite volume model for the thermal process has been developed. Hence, a fluid dynamic thermal simulation of the receiver is presented. The optimized heat sink tube geometries have been manufactured and tested in a lab environment, allowing for a comparison between modelling and experimental test results. Three possible heat sink geometries have been designed and compared regarding their overall heat transfer coefficient with respect to the non-dimensional pumping power, i.e. the ratio between the overall transferred heat and the energy required for pumping. The overall heat transfer coefficient for a finned heat sink has been increased up to 60% with respect to a baseline case without fins under similar conditions. This project has received funding from SOLAR-ERA.NET Cofund 2 joint call undertaking under the European Union’s Horizon 2020 research and innovation programme. This work has also been supported by Scientific and Technological Research Council of Türkiye (TÜBİTAK) under grant number 219M028. The authors acknowledge the Heat and Environment Laboratory, Mechanical Engineering Department, METU, and the help of Bulent G. Akinoglu and Elsen Aydin. D. Santos acknowledges FI AGAUR-Generalitat de Catalunya fellowship (2022FI_B2_00173).

This thesis describes the calculations and proceedings, as well as the theoretical basis behind them, to properly complete the post-process of a CFD analysis of the flow around the McDonnell- Douglas 30P30N airfoil. This completion includes the computations of the relevant magnitudes and the validation of the mesh used comparing the results with the obtained with a finer one and with the results found in the literature. Magnitudes characterizing the boundary layer such as the boundary layer thickness, displacement thickness, momentum thickness and velocity profiles are computed; Also measurements over the shear layer of the slat cove such as the turbulence kinetic energy, velocity magnitude and spanwise vorticity. All the results show sufficient agreement with both a finer mesh that followed the same analysis, as well as the empirical and computational values seen in earlier literature, validating the results and the mesh used.

Programa de doctorat: Doctorat en Enginyeria Tèrmica

Programa de doctorat: Doctorat en Enginyeria Tèrmica

Programa de doctorat: Doctorat en Física Computacional i Aplicada

The behavior of the veins and arteries under some potentially life-threatening situations has been the object of study for a long time. However, these studies have been mostly carried out experimentally, usually conditioned by the situation and procedure of the analysis itself. That is the reason why during the past years, with the development of computational fluid dynamics, there has been a growing interest in creating models to simulate these situations, in order to imitate blood behavior in a generic way. Therefore, during this study, a CFD analysis of the internal flow through the carotid artery bifurcation has been carried out. In first place, starting from a three-dimensional geometry of the artery, a meshing process has been realized using the open-access software Salome, including the optimization of the mesh itself. Later, multiple simulations have been carried out with different characteristics in order to end up obtaining a model as close with the reality as possible, and numeric results have been obtained using the open-access software OpenFOAM. Finally, these results have been compared and validated with the results of previous numerical and experimental projects, and comparisons between different viscosity models have been carried out based on the results obtained.

Baltasar is a new car company from Cerdanyola del Vallès (Barcelona, Spain). Its first car, the Baltasar Revolt, is the first track day registrable car (1). It combines the velocity required for the circuit and the versatility of a registrable car. The company itself has considered relevant to make a CFD study of two new different parts of the car to observe how they affect the aerodynamic development of the Revolt. The first one of these two pieces will be the aerodynamic study using CFD of the body part called “front cover” and the comparison with a new redesign. This item is the equivalent of the bonnet of a passenger car. Moreover, this redesigned front cover must be able to accommodate a small cockpit visor for the pilot and co-pilot, which will also be aerodynamically analysed. The second part of the thesis will consist of the aerodynamic study using CFD of a commercial rear wing and conclude if this aerodynamic device could be beneficial to the performance of the car inside a track. Furthermore, it’s going to be an original study because inside the company there isn’t another aerodynamic thesis and the aerodynamic study of the actual car has to be done to correctly make the comparison of these new two pieces. Finally, this thesis and all the deliverables, information and files of various kinds are confidential.

During this last decade, automotive industry has been focusing on the efficiency of the cars. The fuel consumption regulations, combined with the range of electric cars, have set a critical design factor: fuel consumption. Among all the design areas involved, aerodynamics take a great role in that objective, as more aerodynamic elements are added to everyday cars in order to reduce drag, making cars more efficient. One component that car manufacturers are starting to include is the air curtain, located in the car’s bumper, with different configurations depending on the car manufacturer. In this work, a study of these air curtains will be carried out, from the design of two configurations based on real geometries, to its aerodynamic behaviour to finally understand if their implementation indeed improves the vehicle’s aerodynamic performance against drag, also known as aerodynamic resistance.

Humans have always looked at the heavens and wondered about the nature of the objects seen in the night sky. Discovering the secrets of space has been and is one of humanity’s greatest desires, however, each rocket launch involves a great economic cost. With the development of lighter rockets and the advances in electronics and other technologies in the 20th century, it became possible to lower costs and make rocket launches a common practice. Several methods to reduce the weight of the rockets have been proposed. This document is based on the Vortex-Cooling technique, a variant of film cooling which aims to cool the walls of the combustion chamber by means of a vortex flow in order to reduce its thickness. The main objective is to study the flow behavior of such a simplified vortex-cooled rocket engine in different configurations through numerical simulations. Firstly, a 3D model of Rocketdyne RS-25 (SSME) geometry is created from data sheets published by NASA. Besides, six different geometries of vortex-cooled rocket engines (VCRE) have been 3D modeled in order to carry out four analyses. Secondly, the characteristics of the mesh, a turbulent model for high-velocity flows, as well as the boundary conditions and all the parameters that allow computing the simulations of the SSME and vortex-cooled rocket engine have been selected. Using computational fluid dynamics (CFD) studies on the SSME it has been possible to verify the simulations obtained with Ansys Fluent. Once the parameters used in Fluent have been verified, it is proceeded to compute the simulations of 8 VCRE, modifying the number of inlets that generate the vortex; the angle of incidence of the inlets with respect to the axial direction; the diameter of the inlets; and finally, inlet velocity. After drawing conclusions from each analysis, the VCRE configuration chosen among the 8 cases studied is presented in more detail. The choice is based on the characteristics of the generated vortex, being the one that best meets the purpose of the vortex cooling technique. Comparing this rocket engine with the SSME, an 83% reduction of the flow temperature in the combustion chamber wall stands out. Nevertheless, apart from temperature, results are presented for other physical properties of the flow, such as velocity and pressure.

This study presents an E-FEM/L-S approach for modeling fluid-fluid inter-face deformation in microsystems under electric fields. The approach treats the interface as a zero-thickness boundary and precisely models discontinuities in pressure and elec-tric fields. Numerical results show improved accuracy on coarser meshes compared to conventional methods. The proposed model is useful for problems without guaranteed symmetry of revolution and can efficiently analyze fundamental aspects of multi-phase microsystems.

Floating offshore wind turbines (FOWTs) have the potential to harness wind resources in deepwater, which is so far prohibitive for conventional approaches. This, however, comes at a cost: the platform’s extra degrees of freedom (DoFs) introduce complex aerodynamic and hydrodynamic behaviours. Therefore, FOWTs must be accurately modeled to reduce load uncertainties that ultimately prejudice their economic viability. This project implements a framework for the coupled, high-fidelity simulation of FOWTs in OpenFOAM. The tool is built upon two existing libraries: turbinesFoam [1] —for rotor modeling based on the actuator line method— and waves2Foam [2] —for wave-field generation and absorption based on the relaxation zone method. The multi-phase simulation uses the interFoam solver in combination with a morphing mesh technique and rigid-body model to represent the platform. The mooring restraints are computed with a quasi-steady, catenary model from waves2Foam. The turbinesFoam library, targeted at bottomfixed turbines, is modified so that it can accommodate arbitrary motions along the rigid-body DoFs. The platform-turbine FSI coupling follows a serial sub-iterating strategy based on the PIMPLE scheme. The simulation framework is built in a sequential style. First, the propagation of second-order waves in an empty tank is studied, followed by the decay oscillation of floating buoys from the experiments by Ito and Palm et al. [3, 4]. Then, the modified version of turbinesFoam is tested for the conditions from the OC6 Phase III campaign —a series of wind-tunnel tests carried out at Politecnico di Milano that analyzed the performance of a scaled 10-MW turbine under prescribed motions in pitch and surge [5]. Lastly, the coupled simulation of a 2-DoF (surge and pitch) semi-submersible FOWT under combined wind-wave conditions is achieved. The presented framework proved capable of modeling the aerodynamic performance of turbines under prescribed motion and produced plausible results for a semi-submersible FOWT under combined wind and wave conditions. Once carefully validated, this tool will have the potential to serve as a reliable technique for the advanced modeling of FOWTs.

This work intends to study the effect of aortic annulus eccentricity and leaflet rigidity on the performance, thrombogenic risk and calcification risk in bioprosthetic aortic valve replacements (BAVRs). To address these questions, a two-way immersed fluid–structure interaction (FSI) computational model was implemented in a high-performance computing (HPC) multi-physics simulation software, and validated against a well-known FSI benchmark. The aortic valve bioprosthesis model is qualitatively contrasted against experimental data, showing good agreement in closed and open states. Regarding the performance of BAVRs, the model predicts that increasing eccentricities yield lower geometric orifice areas (GOAs) and higher normalized transvalvular pressure gradients (TPGs) for healthy cardiac outputs during systole, agreeing with in vitro experiments. Regions with peak values of residence time are observed to grow with eccentricity in the sinus of Valsalva, indicating an elevated risk of thrombus formation for eccentric configurations. In addition, the computational model is used to analyze the effect of varying leaflet rigidity on both performance, thrombogenic and calcification risks with applications to tissue-engineered prostheses. For more rigid leaflets it predicts an increase in systolic and diastolic TPGs, and decrease in systolic GOA, which translates to decreased valve performance. The peak shear rate and residence time regions increase with leaflet rigidity, but their volume-averaged values were not significantly affected. Peak solid stresses are also analyzed, and observed to increase with rigidity, elevating risk of valve calcification and structural failure. To the authors' knowledge this is the first computational FSI model to study the effect of eccentricity or leaflet rigidity on thrombogenic biomarkers, providing a novel tool to aid device manufacturers and clinical practitioners. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 713673. The research leading to these results has also received funding from “la Caixa” Foundation, with fellowship ID: LCF/BQ/DI18/11660044, and has been co-funded by the project CompBioMed2 (H2020-EU.1.4.1.3. Grant No. 823712)

Estimations of the grid size and computational cost for direct numerical simulation (DNS) and large-eddy simulation (LES) of Rayleigh-Bénard convection (RBC) are presented in the {Ra, P r} phase space. Computational requirements to reach the so-called asymptotic Kraichnan or ultimate regime of turbulence using DNS are far too expensive. Therefore, we turn to LES to predict the large-scale behavior at very high Ra-numbers. However, a priori alignment studies clearly show why the modelization of the SGS heat flux is the main difficulty that (still) precludes reliable LES of buoyancy-driven flows at (very) high Ra-numbers. This inherent difficulty can be by-passed by carrying out simulations at low-P r numbers where no SGS heat flux activity is expected. This opens the possibility to reach the ultimate regime carrying out LES of RBC at low-P r using meshes of 1010 -1011 grid points. Nevertheless, to do so, we firstly need to combine proper numerical techniques for LES (also DNS) with an efficient use of modern hybrid supercomputers. F.X.T. and A.O. are supported by the Ministerio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00) and by the Generalitat de Catalunya RIS3CAT-FEDER, FusionCAT project (001-P-001722). D.S. and J.A.H. are supported by the FI-AGAUR predoctoral grants 2020FI B 00839 and 2022 FI B1 00204, respectively. Calculations were carried out on MareNostrum 4 supercomputer (projects IM-2022-1-0015 and IM-2022-2- 0019) at BSC. The authors thankfully acknowledge these institutions.

Discrete versions of Poisson’s equation with large contrasts in the coefficients result in very ill-conditioned systems. Thus, its iterative solution represents a major challenge, for instance, in porous media and multiphase flow simulations, where considerable permeability and density ratios are usually found. The existing strategies trying to remedy this are highly dependent on whether the coefficient matrix remains constant at each time iteration or not. In this regard, incompressible multiphase flows with high-density ratios are particularly demanding as their resulting Poisson equation varies along with the density field, making the reconstruction of complex preconditioners impractical. This work presents a strategy for solving such versions of the variable Poisson equation. Roughly, we first make it constant through an adequate approximation. Then, we block-diagonalise it through an inexpensive change of basis that takes advantage of mesh reflection symmetries, which are common in multiphase flows. Finally, we solve the resulting set of fully decoupled subsystems with virtually any solver. The numerical experiments conducted on a multiphase flow simulation prove the benefits of such an approach, resulting in up to 6.6x faster convergences. Adel Alsalti-Baldellou, Xavier Àlvarez-Farré, F. Xavier Trias and Assensi Oliva have been ´ financially supported by two competitive R+D projects: RETOtwin (PDC2021-120970-I00), given by MCIN/AEI/10.13039/501100011033 and European Union Next GenerationEU/PRTR, and FusionCAT (001-P-001722), given by Generalitat de Catalunya RIS3CAT-FEDER. Adel ` Alsalti-Baldellou has also been supported by the predoctoral grants DIN2018-010061 and 2019- DI-90, given by MCIN/AEI/10.13039/501100011033 and the Catalan Agency for Management of University and Research Grants (AGAUR), respectively. Andrey Gorobets has been supported by the RSF project 19-11-00299.

The formulation of multiphase flows emanates from basic conservation laws: mass, momentum and energy. While these are embedded in the celebrated Navier-Stokes equations, none of these properties do necessarily hold when constructing a computational model, unless special care is taken in discretizing the different terms of the governing equations. The conservation of both primary (mass, momentum) and secondary (energy) quantities is not only relevant to mimic the dynamics of the system, but also computationally beneficial. Conservation of such quantities produce an enhanced physical reliability, removing most of the need for stabilization artifacts. In addition, discrete conservation implies numerical stability as well, producing inherently stable problems. Focusing on the capillary force, which is one of the most distinguishable features of multiphase flows, we present here our most recent developments in the quest for conservation. Departing from an inherently mass conservative method, in this work we sketch our previous developments to obtain an energy conservation and next we present our attempt at momentum. By carefully assessing the continuum formulation, we delve into the mathematical properties responsible for the conservation of linear momentum, which we then mimic in the regularized and discrete formulations. We thank the Center for Information Technology of the University of Groningen for their support and for providing access to the Peregrine high performance computing cluster.

Different aspects related to Computational Fluid Dynamics (CDF) will be studied. First, theoretical aspects needed for the resolution of this kind of problems with the finite element method are reviewed. Then, the heat equation is also reviewed. After that, the generic convection diffusion equation is studied, and different convective schemes are used (CDS, UDS, SUDS and QUICK). Numerical experiments are conducted to compare them between each other. Then the problems that arise when trying to numerically solve the Navier Stokes equations are reviewed. These include the checkerboard problem and the fact the the equations are highly coupled. The equations will be solved for different cases, amongst them the Lid-Driven Cavity, using the convective schemes studied previously with a collocated mesh and a fractional step method. Finally, the energy equation is coupled with the Navier-Stokes equations, where the body forces that were previously neglected are also added. This allows the simulation of natural convection cases. The cases studied is that of a cavity with walls at different temperatures, which causes the fluid motion. This case is solved with a Staggered Mesh

Virtual reality has become a significant asset to diversify tools in the support of engineering education and training. The cognitive and behavioral advantages of virtual reality (VR) can help lecturers reduce entry barriers to concepts that students struggle with. Computational fluid dynamics (CFD) simulations are imperative tools intensively utilized in the design and analysis of chemical engineering problems. Although CFD simulation tools can be directly applied in engineering education, they bring several challenges in the implementation and operation for both students and lecturers. In this study, to tackle these challenges, we developed the “Virtual Garage” as a taskcentered educational VR application with CFD simulations. The Virtual Garage is composed of a holistic immersive experience to educate students through a real-life engineering problem solved with CFD simulation data using a VR headset. The prototype is tested by graduate students (n=24). Participants assessed usability, user experience, task load and cybersickness via standard questionnaires together with self-reported questions and a semi-structured interview. Preliminary results reflect that the Virtual Garage is well-received by participants. We identify features that can further enhance the usability and user experience.

The thesis provides the study of a 30P30N wing profile at a high Reynolds (1.7M) and the effects of an active flow control device on its performance. Two different angles of attack will be studied: AOA = 5.5⁰ for the validation process and AOA = 14⁰ for the synthetic jet actuator study. The purpose of the project is to study the performance of the high-lift configuration wing using a Large-Eddy-Simulation turbulence modelling and the effects of the actuator on the lift, drag and the overall efficiency. It has been seen that the mesh has a huge impact on the LES simulations as turbulence phenomena is difficult to predict and requires a lot of computational resources. Under the specific configurations of the synthetic jet actuator used, there is no clear improvement on the performance of the wing even though minor benefits or little differences between the angle of the exhaust velocity can be identified. The memory of the project is differentiated in different chapters. In Chap. 1, all the info about the organization of the project is written. In Chap. 2, you can find all the information about the theoretical basis of the project. Then, Chap. 3 includes the cases studied to validate the meshing, turbulence modelling, etc. Chap. 4 is the object of study of this project, where the reader can find the actuator configuration and the effects of the actuator on the wing’s overall performance. Finally Chap. 5, where the conclusions of the project and future steps will be explained.

The following final degree dissertation composes a study of the computational resolution of the Navier-Stokes equations. The objective of the work is to introduce the student to the field of Computational Fluid Dynamic (CFD) simulations. This is done by creating self-made codes able to solve problems proposed by the Heat and Mass Transfer Technological Center (CTTC) of the Polytechnic University of Catalonia (UPC). The equations of mass, momentum and energy are solved by means of the Finite Volume Method (FVM) with an algorithm based on the Fractional Step Method (FSM) for incompressible fluids. All the codes are self-programmed and verified by the student in C++ language. Emphasis is made in understanding the different theoretical and computational implications of the Navier-Stokes equations. The different problems are chosen specifically to treat as many aspects of the incompressible NavierStokes equations as possible. Their difficulty increases progressively, starting from a simple conduction heat transfer problem and ending in the study of turbulence. The contribution of the convective and diffusive terms is deeply analyzed, firstly by solving a pure diffusion case and then by finding the numerical solution of a general convection-diffusion equation. The Fractional Step Method is employed to solve both internal flow (with forced and natural convection) and external flow (around a square cylinder). Different aspects about turbulence are studied and implemented to the resolution of the Burgers equation and a three-dimensional channel flow. To finalize the work, a proposal for future steps is given in relation to a more advanced research project, based on a deeper study of turbulence models and High Performance Computing (HPC).

Rocket fuel injectors are in charge of propellant atomization and mixing processes, which determine the performance and stability of the liquid rocket engine itself. This small, yet significant components, are in charge of reducing the size and weight of the propellant droplets while mixing them in the shortest possible amount of time. The injector’s design and implementation must be suitable for performing their tasks in order to achieve maximum performance of the nozzle. Aiming to design and test a feasible rocket injector, a conceptual and physical design is presented, using cheap off-the-shelf stream jet nozzles, and its basic behaviour assessed. The numerical approach involves using computational fluid dynamics software (OpenFOAM) while the experimental approach involves lathe machining considerations. The numerical results indicate that the doublet injector design is feasible and mixture quality can be attained but still needs to be crafted and tested using a particular jet nozzle configuration.

Aquesta tesis de màster estableix un estat de l'art sobre diferents models de congelació de l'aigua utilitzant tècniques numèriques per donar resposta a un problema industrial. Mecanismes de transferència de calor com la convecció i la conducció són estudiats per dur a terme la implementació d'aquests models en un solver de CFD . La part final d'aquesta tesis estarà enfocada a produir un solver basat en la transferència de calor conjugada en un marc de multi-regió entre un sòlid i un liquid sotmès a canvi de fase. Esta tesis de màster establece un estado del arte sobre diferentes modelos de congelación de agua usando técnicas numéricas. Mecanismos de transferencia de calor como convección y conducción son estudiados para llevar a cabo la implementación de estos modelos en un solver de CFD. La parte final de esta tesis estará enfocada en producir un solver basado en la transferencia de calor conjugada en un marco de multi-región entre un sólido y un líquido sometido a cambio de fase. This master's thesis establishes a comprehensive state of the art of some water solification models by using numerical techniques. Heat transfer mechanisms as convection and conduction are studied so as to carry out the implementation of such models within a CFD solver framework. The final part of this thesis is devoted to produce a new solver based on the conjugate heat transfer in a multi-region environment between a solid and a two-phase liquid.

Currently, the storage ring vacuum chambers of ALBA are protected by 156 crotch absorbers made of copper and Glidcop. After more than 10 years of operation as a third-generation light source, the ALBA II project arose, aiming to transform this infrastructure into a fourth-generation synchrotron. This introduces new challenges in terms of the thermal and mechanical design of the future absorbers. The absorbers’ cooling channels consist of a set of 8-mm-diameter holes parallel to each other and drilled in the body of the absorbers. In each hole, there is a 6x1 mm stainless steel concentric inner tube coiled in spiral wires, whose aim is to enhance the heat transfer. The convective heat transfer coefficients used for the original design of the absorbers come from experimental correlations from the literature, and are applied as a global value for the whole system. In this work, Heat Transfer-Computational Fluid Dynamics (HT-CFD) studies of the convective heat transfer coefficients and pressure gradients in three different cooling channel geometries are carried out, aiming at leading the way of designing the cooling systems toward the CFD simulations rather than applying global experimental values. This information will be useful for the sizing of the new absorbers for the ALBA II project.

This Bachelor’s thesis proposes a preliminary study on the use of hydrofoils on amphibian aircraft. It contains a summary on seaplane and hydrofoil technology, followed by theoretical introduction in fluid dynamics and the mathematics applied for the study. At the beginning, the no-hydrofoil aircraft configuration is developed, using the theoretical background and XFOIL for the airfoil, to obtain its performance as a base for the hydrofoil study. The study progresses with the geometric configuration of the hydrofoil based on the researched theoretical background. The validation of this calculation is made with the OpenFOAM software based on a 2D flow simulation. The optimization of the foils are made by trial and error in the CFD. Finally, the new hydrofoil platform is designed and later on its performance is compared to the initial configuration to validate the usefulness of the modification. The author of this study intends to improve the TO performance of amphibian aircraft in order to bring up to date their fuel efficiency, as well as to assist amphibian plane electrification.

La comprensió i visualització detallada de la dinàmica de fluids al sistema cardiovascular és una àrea de recerca molt activa en biomedicina. Se sap que el flux sanguini juga un paper clau en el funcionament correcte del sistema cardiovascular i en la progressió de les patologies que l'afecten, per aquesta raó, la dinàmica de fluids computacional i les seves simulacions han anat cobrant importància en el camp de la Medicina Cardiovascular . Atesa la limitació dels mètodes d'imatge actuals, la dinàmica de fluids computacional ha esdevingut una eina poderosa per produir informació detallada i específica de cada pacient. En aquest treball final de grau, les dades específiques d'un pacient, obtingudes mitjançant una ressonància magnètica, han estat utilitzades per crear un model tridimensional, que s'ha utilitzat per dur a terme una simulació computacional del flux sanguini a l'artèria pulmonar, mitjançant l'ús del programari lliure SimVascular. Els principals resultats aconseguits, com el cabal volumètric durant el cicle cardíac o la distribució de velocitats, han estat analitzats i comparats (gràficament i numèricament) amb les dades in vivo del pacient, de manera que, observant la correlació entre tots dos, s'han pogut validar els resultats. La comprensión y visualización detallada de la dinámica de fluidos en el sistema cardiovascular es un área de investigación muy activa en biomedicina. Se sabe que el flujo sanguíneo juega un papel clave en el correcto funcionamiento del sistema cardiovascular y en la progresión de las patologías que le afectan, por esta razón, las simulaciones computacionales de dinámica de fluidos han ido cobrando importancia en el campo de la Medicina Cardiovascular. Dada la limitación de los métodos de imagen actuales, la dinámica de fluidos computacional se ha convertido en una herramienta poderosa para producir información detallada y específica de cada paciente en particular. En este trabajo final de grado, los datos específicos de un paciente, obtenidos mediante una resonancia magnética, han sido utilizados para crear un modelo tridimensional, que se han utilizado para llevar a cabo una simulación computacional del flujo sanguíneo en la arteria pulmonar, mediante el uso del software libre SimVascular. Los principales resultados obtenidos, como el caudal volumétrico durante el ciclo cardiaco o la distribución de velocidades, han sido analizados y comparados (gráfica y numéricamente) con los datos in vivo del paciente, de forma que, observando la correlación entre ambos, se han podido validar los resultados Understanding and detailed visualization of fluid dynamics in the cardiovascular system has become a very active area of research in biomedicine. It is known that blood flow plays a key role in the proper functioning of the cardiovascular system and in the progression of the related pathologies. For this reason, computational fluid dynamics simulations have become increasingly important in the field of Cardiovascular Medicine. Given the limitation of current imaging methods, computational fluid dynamics has become a powerful tool for producing detailed, patient-specific information. In this final degree thesis, patient-specific data, obtained by magnetic resonance imaging, have been used to create a three-dimensional model, which has been used to perform a computational simulation of blood flow into the pulmonary artery, using open source software SimVascular. The principal obtained results, such as the volumetric flow during the cardiac cycle or the velocity distribution, have been analysed and compared (graphically and numerically) with the in vivo data of the patient, observing the correlation between the two, results have been validated.

The present thesis consists on the computational study and development of a 3D reactive flows solver. The main objective is the creation of C++ programming codes able to simulate the combustion chemical reactions. To do so, the project follows a series of steps towards the final goal. First, the theoretical and numerical basis and implementation of the incompressible Navier Stokes equations is addressed. Advanced temporal integration methods are used. Additionally, parallelisation techniques are implemented and tested. Two CFD canonical study cases are presented as a verification for the Navier Stokes solver. Once the first step is completed, mass transport phenomena is tackled. The theoretical framework and numerical implementation of the corresponding equations is addressed. A study case on water vapour transportation in air is presented as a verification process. Finally, the reactive flows solver is completed by adding the chemical reaction phenomena using a one step global mechanism. Premixed CH4/Air combustion in incompressible flow is analysed.

From an engineering, economical and sustainable perspective, performance is always a key factor that must be maximized. When it comes to road mobility, one of the aspects that concern both, the vehicle manufacturer and the user is the fuel consumption. The lower the consumption, the more appealing the product will be to the customers as it has less impact on both, the user’s economy and the environment. As a result, the consumer, the automobile manufacturer, and our planet, Earth are all benefited from a fuel consumption reduction. Thus, it is a win-win situation for all the parties involved. Fuel consumption is caused by several factors, however, one of the most prominent factors is drag. Therefore, an open-source gradient-based optimization framework has been implemented in this project to automatically parametrize the geometry, perform CFD simulations, compute derivatives implementing the discreteadjoint method in order to modify the vehicle shape with the aim of reducing the aerodynamic drag. Finally, the acquired results have been discussed and examined by comparing the drag coefficient, the geometrical shape, and the pressure distribution, velocity field, and the wake between the initial and the optimized case.

The purpose of this work is the development and validation of a Large Eddy Simulation (LES) model for the Aerodynamic’s Section at ESEIAAT in-house Vortex Cloud Method (VCM) fluid mechanics solver. The previous version of the program allowed for a less computationally expensive way to model 2D flow where separation or viscosity effects are important. The addition of a LES subroutine to this program allows studies in conditions where turbulence plays an important role. The subroutine was initially intended to be developed for a Fortran version of the ESEIAAT solver, which was optimised for performance. However, some problems with the Fortran version appearing at the testing stage required a switch to an older MATLAB version. The subroutine was rewritten in MATLAB code and the MATLAB version of the code adapted to accommodate it. Unfortunately, the MATLAB program had a much greater computational cost, which entailed a reduction of the scope of the validation of the program. Consequently, only preliminary validation of the program is carried out in the present work, comparing its performance with bibliographic data, both numerical and experimental. The results obtained are satisfactory and showed that the new LES version of the solver matches closely the results obtained by other similar programs. Despite this, there are still some aspects of the methodology that need more testing and adjustments before it could have full research use. Some questions also arose regarding the adequacy of the implemented LES turbulence model that are discussed in this work. This work has also highlighted the need of properly adjusting the program to each flow situation and has provided information on the different variables to be adjusted. Both physical and numerical (program stability and computational cost) considerations have to be taken into account for a successful adjustment.

This thesis provides an study of the flow around a McDonnell Douglas 30P30N with an Active Flow Control device, a synthetic jet, with different configurations. The flow is two dimensional, with a Reynolds number of 1.7·10⁶ , an Angle of Attack of 5.5⁰ and a Mach of 0.7. The purpose of this device is to reduce the drag and increases the lift by reducing the vortex shedding at the wake by retarding the boundary layer detachment. Several Reynold Averaged Navier-Stokes simulations have been performed to determine the configuration where the maximum aerodynamic efficiency is reach. It has been concluded that the actuation is highly dependant on the position of the actuator; placed after the suction region has been two time more efficient than placed on it. In addition to this, it has been observed that the efficiency increased with the momentum coefficient, the nondimensional frequency and the angle of the exhaust velocity. All of them have played a major role on the actuation.

The main objective of that project is the development and posterior verification of C++ codes able to solve the incompressible Navier-Stokes equations. Nonetheless, before attaching the mainconcerned objective other Computational Fluid Dynamics cases must be solved as a learning tool. That is why the report is structured in chapters, which show both the learning evolution and the physical phenomenons studied. Firstly, an introduction to the project is shown where the main objectives, scope and requirements are established, as well as the justification for its development and the state of the art. Secondly, a numerical methods background, in which all the thesis theoretical basis regarding discretization field are indicated. Thirdly, the resolution of problems in which the energy transport mechanism is the diffusion, focusing on solid materials heat transfer. Fourthly, the numerical resolution of the Convection-Diffusion equation, verifying the simulated results with some benchmark cases proposed by the CTTC. Fifthly, the numerical resolution of the NavierStokes equations using the Fractional Step Method where the classical problems Lid-Driven Cavity, Differentially-Heated Cavity, flow between parallel flat plates and the external flow around a square cylinder will be solved as a verification case. Finally, a study of the turbulence will be undertaken using the Burgers equation in Fourier space, which is a simplified mathematical model that shares many aspects with the Navier-Stokes equations.

In this study Computational Fluid Dynamics (CFD) are used to predict the air flow structure and pollutant retention inside urban canyons taking into account thermal effects due to solar radiation in order to imitate diurnal heating over a city. The diurnal heating induces buoyancy therefore the flow structure and pollutant concentration will be completely different depending on the daytime. The presented method departs from a 2D domain comprised by eight urban canyons of aspect ratio 1 and different 2D transient RANS (Reynolds-Averaged Navier-Stokes) simulations that have been conducted using OpenFoam. Isothermal, single-surface heating and multi-surface heating cases are considered. For all of them, the results obtained have been post-processed using ParaView and Matlab to be able to analyse all the information related with velocity, pressure, temperature, pollutant concentration, streamlines, etc. These results have been also contrasted and validated with data obtained from wind-tunnel experiments and other CFD simulations which have been conducted using a 2D model and conditions similar to the ones used in this thesis. Finally, the conclusions obtained for an airflow with a Reynolds number of 12000 are that for most cases, a single main vortex is formed which is strengthen for the cases where leeward or ground surfaces are heated up compared to the vortex formed in isothermal case. In all cases where the windward side is heated up, a second counter-rotating vortex appears due to buoyancy and displaces the main one from the center of the canyon. Also, when the windward side is heated, the concentration of pollutants increases at the pedestrian level which may be considered as the worst case. Further improvements may be considered to continue with this work such as improving the mesh quality, considering an additional heat source, using another turbulence modelling (LES) or even considering a 3D model since the buoyant flows have an unsteady and 3D characteristc that require complex numerical models to obtain more accurate results

Domain-Specific Languages (DSLs) improve programmers productivity by decoupling problem descriptions from algorithmic implementations. However, DSLs for High-Performance Computing (HPC) have two additional critical requirements: performance and scalability. This paper presents the automated process of generating, from abstract mathematical specifications of Computational Fluid Dynamics (CFD) problems, optimised parallel codes that perform and scale as manually optimised ones. We consciously combine within Saiph, a DSL for solving CFD problems, low-level optimisations and parallelisation strategies, enabling high-performance single-core executions which effectively scale to multi-core and distributed environments. Our results demonstrate how high-level DSLs can offer competitive performance by transparently leveraging state-of-the-art HPC techniques., Comment: 30 pages, 18 figures. Postprint submitted to the Journal of Computational Science (Elsevier). Article updated with reviewers' comments, additional material in section 4.3 including figures and correction of typos

The aerodynamic complexity of the underbody surfaces of conventional road vehicles is a matter of fact. Currently available literature is focused mainly on very simple Ahmed-body geometries as opposed to realistic car shapes, due to their complexity and computational cost. We attempted to understand the flow behaviour around different realistic conventional road car geometries, and we provide an extensive evaluation of the aerodynamic loads generated. The key findings of this article could potentially set a precedent and be useful within the automotive industry’s investigations on drag-reduction mechanisms or sources of downforce generation. The novelty of the work resides in the realistic approach employed for the geometries and in the investigation of barely researched aerodynamic elements, such as front diffusers, which might pave the way for further research studies. A baseline flat-underfloor design, a 7∘ venturi diffuser-equipped setup, a venturi diffuser with diagonal skirts, and the same venturi diffuser with frontal slot-diffusers are the main configurations we studied. The numerical predictions evaluated using RANS computational fluid dynamics (CFD) simulations deal with the aerodynamic coefficients. The configuration that produced the highest downforce coefficient was the one composed of the 7∘ venturi diffuser equipped with diagonal sealing skirts, achieving a CL value of −0.887, which represents an increase of around 1780% with regard to the baseline model. That achievement and the gains in higher vertical loads also entail a compromise with an increase in the overall air resistance. The performance achieved with diffusers in the generation of downforce is, as opposed to the one obtained with conventional wings, a cleaner alternative, by avoiding wake disturbances and downwash phenomena.

The present article presents a new analysis for the dispersion error and the methodology to evaluate it numerically. Here we present the spectral properties of several convective schemes, including non-linear ones, on Cartesian stretched grids for linear advection problems. Results obtained with this method when applied to uniform structured meshes, converge to the results obtained with the classical method for all the studied schemes. Additionally, effects on the time step depending on which scheme is used are considered using the proposed method. The extracted conclusions taken into account both errors and computational cost allow to propose an optimal scheme according to the selected meshing strategy. This work has been financially supported by the Ministerio de Economía y Competitividad, Spain (No. ENE2017-88697-R). J.R.P. is supported by a FI-DGR 2015 predoctoral contract financed by Generalitat de Catalunya, Spain. F.X.T. was supported by a Ramón y Cajal postdoctoral Contract (No. RYC-2012-11996) financed by the Ministerio de Economía y Competitividad, Spain.

In this project a first approach of an in depth study on diffusers and aerodynamics of this elements has been made. This project attempts to rise awareness and interest in this aerodynamic element, but also in Formula SAE racing category. The project and the study carried out, has two parts: theoretic approach and CFD simulations. In order to understand how the car performs with the proposed diffuser, some software adaptation had to be made, and in order to be able to correctly interpret the results obtained, a theoretic approach of diffusers it is presented also. In this project 3 geometries, each one with its own mesh, simulation, and post-process analysis have been performed in order to understand the impact of diffusers, and how other aerodynamic elements interact with it. The results obtained have been validated with similar projects and contrasted with the theoretic results and data. The obtained results can help better understand the influence of diffusers on race prototypes.

Flow turbulence modeling is an expensive computational operation that is often run with simulations using physical simplifications to reduce the cost. Large-eddy simulations (LES) of turbulent flow often make use of wall models in order to lower the computational cost of the simulation in the regions near solid walls, where typically the flow activity contains the smallest structures. Instead of resolving the boundary layer spatiotemporal scales, an algebraic expression for the fluid velocity field yields the wall shear stress, which is then used as a boundary condition to the outer flow, also known as wall modeling. Recently, novel wall model formulations are being developed using data-driven methods which exploit datasets generated with accurate wall-resolved large eddy simulation. However, it is still unclear what dimensional groups should be used to normalise such datasets when the wall shear stress is the target output, rather than a known quantity. The first goal of this dissertation is to explore such relevant groups across the dataset using machine learning applications as multi-expression genetic programming, a tool that allows to derive optimal expressions based on a population of initial candidates and a fitness function. The second goal is to be able to use it to find a correcting expression that yields the wall shear stress given other data and the defined groups.

In the last decades, CFD has become a standard design tool in many fields such as automotive, aeronautical and wind power industries. The driven force behind this is the development of numerical techniques in conjunction with the progress of high performance computing (HPC) systems. However, nowadays we can say that its legacy from the 90-2000s is hindering its progress. The reasons are two-fold: (i) codes designed for CPUs cannot be easily ported and optimized to new architectures (GPUs, ARM...) and (ii) legacy numerical methods chosen to solve (quasi)steady problems using RANS models are not appropriate for more accurate (and more expensive) techniques such as LES or DNS. This work aims to interlace these two pillars with the final goal to enable LES/DNS of industrial applications to be efficiently carried out on modern HPC systems while keeping codes easy to port and maintain. In this regard, a fully-conservative discretization for collocated unstructured grids is used: namely, it exactly preserves the symmetries of the underlying differential operators and is based on only five discrete operators (i.e. matrices): the cell-centered and staggered control volumes (diagonal matrices), ¿c and ¿s, the face normal vectors, Ns, the cell-to-face interpolation, ¿c¿s and the cell-toface divergence operator, M. Therefore, it constitutes a robust approach that can be easily implemented in already existing codes. Then, for the sake of cross-platform portability and optimization, CFD algorithms must rely on a very reduced set of (algebraic) kernels (e.g. sparse-matrix vector product, SpMV; dot product; linear combination of vectors). This imposes restrictions and challenges that need to be addressed such as the inherent low arithmetic intensity of the SpMV, the reformulation of flux limiters and boundary conditions or the efficient computation of eigenbounds to determine the time-step. Results showing the benefits of symmetry-preserving discretizations will be presented together with novel methods aiming to keep a good balance between portability and performance. F.X.T., X.A-F, A.A-B and A.O. are supported by the Ministerio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00). F.X.T. and A.O. are also supported by the Generalitat de Catalunya RIS3CAT-FEDER, FusionCAT project (001- P-001722). A.A-B is also supported by the predoctoral grants DIN2018-010061 and 2019-DI-90, by MCIN/AEI/10.13039/501100011033 and the Catalan Agency for Management of University and Research Grants (AGAUR). Calculations were performed on the MareNostrum 4 supercomputer at the BSC. The authors thankfully acknowledge these institutions.

Finite-volume strategies in fluid-structure interaction problems would be of crucialvimportance in many engineering applications such as in the analysis of reed valves in reciprocating compressors. The efficient implementation of this strategy passes from the formulation of reliable high-order schemes on 3D unstructured meshes. The development of high-order models is essential in bending-dominant problems, where the phenomenon of shear blocking appears. In order to solve this problem, it is possible to either increase the number of elements or increase the interpolation order of the main variable. Increasing the number of elements does not always yield good results and implies a very high computational cost that, in real problems, is inadmissible. Using unstructured meshes is also vital because they are necessary for real problems where the geometries are complex and depart from canonical rectangular or regular shapes. This work presents a series of tests to demonstrate the feasibility of a high-order model using finite volumes for linear elasticity on unstructured and structured meshes. The high-order interpolation will be performed using two different schemes such as the Moving Least Squares (MLS) and the Local Regression Estimators (LRE). The reliability of the method for solving 2D and 3D problems will be verified by solving some known test cases with an analytical solution such as a thin beam or problems where stress concentrations appear. P. Castrillo gratefully acknowledges the Universitat Politècnica de Catalunya and Banco Santander for the financial support of his predoctoral grant FPI-UPC (109 FPI-UPC 2018). The authors are supported by the Ministerio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00).

In this work, a numerical framework aimed at simulating the transport of contaminants and infectious agents within a closed domain is presented. The method employs mature CFD algorithms to calculate air fields with reasonable computational costs. The main objective is to give fast response to stakeholders about air quality indicators in the design phase of HVAC systems. A discussion regarding the size and characteristics of different contaminants is proposed, highlighting the most appropriate methods and coefficients needed to simulate their transport. Next, the methodology employed to evaluate the risk of infection is presented. The numerical set-up, based on the buoyantBoussinesqPimpleFoam solver in OpenFOAM, was tuned by simulating the well-known case of the heated floor cavity, providing accurate results. Hence, the case study of a transport vehicle of generic shape is presented, in order to show possible results in terms of air-age distribution, PM2.5 distribution, and global infection risk matrix. This work has been developed in the context of the Rolen Purifica Bus R&D project, partially financed by INNOTEC (ACCIO - Agència per la Competitivitat de l’Empresa, Generalitat de ´ Catalunya). J. Vera has been financially supported by the Ministerio de Educación y Ciencia (MEC), Spain, (FPI grant PRE2018-084017). N. Morozova is supported by the by the Ministerio de Economía y Competitividad, Spain [FPU16/06333 predoctoral contract].

The implementation of ejectors is getting an increasing interest in many sectors, in order to improve the efficiency and reliability of several systems. The aircraft sector is also integrating ejectors for bleed air management, recirculation or ram-air ventilation. Understanding the physics and the modelling of ejectors in normal mode (on and off design, that is, the generation of the usual characteristic curves) is in a mature development, including several simplified models [1][2] available. They have demonstrated a good accuracy under low computational times, thus being useful for control purposes. However, to achieve a robust and general system level modelling, there is the need for 0D/1D models suitable to tackle abnormal functioning modes (including reverse flow occurrence at any inlet/outlet port of the ejector), and transient operation during transition between different modes of operation. In this sense, the current research done by the authors is focusing on the understanding of the air ejectors under a broader operation perimeter than what can be found in the usual literature, through an operating map covering all modes of interest for aircraft systems. Computational Fluid Dynamics (CFD) simulations and experiments are being collected, to confirm the expected behavior under normal operation, and to extend the knowledge to abnormal operation. All this understanding will be synthetized in new simplified models, that will be implemented in a Modelica ejector object, for its integration in the whole system simulations. The final objective will be to use the tool for design and control purposes at the system level. This paper presents the current status of simulations, experiments and Modelica model developments concerning the ejector working in different steady modes (normal and abnormal).

Currently, the storage ring vacuum chambers of ALBA are protected by 156 crotch absorbers made of copper and Glidcop. After more than 10 years of operation as a third-generation light source, the ALBA II project arose, aiming to transform this infrastructure into a fourth-generation synchrotron. This introduces new challenges in terms of the thermal and mechanical design of the future absorbers. The absorbers’ cooling channels consist of a set of 8-mm-diameter holes parallel to each other and drilled into the body of the absorbers. In each hole, there is a 6x1 mm stainless steel concentric inner tube coiled in spiral wires, whose aim is to enhance the heat transfer. The convective heat transfer coefficients used for the original design of the absorbers come from experimental correlations from the literature, and are applied as a global value for the whole system. In this work, Heat Transfer-Computational Fluid Dynamics (HT-CFD) studies of the convective heat transfer coefficients and pressure gradients in three different cooling channel geometries are carried out, aiming at leading the way of designing the cooling systems toward the CFD simulations rather than applying global experimental values. This information will be useful for the sizing of the new absorbers for the ALBA II project., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

Planetary boundary layer phenomena, such as slope winds and nocturnal low-level jets, are frequent on Mars, e.g., whenever diurnal surface temperature variations are significant, and large-scale winds are weak. In particular, thermally-driven winds are very common over sloping regions; they can reach significantly high speeds for steep slopes, and govern near-surface winds in many sites. Simulations of this type of flows on Earth have usually been conducted using commercial computational fluid dynamics software (CFD). In this work, a set-up is described for numerically simulating slope flows on Mars within the framework of the open-source code OpenFOAM CFD. For this purpose, thermally-driven winds are analysed in an idealised 2D Martian mountain-valley system with realistic values of parameters such as the slope angle, temperature difference between the atmosphere and slope, etc. The fluid domain dimensions, and the temperature and pressure initial and boundary conditions used in this work were tested and proven suitable in previous similar investigations for Earth applications. Large-eddy simulations (LES) turbulent model is used here. The results obtained for velocity and temperature profiles are shown both for anabatic (up-slope) and katabatic (down-slope) winds. This study shows the potential of the proposed methodology and the performance of OpenFOAM. Future work aims at facilitating the screening of large numbers of candidate landing sites for future Mars missions, and at estimating the thermally-driven wind speeds for wind resource assessment. This work has been supported by the project PID2019-105162RB-I00 funded by MCIN/AEI/10.13039/501100011033 and by the project 2017 SGR 1278 from the AGAUR from Generalitat de Catalunya Peer Reviewed Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.2 - Per a 2030, augmentar substancialment el percentatge d’energia renovable en el con­junt de fonts d’energia Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.1 - Per a 2030, garantir l’accés universal a serveis d’energia assequibles, confiables i moderns

In this work, we present a multi-fidelity machine learning surrogate model, which predicts comfort-related flow parameters in a ventilated room with a heated floor. The model uses coarse- and fine-grid CFD data obtained using LES turbulence models. The dataset is created by changing the width aspect ratio of the rooms, inlet flow velocity, and temperature of the hot floor. The surrogate model takes the values of temperature and velocity magnitude at four different cavity locations as inputs. These probes are located such that they could be replaced by actual sensor readings in a practical case. The model’s output is a set of comfort-related flow parameters. We test two multi-fidelity approaches based on Gaussian process regression (GPR), among them GPR with linear correction (LC GPR), and multi-fidelity GPR (MF GPR) or cokriging. The computational cost and accuracy of these approaches are compared with GPRs based on single-fidelity data. All of the tested multi-fidelity approaches successfully reduce the computational cost of dataset generation compared to high-fidelity GPR while maintaining the required level of accuracy. The co-kriging approach demonstrates the best trade-off between computational cost and accuracy. This work has been financially supported by the project RETOtwin [PDC2021-120970-I00] funded by MCIN/AEI/10.13039/501100011033 and European Union Next Generation EU/PRTR. N. Morozova is supported by the by the Ministerio de Economía y Competitividad, Spain [FPU16/06333 predoctoral contract]. E. Burnaev is supported by RFBR grant 21-51-12005 NNIO a. C. Oliet, is suppoted by the Catalan Government as a Serra Húnter lecturer. The calculations were performed on the MareNostrum 4 supercomputer at the Barcelona Supercomputing Center [RES project IM-2021-1-0015]. The authors thankfully acknowledge these institutions.

The use of flux limiters is widespread within the scientific computing community to capture shock dis- continuities and are of paramount importance for the temporal integration of high-speed aerodynamics, multiphase flows and hyperbolic equations in general. Meanwhile, the breakthrough of new computing architectures and the hybridization of supercomputer systems pose a huge portability challenge, particularly for legacy codes, since the computing subroutines that form the algorithms, the so-called kernels, must be adapted to various complex parallel program- ming paradigms. From this perspective, the development of innovative implementations relying on a minimalist set of kernels simplifies the deployment of scientific computing software on state-of-the-art supercomputers, while it requires the reformulation of algorithms, such as the aforementioned flux lim- iters. Equipped with basic algebraic topology and graph theory underlying the classical mesh concept, a new flux limiter formulation is presented based on the adoption of algebraic data structures and kernels. As a result, traditional flux limiters are cast into a stream of only two types of computing kernels: sparse matrix-vector multiplication and generalized pointwise binary operators. The newly proposed formulation eases the deployment of such a numerical technique in massively parallel, potentially hybrid, computing systems and is demonstrated for a canonical advection problem. The work of N. V. and X. Á. F. has been supported by the Government of Catalonia, FI AGAUR predoctoral grants 2019FI_B2_ 000104 and 2019FI_B2_00076. N. V., X. Á. F., J. C., A. O. and F. X. T. have been funded by the Spanish Research Agency, ANUMESOL project ENE2017-88697-R. J. C. has also been funded by Spanish Research Agency, GALIFLOW project ENE2015-70662-P. The studies of this work have been carried out using the MareNostrum 4 supercomputer of the Barcelona Supercomput- ing Center, projects IM-2019-3-0026 and IM-2020-1-0006; the TSUBAME3.0 supercomputer of the Global Scientific Information and Computing Center at Tokyo Institute of Technology; the Lomonosov-2 supercomputer of the shared research facilities of HPC computing resources at Lomonosov Moscow State University; the K-60 hybrid cluster of the collective use center of the Keldysh Institute of Applied Mathematics; the JFF cluster of the Heat and Mass Transfer Technological Center at Technical University of Cat- alonia. The authors thankfully acknowledge these institutions for the compute time and technical support.

CFD users of supercomputers usually resort to rule-of-thumb methods to select the number of subdomains (partitions) when relying on MPI-based parallelization. One common approach is to set a minimum number of elements or cells per subdomain, under which the parallel efficiency of the code is "known" to fall below a subjective level, say 80%. The situation is even worse when the user is not aware of the "good" practices for the given code and a huge amount of resources can thus be wasted. This work presents an elastic computing methodology to adapt at runtime the resources allocated to a simulation automatically. The criterion to control the required resources is based on a runtime measure of the communication efficiency of the execution. According to some analytical estimates, the resources are then expanded or reduced to fulfil this criterion and eventually execute an efficient simulation., Comment: 27 pages, 15 figures

Commercial computational fluid dynamics (CFD) codes have often been used for simulation of atmospheric boundary layer (ABL) flows. The present work explores the potential of the open-source CFD software OpenFOAM for simulating thermally-driven winds, by comparing several turbulence models. Indeed, in ABL and other large-scale flows, turbulence is critical to the mixing process of momentum and buoyancy, and simulations with commercial CFD codes have usually been done with Reynolds-Averaged Navier-Stokes (RANS) turbulence modelling. In this work, the formation of thermally-driven winds is studied in an idealised mountain-valley system, with realistic values of parameters such as the slope angle, the diurnal temperature cycle, etc. Performances of various OpenFOAM RANS turbulence models (k–e, re-normalisation group (RNG) k–e, k–¿ shear stress transport (SST)) are compared. A preliminary study of LES using Smagorinsky closure is also contemplated. Velocity contours, velocity and temperature profiles, the shapes of vortexes/convective cells, and the computational times are presented for all the studied turbulence models, to help identify the most suitable one for simulation of thermally-driven winds. This work is supported by the project PID2019-105162RB-I00 funded by MCIN/AEI/10.13039/501100011033 and by the project 2017 SGR 1278 from the AGAUR Generalitat de Catalunya. Peer Reviewed Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.1 - Per a 2030, garantir l’accés universal a serveis d’energia assequibles, confiables i moderns Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.2 - Per a 2030, augmentar substancialment el percentatge d’energia renovable en el con­junt de fonts d’energia

In this work, the symmetry-preserving method [1, 2, 3] is extended to include magnetohydrodynamic effects, using the collocated grid arrangement of Ni et al. [4, 5]. The electromagnetic part is solved explicitly using the induction-less approximation and an electric potential Poisson equation. The proposed solver is implemented in OpenFOAM and tested for accuracy and stability, and compared to the method of Ni et al. [4, 5]. A new benchmark case using a Taylor-Green vortex in a transverse magnetic field is used, for which kinetic energy budget terms are compared to the analytical solutions. Finally, Hunt’s case is used to compare flow profiles to the analytical solutions. Influence of the spatial discretisation on accuracy and stability is also examined by solving both cases on meshes with variable degrees of distortion. The symmetry-preserving method showed accuracy on Cartesian meshes and stability even on extremely distorted meshes, whereas the method of Ni et al. [4, 5] showed less accurate conservation of current density and was not able to produce stable solutions on the extremely distorted meshes. This project is part of the RIS3CAT-FEDER, FusionCAT project (001- P-001722) of Generalitat de Catalunya and the RETOtwin project (PDC2021-120970-I00) of Ministerio de Economía y Competitividad, Spain. J.A.H. is supported by the predoctoral grant FI 2022 (2022 FI B1 00204) of the Catalan Agency for Management of University and Research Grants (AGAUR).

The breeding blanket is one of the fundamental components of a nuclear fusion reactor and is responsible for the fuel production, generating tritium through neutronic capture reaction between lithium and neutrons. Lithium is a liquid PbLi alloy and the helium formed as reaction by-product can coalesce into bubbles, generating a two-phase mixture with a high-density ratio (ηρ∼O5). These bubbles can accumulate and stagnate within the blanket channels with potentially harmful consequences. In this work, the interIsoFoam solver of OpenFOAM v2012 is used to simulate bubble motion for a two-phase mixture representative of the He-PbLi system to test its potential for future developments in the field of fusion. In a first phase, several traditional benchmarks were carried out, both 2D and 3D, and considering the two variants of the VOF method implemented in the solver, isoAdvector and plicRDF. Subsequently, He bubbles of different diameters rising in liquid PbLi (ηρ=1.2×105) were analysed to investigate different regimes. For a Eötvös number (Eo) greater than 10, it was possible to recreate the axisymmetric, skirted, oscillatory regimes and the peripheral and central breakup regimes. For Eo < 10, non-physical deformations of the interface are observed, probably generated by spurious velocities that have a greater impact on the solution for very small bubbles and rising velocities.

Mechanical forces on the nucleus of a biological cell can play a significant role in its positioning within the cell. Some models for the migration of the nucleus are based on biochemical signals, which have a very slow response, and others on the mechanical actions of direct pushing–pulling forces, relying on the cytoskeleton and cell contractility. In the present work, hydrodynamic drag forces, produced by the surrounding membrane movement and acting on a spherical model of the nucleus, are numerically investigated. The space distributions of force and drag resistance are estimated by unsteady three-dimensional computational fluid dynamics simulations. These distributions are functions of sphere size and position, and they are fitted to mathematical models. Dynamic simulations are performed using these mathematical models to compute the trajectory of the microsphere under the action of thousands of position-dependent pushing–pulling forces. A characteristic time is estimated by linearization of the equation, and a dynamic simulation is performed with one-dimensional finite differences. With this approach, it takes a few minutes to simulate several hours of nuclear migration. It is shown that in the long term, nuclei tend to migrate to the center of the cell, especially when it is small compared with the cell size. The model presented here, based exclusively on hydrodynamic interaction in a Newtonian fluid, shows that pushing–pulling forces on the nucleus, produced by periodic contractions of the cell membrane, will cause it to migrate to the center of the cell in a manner that is in agreement with experimental observations.

A numerical model for studying a storage tank for concentrated solar power is presented. The model consists of solving the heat equation for the solid part made from ceramic materials, a one-dimensional model for the molten salt circulating inside the solid, and a coupling between them. Then, some results are presented for a reference case with some typical parameters for the storage system. This work has been carried out in the framework of the Newcline project, which is supported under the umbrella of CSP-ERA.NET 1st Cofund Joint Call, and by the following National Agencies: AEI (Spain), CDTI (Spain), Jülich (Germany), SFOE (Switzerland). CSP-ERA.NET is supported by the European Commission within the EU Framework Programme for Research and Innovation HORIZON 2020 (Cofund ERA-NET Action, N° 838311). J. Vera has been financially supported by the Ministerio de Educación y Ciencia (MEC), Spain, (FPI grant PRE2018-084017).

A 2D aerodynamic study of the NASA&rsquo, s X-43A hypersonic aircraft is developed using two different approaches. The first one is analytical and based on the resolution of the oblique shock wave and Prandtl&ndash, Meyer expansion wave theories supported by an in-house program and considering a simplified aircraft&rsquo, s design. The second approach involves the use of a Computational Fluid Dynamics (CFD) package, OpenFOAM and the real shape of the aircraft. The aerodynamic characteristics defined as the lift and drag coefficients, the aerodynamic efficiency and the pitching moment coefficient are calculated for different angles of attack. Evaluations are made for an incident Mach number of 7 and an altitude of 30 km. For both methodologies, the required angles of attack to achieve a Vertical Force Balance (VFB) and a completely zero pitching moment conditions are considered. In addition, an analysis to optimise the nose configuration of the aircraft is performed. The mass flow rate throughout the scramjet as a function of the angle of attack is also presented in the CFD model in addition to the pressure, density, temperature and Mach fields. Before presenting the corresponding results, a comparison between the aerodynamic coefficients in terms of the angle of attack of both models is carried out in order to properly validate the CFD model. The paper clarifies the requirements needed to make sure that both oblique shock waves originating from the leading edge meet just at the scramjet inlet clarifying the advantages of fulfilling such condition.