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To activate the boundary layer in Active Flow Control (AFC) applications, the use of pulsating flow has notable energy advantages over constant blowing/suction jet injections. For a given AFC application, five parameters, jet location and width, inclination angle, frequency of injection, and the momentum coefficient, need to be tuned. Presently, two main devices are capable of injecting pulsating flow with a momentum coefficient sufficient to delay the boundary layer separation: these are zero-net-mass-flow Actuators (ZNMFAs) and fluidic oscillators (FOs). In the present study, a novel FO configuration is analyzed for the first time at relatively high Reynolds numbers, and fluid is considered to be incompressible. After obtaining the typical linear correlation between the incoming Reynolds number and the outlet flow oscillating frequency, the effects of dimensional modifications on outlet width and mixing chamber wedge inclination angle are addressed. Modifications of the outlet width were observed to create large variations in FO performance. The origin of self-sustained oscillations is also analyzed in the present manuscript and greatly helps in clarifying the forces acting on the jet inside the mixing chamber. In fact, we can conclude by saying that the current FO configuration is pressure-driven, although the mass flow forces appear to be much more relevant than in previously studied FO configurations, Postprint (published version)

The physics and modeling of air ejectors in on- and off-design conditions have been extensively addressed in the past, reaching a good level of maturity. However, to achieve a robust model at system scale integrating an ejector, there is a need for developing 0D models suitable to tackle abnormal functioning modes, such as cases where the ejector works in subsonic conditions, reverse flows, or closed ports. Based on state-of-the-art models, a new 0D model has been built in the current work, where proper subroutine implementations allow covering normal and abnormal operations of convergent nozzle ejectors. The current research also focuses on understanding the physical behavior of air ejectors for aeronautical applications. In this regard, CFD-RANS simulations are used to perform model verification and calibration as well as to gain knowledge of the whole operational envelope. The regimes reproduced by the 0D model, and validated by CFD, cover the normal operating mode with different choking regimes, the occurrence of a subsonic primary flow, and the closed secondary port case. Additional cases are run to assess other features of the ejector behavior, such as the influence of geometrical properties on the choking mechanism and the impact of thermal effects., Peer Reviewed, Postprint (published version)

Programa de doctorat: Doctorat en Enginyeria Tèrmica

Due to its high power density, the aviation sector is pushing towards its electrification by designing new propulsion systems based on cryogenic liquid hydrogen (LH2). From an engineering point of view, the efficient design of tanks constitutes one of the main challenges. They must guarantee efficient isolation of the hydrogen to avoid its evaporation and, at the same time, ensure reasonable weight and dimensions. Using numerical simulations is fundamental for studying heat transfer between the external environment and the liquid fuel. This work focuses on the simulation of multiphase liquid-gas flow to analyze the tank pressurization as a function of different external heat fluxes. A commercial CFD software is used to perform simulations. VOF simulations with the Lee mass transfer model are employed, and some critical numerical parameters are analyzed to provide information on the optimal values., The project H2ELIOS is supported by the Clean Aviation Joint Undertaking and its members. Carles Oliet,as a Serra-Hunter Associate Professor, acknowledges the Catalan Government for the support through this Programme., Peer Reviewed, Postprint (published version)

This thesis consists in the development and implementation of a series of numerical codes for the resolution of the fundamental equations governing the conservation of mass, momentum, and energy in fluid dynamics and heat transfer problems. Through the utilization of the Finite Volume Method (FVM) and an algorithm based on the Fractional Step Method (FSM) for incompressible fluids, the codes are progressively refined to address key fluid dynamics challenges, including diffusion, convection, the calculation of flow fields, and the simulation of turbulence phenomena. Special emphasis is placed on the field of aerodynamics, given its significance in aeronautical and aerospace solutions. The relationship between Computational Fluid Dynamics and Heat Transfer (CFD&HT) and High-Performance Computing (HPC) is explored, recognizing the potential applications and limitations of the developed codes. Each implementation undergoes rigorous validation, including comparison with analytical solutions or benchmark cases, ensuring mathematical rigor and comprehensive understanding. The project is supervised by the Heat and Mass Transfer Technological Center (CTTC) at Universitat Politècnica de Catalunya (UPC) and sets the stage for exploring more advanced techniques and practical applications in fluid dynamics and heat transfer. The study provides hands-on coding experience in C++, valuable understanding of fluid dynamics intricacies, and paves the way for future learning and exploration in computational fluid dynamics.

The influence of cavitation on the wake behind a NACA 0009 hydrofoil with a truncated trailing edge has been numerically investigated using a homogeneous mixture model coupled with a controlled decay SST ¿-Re¿t turbulence model. Using optimal definitions of the inlet turbulent intensity and the empirical condensation and vaporization coefficients of the cavitation model, simulated results have shown a good agreement with experimental data. Notably, the numerical results exhibit negligible deviations of less than 0.7% in vortex shedding frequencies for different cavitation levels. If the size of the vortex cavitation grows, a substantial increase in both lift and drag coefficients on the hydrofoil is observed. Furthermore, the influence of cavitation on the trajectories of vortex centers and the morphology of the primary shedding vortices has been revealed using a vortex identification method. The findings highlight that the cavitation development enhances the advected velocity of the shedding vortices while decreasing the streamwise inter-vortex spacing. Consequently, both factors are found to contribute to the increase of the vortex-shedding frequency behind the NACA 0009 hydrofoil with the truncated trailing edge when cavitation appears and develops., Peer Reviewed, Postprint (published version)

Nasal airflow obstruction correlates with several ailments, such as higher patency, increased friction at the mucosal wall or the so-called Little's area, improper air conditioning, and snoring. Nasal dilators are frequently employed, mainly due to their ease of access and use, combined with their non-permanent and non-surgical nature. Their overall efficacy, however, has not been clearly demonstrated so far, with some studies reporting conflicting outcomes, mainly because being based on subjective evaluations. This study employs Computational Fluid Dynamics simulations to analyze the flow inside a real nose, performs an objective assessment of a nasal dilator's effect in terms of airflow and air conditioning, reporting flow paths, friction levels, heat and water fluxes and detailed temperature and humidity distributions. Coincidentally, the studied nose presents a septal deviation, with one nostril being wider than the other. The tubes of the dilator used in both nostrils are identical, as with any standard commercial dilator. Consequently, the dilator widens one nostril, as intended, but results in an obstruction in the other. This allows simultaneously addressing two situations, the nominal function of the dilator, as well as an off-design case. Results indicate a 24 % increase in nasal patency in the design situation. The effect, however, is limited, as quantified by appropriate measures, such as the flow-generated friction at the nose surfaces and the temperature fluxes. Hence, the effect of such a dilator in nominal conditions is perhaps not as large as might be hoped. In the off-design situation, nasal resistance increases by 62 %, an undesirable effect, illustrating the consequences of using an inappropriate dilator., Peer Reviewed, Postprint (published version)

An experimental and numerical fluid flow study in a micro heat exchanger (microHEX) is conducted in this work to investigate its pressure drop characteristics. The microHEX sample presents a high degree of protrusion of the microchannels inside the inlet and outlet manifolds (Lprot /Dm =0.7), a large manifold-to-branch area aspect ratio (Am/As =37), and a very small flow division in each microchannel (ß=1/34). These features configure T-junctions that differ completely from the conventional ones in the literature. Experimental measurements of the microHEX pressure drop were performed for the Reynolds range of 300-2200. The numerical methodology combines a 1D model to evaluate the pressure drop of a single microchannel, adding minor entrance and exit losses due to the change of section and direction. A CFD numerical simulation evaluates the particular pressure loss associated with the dividing T-junction towards the protruded microchannel. The numerical simulation pressure drop results compared well with the experimental measurements, reporting a difference of 3.11% for an initial case. An extension of the work will soon cover all the collected experimental cases., Carles Oliet, as a Serra-Húnter Associate Professor, acknowledges the Catalan Government for the support through this Programme., Peer Reviewed, Postprint (published version)

A comprehensive analysis of the unsteady flow dynamics past the 30P30N three-element high lift wing is performed by means of large eddy simulations at different angles of attack (AoA=5°, 9°, and 23°) and at a Reynolds number of Rec=750.000 (based on the nested chord). Results are compared with experimental and numerical investigations, showing a quantitatively good agreement and, thus, proving the reliability and accuracy of the present simulations. Within the slat and main coves, large recirculation bubbles are bounded by shear layers, where the onset of turbulence is triggered by Kelvin–Helmholtz instabilities. In the energy spectrum of the velocity fluctuations, the footprint of these instabilities is detected as a broadband peak; its frequency being moved toward lower values as the angle of attack increases. Kelvin–Helmholtz vortices roll-up and break down into small scales that eventually impinge into the slat and main coves lower surfaces. The slat impingement shows to be more prominent, and hence, larger velocity and pressure fluctuations are observed. The impingement strength diminishes with the angle of attack in both coves, while higher fluctuations are originated on the slat and main respective suction sides, leading to larger boundary layers. This is associated with the displacement of the stagnation point with the angle of attack. Another salient feature observed is the laminar-to-turbulent flow transition in the main and flap leading edges although the average location of this transition seems to not be affected by the angle of attack. Tollmien–Schlichting instabilities precede this transition, with the disturbances amplified by the inviscid mode at low angles of attack, while at AoA=23°, the local Reynolds number on the main suction side is incremented and the viscous mode becomes important. The analysis shows that the turbulent wake formed at the trailing edge of all elements dominates the dynamics downstream. This is especially true at the higher an, This research has received financial support from the Ministerio de Ciencia e Innovación of Spain (Nos. PID2020-116937RB-C21 and PID2020-116937RB-C22). Furthermore, simulations were conducted with the assistance of the Red Española de Supercomputación (RES) and the EuroHPC JU, who granted us computational resources at the HPC facilities of MareNostrum IV, at Barcelona Supercomputing Center (IM-2022-3-0005), and Vega, at Institute of Information Science (REG- 2022R01-030), respectively. Finally, R. Montalà would like to express gratitude to the Departament de Recerca i Universitats de la Generalitat de Catalunya for awarding the FI-SDUR grant (No. 2022 FISDU 00066), which supports his doctoral studies., Peer Reviewed, Postprint (published version)

A direct numerical simulation of a three-dimensional diffuser at Reynolds number Re = 10,000 (based on inlet bulk velocity) has been performed using a low-dissipation finite element code. The geometry chosen for this work is the Stanford diffuser, introduced by Cherry et al. (Int. J. Heat Fluid Flow 29:803–811, 2008). Results have been exhaustively compared with the published data with a quite good agreement. Additionally, further turbulent statistics have been provided such as the Reynolds stresses or the turbulent kinetic energy. A proper orthogonal decomposition and a dynamic mode decomposition analyses of the main flow variables have been performed to identify the main characteristics of the large-scale motions. A combined, self-induced movement of the large-scales has been found to originate in the top-right expansion corner with two clear features. A low-frequency diagonal cross-stream travelling wave first reported by Malm et al. (J. Fluid Mech. 699:320–351, 2012), has been clearly identified in the spatial modes of the stream-wise velocity components and the pressure, associated with the narrow band frequency of St¿[0.083,0.01] . This movement is caused by the geometrical expansion of the diffuser in the cross-stream direction. A second low-frequency trait has been identified associated with the persisting secondary flows and acting as a back and forth global accelerating-decelerating motion located on the straight area of the diffuser, with associated frequencies of St<0.005 . The smallest frequency observed in this work has been St=0.0013 . This low-frequency observed in the Stanford diffuser points out the need for longer simulations in order to obtain further turbulent statistics., The research leading to this work has been partially funded by the European Project NextSim which has received funding from the European High-Performance Computing Joint Undertaking (JU) under grant agreement No 956104 and co-founded by the Spanish Agencia Estatal de Investigacion (AEI) under grant agreement PCI2021-121962. Benet Eiximeno also acknowledges the financial support by the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (Refs: PID2020-116937RB-C21 and PID2020-116937RB-C22). Oriol Lehmkuhl has been partially supported by a Ramon y Cajal postdoctoral contract (Ref: RYC2018-025949-I). He also acknowledges the support of the European Project HiFi-TURB which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 814837. We also acknowledge the Barcelona Supercomputing Center for awarding us access to the MareNostrum IV machine based in Barcelona, Spain. The authors acknowledge the support of Departament de Recerca i Universitats de la Generalitat de Catalunya to the Research Group Large-scale Computational Fluid Dynamics (Code: 2021 SGR 00902)., Peer Reviewed, Postprint (published version)

Swimming pools are characterized by high humidity, high temperature, an aggressive environment caused by disinfection processes, high energy consumption, and extensive technical infrastructure. As a result, these facilities are at high risk of damage, including structural damage, which can have catastrophic consequences in extreme cases. Adequate ventilation plays a critical role in natatoria. The technical condition of swimming pool facilities should be regularly inspected to ensure that they are safe for the public. Various techniques can be used for this purpose. In the present study, a numerical analysis of ventilation performance and an assessment of the risks associated with design and construction errors in the ventilation system were conducted in a swimming pool building with a volume of 116,280.7 m3 and a surface area of 20,188.6 m2 in north-eastern Poland. Two scenarios were analyzed: the performance of the originally designed ventilation system and the performance of the ventilation system installed in the examined facility. The analyses were conducted with the use of computational fluid dynamics (CFD) tools in the FloVent program. Air distribution was determined by measuring ventilation parameters and conducting smoke tests in analyzed natatorium, and the obtained data were consistent with the results of the numerical analysis. The analyzed scenarios were compared to identify the risks associated with inadequate ventilation in swimming pools. A significant decrease in air supply led to local increases in air velocity and worsened thermal comfort parameters in the real-world facility. In addition, the decrease in air supply induced changes in air distribution and prevented air streams from reaching all parts of the natatorium. Some areas were inadequately ventilated, and condensed water vapor settled on the building’s glass facade and the roof, posing a direct threat to structural elements. The results of the numerical analysis were congruent with on-site, Peer Reviewed, Objectius de Desenvolupament Sostenible::11 - Ciutats i Comunitats Sostenibles, Preprint

We propose a finite-element formulation for simulating multi-component flows occupying the same domain with spatially varying concentrations. Each constituent is assumed to behave as an incompressible Newtonian fluid, and solutions are sought for the velocities and volume fractions of each phase, as well as the common pressure. Stabilization terms are derived within the framework of the variational multiscale method based on an approximation of the finite-element residual to achieve control of the pressure and volume fractions. We utilize the concept of term-by-term stabilization in conjunction with orthogonal subgrid scales, thus incorporating only those terms of the residual essential to obtain stability and projecting them on a space orthogonal to the finite element space. The resulting system of equations is solved in a monolithic manner, requiring a small number of nonlinear iterations. Several benchmark tests have been performed to confirm the stability and optimal asymptotic convergence rates for linear and higher-order elements using the proposed formulation., H. Gravenkamp acknowledges grant CEX2018-000797-S funded byMCIN/AEI/ 10.13039/501100011033. R. Codina acknowledges the support received fromthe ICREA Acadèmia Research Program of the Catalan Government. J. Principe acknowledges grant PID2021-123611OB-I00 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe.”, Peer Reviewed, Postprint (author's final draft)

This bachelor’s thesis focuses on the resolution of the mass, momentum and energy conservation equations through the resolution of several heat transfer and fluid flow benchmark problems. Its resolution is used as in introduction to the topic of computational fluid dynamics. To solve them, a progressive difficulty approach is taken, firstly introducing heat conduction, then convection through the general convection-diffusion equation and the eventual the resolution of the (incompressible) Navier-Stokes equations, finally adding an introduction into turbulent models and turbulence. Each one of the listed topics has its own chapter, in which the relevant equation is introduced theoretically and mathematically and solved using the finite volume method. The resolution of each code is done by developing a self-written code in C++. The aim of these benchmark cases is to reproduce and analyze the results for a further understanding of fluid dynamics and to verify the code. The verification cases for the heat conduction and convection-diffusion equation resolution focuses in a one-dimensional and two-dimensional cases, that help understand and characterize both phenomena. Also, studies are performed analyzing the effect of several parameters and schemes. For the resolution of the Navier-Stokes equation, the fractional step method is applied to solve said equations, that are verified with classical scenarios like the lid-driven and differentially heated cavities, flow between parallel plates and the square cylinder problem. Each one of this verification cases characterizes one specific topic of the Navier-Stokes equation: its resolution, resolution of free convection and introduction of geometries. The thesis culminates with the resolution of Burger’s equation in Fourier space, which is used as an introduction to turbulence and LES modelling.

This thesis explores the numerical resolution of mass, momentum, and energy conservation equations, with a focus on their application in absorption and adsorption refrigeration systems. It begins with establishing the fundamentals of computational fluid dynamics, with classic cases examined and validated through custom C++ code. The research then moves to practical case study: falling film heat transfer, a key process in absorption and absorption refrigeration. The simulation of liquid ethylene glycol film flow over a vertical plate is performed using the open foam solver interFoam. The simulation results are validated against both experimental and numerical data, and boundary conditions are optimized for accuracy. Additionally, a new solver, interTempFoam, is introduced to incorporate the temperature transport equation. The heat transfer phenomenon occurring in falling film is analysed and assessed.

A strategy to improve the performance and reduce the memory footprint of simulations on meshes with spatial reflection symmetries is presented in this work. By using an appropriate mirrored ordering of the unknowns, discrete partial differential operators are represented by matrices with a regular block structure that allows replacing the standard sparse matrix–vector product with a specialised version of the sparse matrix-matrix product, which has a significantly higher arithmetic intensity. Consequently, matrix multiplications are accelerated, whereas their memory footprint is reduced, making massive simulations more affordable. As an example of practical application, we consider the numerical simulation of turbulent incompressible flows using a low-dissipation discretisation on unstructured collocated grids. All the required matrices are classified into three sparsity patterns that correspond to the discrete Laplacian, gradient, and divergence operators. Therefore, the above-mentioned benefits of exploiting spatial reflection symmetries are tested for these three matrices on both CPU and GPU, showing up to 5.0x speed-ups and 8.0x memory savings. Finally, a roofline performance analysis of the symmetry-aware sparse matrix–vector product is presented., A.A.B., X.A.F., G.C., C.D.P.S., A.O. and F.X.T. have been financially supported by two competitive R+D projects: RETOtwin (PDC2021120970-I00), given by MCIN/AEI/10.13039/501100011033 and European Union Next GenerationEU/PRTR, and FusionCAT (001P-001722), given by Generalitat de Catalunya RIS3CAT-FEDER. A.A.B. 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. The numerical experiments have been conducted on the Marenostrum4 supercomputer at the Barcelona Supercomputing Center under the project IM-2022-3-0026. The authors thankfully acknowledge these institutions., Peer Reviewed, Postprint (published version)

Over the last years, the reduction of CO2 pollution has been of a major concern in the automotive industry, since it is one of the biggest contributors to the overall emissions of this gas. Besides, the European Union is taking action to reduce its emissions. On the other hand, exposure to high levels of noise is considered harmful to health, of which the automotive industry is also a major contributor, especially in large cities. Both problems can be addressed by performing scale-resolving simulations of the flow over a passenger car, whose results can be used to reduce the aerodynamically generated drag and noise. Nevertheless, in recent years this type of simulations have not been feasible due to their high computational requirements. Nowadays, with the emergence of supercomputing architectures together with the use of high-order methods, allowing the algorithm to be parallelized, the computational resources of this models have been optimized. The high-order methods stand out since they can obtain solutions with higher accuracy and with less computational cost than linear methods. However, the lack of having a robust method to generate 3D curvilinear meshes has made difficult their implementation into real life cases. Therefore, the present work aims to establish a suitable quality criteria, which allows to obtain high-order 3D meshes in a robust way, by choosing the adequate methodologies already present in the literature. The main drawbacks of the present approach are also analyzed. The aerodynamic analysis is performed with the DrivAer geometry, which represents a generic but realistic car configuration which was specifically created to have a wide range of representative results to validate CFD algorithms when dealing with complex geometries. The results show good agreement with the literature, which opens the way for further research to get the most out of high order methods, and reduce the computational demands of scale-resolving simulations.

This master’s thesis has been carried out in relation to the studies of the master’s degree in industrial engineering. The project comes from an interest in understanding how electrolyzers work to produce hydrogen and to develop a model that analyzes, by means of computational fluid dynamics (CFD), the heat management of these devices. Therefore, it will first be necessary to study the physical and chemical fundamentals of elec- trolysis and and the different types of water electrolyzers for hydrogen production that exist nowadays. In addition, since the study focuses on the use of CFD for thermal analysis, it will be explained what the physical fundamentals of computational fluid dynamics are, and how the computer programs that allow these fundamentals to be applied work. In particular, the project focuses on a SOEC type electrolyzer. In order to model these devices, it has been necessary to understand which layers of materials make up these devices, as well as the function that each of them performs in the electrolysis of water. Some of the thermodynamic and electrical properties of the different layers have been obtained as a function of temperature. Subsequently, the different SOEC configurations that currently exist have been analysed, and the geometry to be modelled has been defined. The physical-theoretical equations that allow the behaviour of these devices to be simulated have also been analyzed. These equations have been explained in detail. They have then been simplified in order to be able to apply them to the model that has been necessary to carry out the simulation of this project and, in this way, to obtain a first approximation of the thermal analysis of this type of electrolyzers. In order to check whether the modelling idea proposed is correct, a base model has first been developed to verify the correct use of the tools provided by the OpenFOAM software. In this base model, three solid regions emulating the SOEC regions have been defined. The meshi

El resum del següent Treball de Final de Grau (TFG) es centra en l'anàlisi d'una cavitat de flux utilitzant un codi en Fortran. La cavitat amb moviment de tapa (lid-driven cavity en anglès) és un problema clàssic en la mecànica de fluids que implica el flux d'un fluid confinat en una cavitat rectangular on la cara superior es desplaça a una velocitat constant, la qual cosa indueix un flux a l'interior de la cavitat. La tapa lateral i la inferior es mantenen estacionàries, i les parets de la cavitat es consideren impermeables i no lliscants. L'objectiu principal de l'estudi va ser investigar i comprendre el comportament del flux a la cavitat tancada i avaluar els efectes de diferents paràmetres en el flux i les característiques de la cavitat. Per aconseguir-ho, vam utilitzar un codi en llenguatge de programació Fortran, que permet simular i analitzar el flux a la cavitat. El codi es basa en el mètode de volums finits i l'algorisme SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) que s'utilitza per resoldre problemes de flux de fluids incompressibles. Vam realitzar simulacions computacionals utilitzant el codi, variant alguns paràmetres com el nombre de Reynolds i el nombre de nodes a la malla. Analitzarem els resultats comparant-los amb els resultats de referència., El resumen del siguiente trabajo de TFG se centra en el análisis de una cavidad de flujo utilizando un código de fortran. La cavidad con movimiento de tapa (lid-driven cavity en inglés) es un problema clásico en la mecánica de fluidos, trata del flujo de un fluido confinado en una cavidad rectangular donde la cara superior se desplaza a una velocidad constante, lo que induce un flujo en el interior de la cavidad. La tapa lateral y la inferior se mantienen estacionarias, y las paredes de la cavidad son consideradas impermeables y no deslizantes. El objetivo principal del estudio fue investigar y comprender el comportamiento del flujo en la cavidad cerrada y evaluar los efectos de diferentes parámetros en el flujo y las características de la cavidad. Para ello, utilizamos un código en lenguaje de programación de Fortran, en el que permite simular y analizar el flujo de la cavidad. El código de basa en el método de volúmenes finitos y el algoritmo SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) que es utilizado para resolver problemas de flujo de fluidos incompresibles. Llevamos a cabo simulaciones computacionales utilizando el código, variando algunos parámetros como el número de Reynolds y el número de nodos en la malla. Analizaremos los resultados comparándolos con los resultados de Benchmark., Certainly, here's the summary of the following Bachelor's Thesis (TFG), which focuses on the analysis of a flow cavity using a Fortran code. The lid-driven cavity is a classic problem in fluid mechanics that involves the flow of a confined fluid in a rectangular cavity where the top face moves at a constant speed, inducing flow inside the cavity. The side and bottom lids are held stationary, and the cavity walls are considered impermeable and non-slip. The main objective of the study was to investigate and understand the behavior of flow in the closed cavity and evaluate the effects of different parameters on the flow and cavity characteristics. To achieve this, we employed a Fortran programming language code, which allowed us to simulate and analyze the cavity flow. The code is based on the finite volume method and the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm, commonly used to solve incompressible fluid flow problems. We conducted computational simulations using the code, varying parameters such as the Reynolds number and the number of nodes in the mesh. We will analyze the results by comparing them with benchmark results.

Preserving the operators’ symmetries at the discrete level is crucial to enable reliable DNS and LES simulations of turbulent flows. Moreover, real-world applications demand robust and stable numerical methods suitable for complex geometries. In this regard, this work describes TFA, our novel in-house code, which relies on a symmetry-preserving discretisation for unstructured collocated grids that, apart from being virtually free of artificial dissipation, is shown to be unconditionally stable. To ensure cross-platform portability, the implementation of such a discretisation relies on a minimal set of algebraic kernels. Doing this poses challenges that need to be addressed, like the low arithmetic intensity of the sparse matrix-vector product, the reformulation of boundary conditions and flux limiters, or the efficient computation of eigenbounds to determine the time-step. With the aim of analysing the advantages and disadvantages of this “algebraic” approach, a comparison with OpenFOAM, probably the most widespread open-source CFD code, will be made. Finally, a relevant case from the CSP industry will be presented in order to assess the feasibility of overnight industrial simulations., A.A.B., G.C., J.A.H., F.X.T., C.D.P.S. and A.O.were financially supported by the competitive R+D project RETOtwin (PDC2021-120970-I00), given by MCIN/AEI/10.13039/501100011033 and European Union Next Generation EU. A.A.B. has also been supported by the predoctoral grants DIN 2018-010061 and 2019-DI-90, given by MCIN/AEI/10.13039/501100011033 and the Catalan Agency for Management of University and Research Grants (AGAUR), respectively. J.A.H.has also been supported by the predoctoral grant 2022 FI_B1 00204, given by AGAUR. Calculations were carried out on Snellius supercomputer at SURF. The authors thankfully acknowledge these institutions., Postprint (published version)

Preserving the operators symmetries at discrete level is the key aspect to enable reliable DNS and LES simulations of turbulent flows. On the other hand, realworld applications demand robust and stable numerical methods suitable for complex geometries. In this regard, this work presents a symmetry-preserving discretization for unstructured collocated grids that, apart from being virtually free of artificial dissipation, it is shown to be unconditionally stable. Special attention is given to the (de)construction of the discrete operators into fundamental geometrical entities. On this basis, a new inexpensive method to compute strict eigenbounds for the convective and diffusive operators, which are needed to determine the time-step à la CFL, is presented. Apart from providing better estimations than previous methods, it is relies on a sparsematrix vector product where only vectors change on time. Hence, both implementation in existing codes and cross-platform portability are straightforward. Altogether leads to a simple and robust approach for DNS and LES simulations of complex turbulent flows., F.X.T., J.A.H,D. Sand A.O. are supported by the Ministerio de Economía y Competitividad, Spain, RETO twin project (PDC2021-120970-I00). Calculations were carried out on MareNostrum4 supercomputer at BSC.We thankfully acknowledge these institutions., Postprint (published version)

In this work, we aim to shed light to the following research question: is the complexity of numerically solving Poisson’s equation increasing or decreasing for very large DNS and LES simulations of incompressible flows? Physical and numerical arguments are combined to derive power-law scalings at very high Reynolds numbers. Preliminary results of forced homogeneous isotropic turbulence seems to confirm these scalings. Theoretical convergence analysis for both Jacobi and multigrid solvers defines a two-dimensional phase space divided into two regions depending whether the number of solver iterations tend to decrease or increase with the Reynolds number. Preliminary results seem to confirm that we are indeed in the second region., This work was financially supported by two competitive R+D projects: RETOtwin (PDC2021- 120970-I00), given by MCIN/AEI/ 10.13039/501100011033 and European Union Next GenerationEU, and FusionCAT (001-P-001722), given by Generalitat de Catalunya RIS3CATFEDER. A` del 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., Postprint (author's final draft)

This paper presents a study on the falling film characteristics of vertical tubes and inclined plates, focusing on film thickness and wave frequency. A numerical modelling approach was employed using OpenFOAM’s InterFOAM solver to calculate the fluid dynamic characteristics of the falling film. The experiments were conducted using a variety of liquids, including water, ethylene glycol, and LiBr solution, and in the range of Reynolds numbers (Re) up to 1000. The film thickness was measured using two methods i. e., the contact point method and the holding method. Besides, a laser distance sensor was employed to detect the wave frequency of the falling film. The results indicated that the numerical model was able to accurately predict the mean film thickness in the complete turbulent film regime. The film thickness measured in this study was in the 0.3-1.0 mm range, with the average film thickness of the horizontal falling film being higher than that of the vertical falling film., Research project PID2020-115837RB-I0 funded by MCIN/AEI/10.13039/501100011033. J. Zheng holds a China Scholarship Council Studentship with the Polytechnical University of Catalonia., Postprint (published version)

Direct numerical simulations of the incompressible Navier-Stokes equations at high Reynolds numbers are not yet feasible, so dynamically less complex mathematical formulations such as Large Eddy Simulation (LES) have been developed. For the well-known eddy viscosity models for LES, the computational method is based on the combination of invariants of a symmetric tensor that depends on the gradient of the resolved velocity field, G = ¿u. Several models (namely S3PQR) have been developed using the first three principal invariants of the symmetric tensor GGT with excellent results. Therefore, in this work, we will focus on the application of the S3PQR and other LES models on the free boundary layer case. Then, we will test their performances over a fully developed boundary layer wind farm, using a simplified wind turbine model., Postprint (published version)

The problem of naval ship resistance prediction is a critical issue in naval architecture and ship design. It involves estimating the amount of resistance a ship will encounter as it moves through the water under various conditions. This prediction is crucial because it allows ship designers and engineers to determine the power requirements, fuel efficiency, and other characteristics. Computational Fluid Dynamics (CFD) is the most reliedupon prediction technique in hull-form design and analysis for new vessels. It is used to perform fluid flow analyses to help designers extract the highest possible performance in nearly every functional aspect of a naval vessel, including ship resistance. Modern CFD methods are certainly capable of delivering highly accurate results in this domain, but they are fundamentally based on numerical methods that solve the relative equations of mass, energy, and momentum conservation resulting in CPU-intensive mathematical procedures and this hinders their convenient use and integration within the conventional ship design loop. Machine Learning (ML) is rapidly becoming a core technology for scientific computing and its use in CFD is a wide research topic. In this work, we propose data-driven surrogate models, based on stateof-the-art ML algorithms, to speed up the exploration of several hull forms at an early design stage. Results showed that the employed approaches can be a valuable and more efficient solution for the initial qualitative analysis of several hull shapes in order to find the most efficient one for every scope of use., El problema de la predicci´o de la resist`encia dels vaixells ´es un tema cr´ıtic en l’arquitectura naval i el disseny de vaixells. Implica estimar la quantitat de resist`encia que trobar`a un vaixell quan es mou per l’aigua en diverses condicions. Aquesta predicci´o ´es crucial perqu`e permet als dissenyadors i enginyers de vaixells determinar els requisits d’energia, l’efici`encia del combustible i altres caracter´ıstiques. Computational Fluid Dynamics (CFD) ´es la t`ecnica de predicci´o m´es fiable en el disseny i l’an`alisi de la forma del casc per a vaixells nous. S’utilitza per realitzar an`alisis de flux de fluids per ajudar els dissenyadors a extreure el m`axim rendiment possible en gaireb´e tots els aspectes funcionals d’un vaixell naval, inclosa la resist`encia del vaixell. Els m`etodes CFD moderns s´on certament capa¸cos d’oferir resultats altament precisos en aquest domini, per`o es basen fonamentalment en m`etodes num`erics que resolen les equacions relatives de conservaci´o de massa, energia i moment, donant lloc a procediments matem`atics intensius en CPU i aix`o dificulta. el seu ´us convenient i la seva integraci´o dins del bucle de disseny de vaixells convencional. Machine Learning (ML) s’est`a convertint r`apidament en una tecnologia b`asica per a la inform`atica cient´ıfica i el seu ´us a CFD ´es un tema de recerca ampli. En aquest treball, proposem models substituts basats en dades, basats en algorismes ML d’´ultima generaci´o, per accelerar l’exploraci´o de diverses formes de casc en una fase inicial de disseny. Els resultats van mostrar que els enfocaments emprats poden ser una soluci´o valuosa i m´es eficient per a l’an`alisi qualitativa inicial de diverses formes de casc per tal de trobar la m´es eficient per a cada `ambit d’´us

An Inertia Particle Separator (IPS) is a device situated in the intake of the aircraft to remove particles from entering the engine; consisting of two types of outlets: core and scavenge. This work presents the numerical simulation of the Reynolds-Average Navier-Stokes (RANS) and the Large Eddy Simulation (LES) approach for an IPS. First, the fluid comparison of RANS and LES is carried out for three cases. Results show that the profile velocities of the continuous phase for RANS and LES are in good agreement. In the following, the separation efficiency is analyzed through different Reynolds numbers (Re) of 52K, 83K and 110K and coreto-scavenge mass flow rate ratios (ß = 0.10,0.12,0.18) for two types of particles with diameters of d p = 10µm and d p = 120µm. The elastic particle-wall collision is considered here. The particles with the smallest Stokes number (St <<1) show the smallest separation efficiency because they follow the flow pattern. Their separation efficiency decreases in lower flow split values. Also, decreasing the Re decreases the separation efficiency for the particle diameter of 10 µm. The particles with 120 µm have a separation efficiency almost near one, which does not change with decreasing the flow split. The discrepancies in separation efficiency between RANS and LES show up in the small particles where these particles are affected by the flow unsteadiness. In contrast, RANS can predict the separation efficiency correctly for the particles with large inertia and Stokes numbers (St >> 1)., This work has been developed within the EU H2020 Clean Sky 2 research project “A New proTection devIce for FOD - ANTIFOD” (grant agreement Nº 821352). Linda Bahramian acknowledges the financial support from the Secretariat of Universities and Research of the Generalitat de Catalunya and the European Social Fund, FI AGAUR Grant (2019 FI B 01205) and the UPC-Santander Grant. Carles Oliet, as a Serra Húnter associate professor, acknowledges the Catalan Government for the support through this Programme., Postprint (published version)

This work presents a massively parallelized Unstructured Conservative Level-Set (UCLS) method for two-phase flow with variable surface tension, i.e., Marangoni effect. This method employs a multiple-marker UCLS approach to circumvent the numerical coalescence of fluid particles (bubbles and droplets). The finite-volume method discretizes transport equation on three dimensional collocated unstructured meshes. The fractional-step projection method solves the pressure-velocity coupling in momentum equation. The convective term of the mo- mentum transport equation, level-set advection equations, energy equation or surfactant trans- port equation is discretized by unstructured flux-limiters schemes to avoid numerical oscilla- tions around discontinuities and to minimize the numerical diffusion. Such a combination of numerical techniques preserves the numerical stability and accuracy in two-phase flows with variable surface tension., Postprint (published version)

Background Computational Fluid Dynamics (CFD) are usually carried out with the rigid walls assumption. In some cases, this is a valid approach however, blood does not flow inside rigid vessels instead, they have some degree of elasticity. This elasticity is vital in some studies into the stiffness of the walls due to either natural aging or some cardiovascular disease. Materials and methods Parting with an already prepared 3D simVascular model of an healthy pulmonary artery, we started with deciding the characteristics of the mesh. Later, we decided the boundary conditions and the wall properties. Results Results show us the differences of a rigid wall simulation compared to one with deformable walls. At first glance, there might seem that the difference is negligible but, the reality is, that the values like wall shear stress (WSS) or the vorticity held great importance in the ambit of cardiovascular disease. Conclusions CFD is a powerful tool to investigate flow patterns in blood vessels and in the prediction of dangerous situations. Even though all the simulations have many assumptions and are not perfect, their advanced techniques that are continuously being introduced and improved are essential for the betterment of our life.

Postprint (published version)

Reversible Pump- Turbine turbomachines are frequently used in peaking load hydroelectrical power plants. These plants contribute to satisfy the energy demand during peak load periods. Its dynamic behaviour is influenced by the flow structures developed along the distributor and runner vanes. This behaviour depends both on the internal design of the machine and its operating conditions, whether it works as pump or as a turbine. High-amplitude pressure pulsations appear due to the rotor-stator interaction phenomenon, and under certain operating conditions, the frequency of these pulses can seriously affect the structural integrity of the machine. The goal of this study is to assess the dynamic behaviour of a low-specific-speed reversible pump-turbine when working both as a turbine or as a pump. For validation, the obtained numerical results for the pressure pulsation in both operating modes are compared to experimental results measured on a working prototype. These results could be useful to enhance the knowledge concerning the behaviour of the pressure pulsations due to the internal flow dynamics into the machine., Peer Reviewed, Postprint (author's final draft)

El presente Trabajo de Final de Máster se enfoca principalmente en la implementación de un sistema de acoplamiento multiescalar utilizando la metodología Pseudo-DNS, con el objetivo de reducir la complejidad de los modelos de simulación de dinámica de fluidos. El sistema involucra la confección y simulación de modelos de Elementos Volumétricos Representativos (RVE), que permiten la obtención de los efectos o patrones de las turbulencias de pequeña escala que no pueden resolver las mallas gruesas. A través de esta información, en el proyecto se ha confeccionado una nueva ley constitutiva modifica la viscosidad en cada caso para que se puedan insertar de forma adecuada estos efectos capturados previamente. A partir de aquí, se han verificado los resultados obtenidos a través de la creación de un modelo de una tubería, el cual se poseen los resultados de referencia. Por último, se ha utilizado la nueva ley constitutiva para la simulación del paso de un flujo de agua por una de las fundamentaciones que sustentan el Puente de la Bahía de Hangzhou para poder comprender la aplicabilidad real que tiene el sistema confeccionado dentro del campo de la ingeniería civil., This Master's thesis focuses mainly on the implementation of a multiscale coupling system using the Pseudo-DNS methodology, with the aim of reducing the complexity of fluid dynamics simulation models. The system involves the creation and simulation of Representative Volume Element (RVE) models, which allow the effects or patterns of small-scale turbulence that cannot be resolved by coarse meshes to be obtained. Using this information, the project has created a new constitutive law that modifies the viscosity in each case so that these previously captured effects can be inserted appropriately. From here, the results obtained have been verified by making a model of a pipe, which has the reference results. Finally, the new constitutive law has been used to simulate the passage of a water flow through one of the foundations supporting the Hangzhou Bay Bridge in order to understand the real applicability of the system created within the field of civil engineering., Aquest Treball de Final de Màster s'enfoca principalment a la implementació d'un sistema d'acoblament multiescalar utilitzant la metodologia Pseudo-DNS, amb l'objectiu de reduir la complexitat dels models de simulació de dinàmica de fluids. El sistema involucra la confecció i simulació de models d'Elements Volumètrics Representatius (RVE), que permeten l'obtenció dels efectes o els patrons de les turbulències de petita escala que no poden resoldre les malles més grosses. A través d'aquesta informació, al projecte ha confeccionat una nova llei constitutiva modifica la viscositat en cada cas perquè es puguin inserir de forma adequada aquests efectes capturats prèviament. A partir d'aquí, s'han verificat els resultats obtinguts a través de la creació d'un model d'una canonada, el qual es posseeixen els resultats de referència. Finalment, s'ha utilitzat la nova llei constitutiva per a la simulació del pas d'un flux d'aigua per una de les fonamentacions que sustenten el pont de la badia de Hangzhou per poder comprendre l'aplicabilitat real que té el sistema confeccionat dins del camp de l'enginyeria civil.

The operation of heat exchangers in a thermal system can be described by simple heat transfer equations. The heat exchange is normally defined by a global heat transfer coefficient, which takes into account internal and external convection and the characteristics of the pipe. This coefficient can be obtained analytically only for heat exchangers with standard and simple geometric characteristics otherwise leading to unacceptable inaccuracies when employed in lower-order models. An effective solution for calculating the global heat transfer coefficient for a particular heat exchanger consists of the full simulation of the heat exchange, including internal pipes and external fluid, through CFD&HT simulations. In this work, simulations of a coil heat exchanger are carried out, and the values obtained are compared with those of a reference work obtained experimentally [1]. In this work, the results obtained by means of different turbulence models present in the OpenFOAM software are compared., The author Jordi Vera has been financially supported by the Ministerio de Educaci´ on y Ciencia (MEC), Spain, (FPI grant PRE2018-084017). The author E.Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060)., Peer Reviewed, Postprint (published version)

In the present paper, a new strategy of multigrid method is introduced to accelerate the convergence speed of numerical simulations via lattice Boltzmann method. Based on the popular V-cycle multigrid algorithm, a simplified multigrid algorithm is presented and validated through the simulations of the classic lid-driven cavity flow for steady states. The novelty of the present algorithm resides in the construction of the information transferring process, in which, for a full cycle, the numerical simulation starts on the coarse mesh, transferring to medium mesh and then streams to the fine mesh through a prolongation operator. Afterwards, instead of using a restriction operator, the fluid information jumps back directly from the fine mesh to the coarse mesh via an assignment operator. The accuracy and efficiency of the simplified new algorithm are validated by comparing the results obtained when employing the classic V-cycle multigrid algorithm and the traditional lattice Boltzmann method with uniform Cartesian grid, This research was founded by the project provided by the Northwestern Polytechnical University (G2021KY05103). This work was also sponsored by the foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research (No. 614220121030101), Postprint (author's final draft)

This paper presents a strategy to accelerate virtually any Poisson solver by taking advantage of s spatial reflection symmetries. More precisely, we have proved the existence of an inexpensive block diagonalisation that transforms the original Poisson equation into a set of 2s fully decoupled subsystems then solved concurrently. This block diagonalisation is identical regardless of the mesh connectivity (structured or unstructured) and the geometric complexity of the problem, therefore applying to a wide range of academic and industrial configurations. In fact, it simplifies the task of discretising complex geometries since it only requires meshing a portion of the domain that is then mirrored implicitly by the symmetries’ hyperplanes. Thus, the resulting meshes naturally inherit the exploited symmetries, and their memory footprint becomes 2s times smaller. Thanks to the subsystems’ better spectral properties, iterative solvers converge significantly faster. Additionally, imposing an adequate grid points’ ordering allows reducing the operators’ footprint and replacing the standard sparse matrix-vector products with the sparse matrixmatrix product, a higher arithmetic intensity kernel. As a result, matrix multiplications are accelerated, and massive simulations become more affordable. Finally, we include numerical experiments based on a turbulent flow simulation and making state-of-theart solvers exploit a varying number of symmetries. On the one hand, algebraic multigrid and preconditioned Krylov subspace methods require up to 23% and 72% fewer iterations, resulting in up to 1.7x and 5.6x overall speedups, respectively. On the other, sparse direct solvers’ memory footprint, setup and solution costs are reduced by up to 48%, 58% and 46%, respectively., This work has 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. Àdel 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., Peer Reviewed, Postprint (published version)

This paper presents and discusses the results of the "2022 International Computational Fluid Dynamics Challenge on violent expiratory events" aimed at assessing the ability of different computational codes and turbulence models to reproduce the flow generated by a rapid prototypical exhalation and the dispersion of the aerosol cloud it produces. Given a common flow configuration, a total of seven research teams from different countries have performed a total of eleven numerical simulations of the flow dispersion by solving the Unsteady Reynolds Averaged Navier-Stokes (URANS) or using the Large-Eddy Simulations (LES) or hybrid (URANS-LES) techniques. The results of each team have been compared with each other and assessed against a Direct Numerical Simulation (DNS) of the exact same flow. The DNS results are used as reference solution to determine the deviation of each modeling approach. The dispersion of both evaporative and non-evaporative particle clouds has been considered in twelve simulations using URANS and LES. Most of the models predict reasonably well the shape and the horizontal and vertical ranges of the buoyant thermal cloud generated by the warm exhalation into an initially quiescent colder ambient. However, the vertical turbulent mixing is generally underpredicted, especially by the URANS-based simulations, independently of the specific turbulence model used (and only to a lesser extent by LES). In comparison to DNS, both approaches are found to overpredict the horizontal range covered by the small particle cloud that tends to remain afloat within the thermal cloud well after the flow injection has ceased, This study was funded by the Spanish Ministerio de Ciencia, Innovación y Universidades through the grants PID2020-113303GB-C21 and RTI2018-100907-A-I00 (MCIU/AEI/FEDER) and by the Generalitat de Catalunya through the grant 2017-SGR-1234. M. Z, V. R. and N. I. acknowledge the Super Computer Center (SCC) «Polytechnic» for providing computational resources. J.W., M.Š. and J.R. would like to thank the valuable insights given by professors Paul Steinmann and Matjaž Hriberšek and the financial support of the Deutsche Forschungsgemeinschaft, Germany under project STE 544/58-2 and the Slovenian Research Agency under project No. P2-0196, Postprint (author's final draft)

In microbiology laboratories, it is essential to obtain high-quality samples where the culture media are completely homogeneous. The pour plate method includes having to melt the culture media—a mixture of agar and a nutrient solution—before seeding. A static mixer is designed for this purpose, employing CFD (Computational Fluid Dynamics) with the software OpenFOAM to simulate the fluid’s behaviour in a helical static mixer with both internal and external configurations. The objective is to validate the CFD model by comparing it with the literature and provide a first approach to the mixer design. After satisfactory validation of the model, the results of the initial designs for Reynolds number 14 reveal the notably different behaviour of the fluids during mixing due to their differing properties, since agar and the nutrient solution present a high viscosity ratio. While the mixing efficiency is similar for the internal and external mixers, improved performance is demonstrated in the internal mixer, even for a shorter version. The external version of the mixer reaches a value for a mixing efficiency of 0.89 whereas the internal version performs more homogeneous mixing for the same number of mixing elements. This evaluation is based on a simplified internal design for computational simulations, whereas the external mixer is easier to manufacture but more complex to implement computationally. Finally, homogeneous mixing is achieved for the internal mixer configuration when adjusting its dimensions to those available on the market, This research was funded by Doctorats Industrials from Generalitat de Catalunya grant number 2021 DI 42. The APC was funded by Doctorats Industrials from Generalitat de Catalunya, Postprint (published version)

Al llarg del present treball s’estudien els efectes que té la influència del vent sobre cobertes cor- bades lleugeres. L’objectiu principal de l’estudi és l’observació del fenomen d’interacció fluid- estructura (FSI) i, per tant, s’analitzen les distribucions de pressió al voltant de la coberta, les deformacions que aquestes provoquen i com això afecta al flux de tornada. Per tal de realitzar un estudi d’aquest tipus s’utilitza el software de simulació de fluids ANSYS Fluent i el corresponent ”solver” d’elements finits ANSYS Mechanical, del qual només s’utilitza la seva variant estructural. La plataforma de treball que permet integrar les dades provinents de diversos tipus de simulacions és l’ANSYS Workbench. Amb la finalitat d’obtenir uns resultats consistents, es realitzen una sèrie de simulacions per tal de validar el procediment seguit, així com un estudi de la influència que té el la greca de la coberta sobre el flux. Aquest treball previ permet arribar a confeccionar el model simplificat final, a partir del qual s’estudia els efectes la interacció fluid-estructura, A lo largo del presente trabajo se estudian los efectos que tiene la influencia del viento sobre cubiertas curvadas ligeras. El objetivo principal del estudio es la observación del fenómeno de interacción fluido-estructura (FSI) y, por lo tanto, se analizan las distribuciones de presión alrededor de la cubierta, las deformaciones que estas provocan y cómo esto afecta al flujo de vuelta. Para llevar a cabo un estudio de este tipo, se utiliza el software de simulación de fluidos ANSYS Fluent y el correspondiente ”solver” de elementos finitos ANSYS Mechanical, del cual solo se usa su variante estructural. La plataforma de trabajo que permite integrar los datos provenientes de diversos tipos de simulaciones es ANSYS Workbench. Con el fin de obtener resultados consistentes, se realizan una serie de simulaciones para validar el procedimiento seguido, así como un estudio de la influencia que tiene el grecado de la cubier- ta sobre el flujo. Este conjunto de simulaciones previas permiten llegar a confeccionar el modelo simplificado final, a partir del cual se estudian los efectos de la interacción fluido-estructura, Through the course of this project, the effects of wind influence on lightweight long-span vaul- ted roofs are studied. The aim of the study is to observe the phenomenon of fluid-structure interaction (FSI), and therefore, the pressure distribution around the structure, the deformati- ons they cause to the roof, and how this affects the flow in return, are analyzed. To perform such a study, the fluid simulation software ANSYS Fluent and the corresponding fi- nite element solver, ANSYS Mechanical, are used. The working platform that allows integrating data from various types of simulation is ANSYS Workbench. In order to obtain consistent results, some simulations are carried out to validate the procedure followed, as well as a study of the influence that the roof’s strain has on the flow. This set of preliminary simulations lead to the construction of the final simplified model, from which the effects of fluid-structure interaction are studied

This work presents a preliminary assessment of the suitability of the immersed boundary method (IBM) for high-fidelity direct sound computations. A ghost-cell IBM is implemented in conjunction with a recently developed non-dissipative and robust numerical framework based on kinetic-energy and pressure-equilibrium preserving discretizations. The strategy is validated using the well-known canonical benchmark of acoustic scattering of a steady cylinder, providing accurate results and thus holding great promise for its application in complex scenarios., Postprint (published version)

Aquest treball de fi de grau del Grau en Enginyeria Mecànica posa a prova la simulació computacional de dinàmica de fluids mitjançant el programari OpenFOAM on s'intentarà esbrinar si es pot reproduir casos d’experiments reals amb precisió i realisme. S'avaluarà, sota diverses velocitats, el comportament d'un túnel de vent real proporcionat pel Departament de Mecànica de Fluids. Les dades obtingudes durant els experiments s'usaran per calcular una possible simulació i aquesta es durà a terme. D'aquesta manera esbrinarem si realment és possible simular la realitat d'aquests casos. En últim lloc, es deixarà una base sòlida per a la continuació d'aquest treball ampliant els coneixements i afegint camps com l'estudi d'objectes i proves aerodinàmiques., Este trabajo de fin de grado del Grado en Ingeniería Mecánica pone a prueba la simulación computacional de dinámica de fluidos mediante el software OpenFOAM donde se intentará averiguar si se puede reproducir casos de experimentos reales con precisión y realismo. Se evaluará, bajo varias velocidades, el comportamiento de un túnel de viento real proporcionado por el Departamento de Mecánica de Fluidos. Los datos obtenidos durante los experimentos se usarán para calcular una posible simulación y esta se llevará a cabo. De este modo averiguaremos si realmente es posible simular la realidad de estos casos. En último lugar, se dejará una base sólida para la continuación de este trabajo ampliando los conocimientos y añadiendo campos como el estudio de objetos y pruebas aerodinámicas., This final thesis of the Degree in Mechanical Engineering tests the computational simulation of fluid dynamics using OpenFOAM software where we will try to find out if it is possible to reproduce cases of real experiments with accuracy and realism. The behavior of a real wind tunnel provided by the Fluid Mechanics Department will be evaluated under various velocities. The data obtained during the experiments will be used to calculate a possible simulation and this will be carried out. In this way we will find out if it is possible to simulate the reality of these cases. Finally, a solid base will be left for the continuation of this work by extending the knowledge and adding fields such as the study of aerodynamic objects and tests

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., Postprint (published version)

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)., Peer Reviewed, Postprint (published version)

Programa de doctorat: Doctorat en Física Computacional i Aplicada

Programa de doctorat: Doctorat en Enginyeria Tèrmica

The concept of a vortex, characterised by the rotational or swirling motion of fluids, lacks a universally accepted definition in the field of fluid mechanics. This ambiguity hampers turbulence research and can lead to confusion and misunderstandings. This thesis addresses this issue by implementing a newly proposed vortex identification criterion, namely the triple decomposition method (TDM), on direct numerical simulation (DNS) data of a linear compressor cascade. The criterion is compared to established techniques, and two different algorithms for its implementation are introduced. Notably, a newer algorithm significantly reduces computational time compared to the original one proposed by Koláˇr (2007), addressing a key limitation of the method. The results obtained through this criterion highlight its main purpose of eliminating shear contamination, which the other methods suffer from. This development expands the potential usage of the TDM in a wider range of cases and can make it more commonly employed in the literature.

The present bachelor’s final thesis consists of a study for the resolution based on computational numerical methods of several problems related to the fluid mechanics and heat transfer field. The main objective of the study is to introduce the author to Computational Fluid Dynamics (CFD), allowing him to achieve advanced knowledge in numerical methods. Therefore, for this purpose, the project is based on the implementation and development of software, using C++ language, able to solve the different proposed problems considering its particularities. As can be seen throughout the document, the cases chosen are basically benchmark problems widely studied in order to be able to compare and verify the efficiency and accuracy of the solutions reached. In addition, the complexity of the studied problems increases throughout the project to assure the proper attainment of the objectives. The thesis is largely focused on the resolution of the mass, momentum, and energy conservation equation, also known as Navier-Stokes equations, for incompressible and laminar flows. Lastly, an introduction to the study of the turbulence phenomena is included, based on the analysis of preliminary cases.

The intermediate wake of a circular cylinder at Reynolds number 3300 is studied by means of highly resolved large eddy simulations. The domain spans 100 cylinder diameters D downstream of the cylinder. The intermediate wake is taken here as the region between 6D and 50D downstream of the cylinder. Initially, the velocity deficit shows a marked decrease. This rate of decrease diminishes in the region 10D to 20D downstream of the cylinder centre. Thereafter, it starts to evolve following the far-wake behaviour. The wake half-width exhibits a marked increase in the region 20D to 40D. It is in this region where the von Kárman vortex street looses its coherence., J. Fröhlich and J. Ventosa-Molina acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG), through FR1593/15-1 within PAK948. I. Rodriguez and J. Ventosa-Molina acknowledge funding by AGAUR through grant 2021 SGR 01051. The authors thank Jan Wissink for providing the inflow signals. Part of the computations were performed at the Centre for Information Services and High Performance Computing (ZIH) at TU Dresden. The authors thankfully acknowledges the computer resources at Altamira and MareNostrum Supercomputers and technical support provided by Santander Supercomputacion support group at the University of Cantabria and Barcelona Supercomputing Center - Centro Nacional de Supercomputación (RES-IM-2023-1-0016, RES-IM-2023-2-0021)., Peer Reviewed, Postprint (published version)

This project is about the usage of Runge Kutta methods in Shallow Water simulations. This is not widely used in any case as other methods such as Euler or Adam Bashforth. Runge Kutta is a predictive method that is believed to be better than other numerical methods in this kind of simulation. This project has the aim to analyze how far its capacities are in Shallow Water simulations and compare them to other numerical methods. For this goal several steps are going to be taken, as the physical description of Shallow Water, how each method works, how can we simulate them, etc. After all its expected to have several results, such as time of simulation or quality of it, to determine if its performance is really better than other ones.