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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)

Aquest projecte tracta de dur a terme el disseny i fabricació d’un prototip de dosificador com a part d’un projecte major que té com a finalitat fabricar una màquina capaç d’automatitzar la tasca d’omplir plaques Petri amb una mescla homogènia de fase de nutrients i agar. El projecte inclou un disseny detallat del dosificador a partir de diferents propostes inicials, la fabricació del prototip mitjançant la tecnologia d’impressió 3D, diferents experimentacions una vegada aquest estigui fabricat, i per últim un estudi fluidodinàmic relacionat amb com poden afectar les imprecisions relacionades amb mètode de fabricació al funcionament del prototip. Per tal de realitzar aquestes tasques de manera satisfactòria també s’ha fet un aprenentatge previ per tal d’entendre el context en què es desenvolupa el projecte i quins són els seus objectius, així com també realitzar una cerca d’informació important relacionada amb la tecnologia de fabricació utilitzada, en aquest cas l’expressió 3D. El dosificador serà el component encarregat d’extreure la quantitat desitjada de fluid del recipient que el conté i dosificar-la amb una certa pressió a la sortida. Com que en un futur s’haurà d’automatitzar el procés l’accionament ha de ser el més simple possible i en aquest cas serà un accionament lineal. En aquest projecte s’ha pres especial atenció en les conclusions extretes en els treballs previs relacionats amb el contingut d’aquest. El resultat global del projecte ha estat positiu, tot i aixà, en alguns punts han sorgit problemàtiques les quals algunes ja eren esperades. Finalment, s’ha fet un informe amb els inconvenients trobats i les possibles solucions a dur a terme en un futur.

It is believed that Killer whales, Dusky dolphins and other aquatic animals can take advantage of their skin color to provide drag reduction owing to preferential solar heating. In this work, however, in addition, a first assessment is made on the possible additional effect of the precipitation of the oxygen form the supersaturated ocean over the superhydrophobic skin of those aquatic animals. With such goal, two different approaches are addressed, namely, the most probable homogeneous model situation in which the precipitated oxygen is homogeneous combined with the water in the boundary layer, and the heterogeneous model in which the oxygen is covering the skin surface forming a very thin microfilm, Portions of this research were supported by the Spanish Ministry of Economy and Competitiveness under fellowship grant Ramon y Cajal: RYC-2013-13459. The assistance of Professor M. Scott in the calculations is gratefully acknowledged, Preprint

The articles presented in this book are related to novel flow control technologies and are divided into four main categories. The first one presents several Passive Flow Control (PFC) applications, which highlights the current relevance of passive methodologies. PFC is used to improve the performance of axial compressors by reducing the generation of shedding vortices at the trailing edge of a blade. This was initially investigated by Gao et al. [1]. Flow field improvements in highly loaded compressors and aeroengines are studied numerically by Xu et al. [2] and Lei et al. [3], respectively. In both cases, it was observed that appropriate three-dimensional blading resulted in an increase in static pressure and a reduction in the influence of secondary flows. A flapping bionic wavy leading edge wing is studied by Bai et al. [4], and they realized that it generated a higher lift than conventional airfoils. The passive flow-control application section is closed by the transient analysis undertaken on cruise missiles’ submerged inlet by Zhang and Mi [5]. The use of a distributed submerged inlet proved to have clear advantages compared to the conventional inlet. Closely related to passive flow control applications is the section related to surface micro-machining, where micro-texturing is employed by Shang et al. [6], to reduce the drag and modify the cavity area in hydrostatic bearings. Micro-groves are used by Cacciatori et al. [7], to reduce the drag in an Unmanned Aerial Vehicle’s (UAV’s) fixed wing. The next section presents several Active Flow Control (AFC) investigations. In the research by Carbajosa et al. [8], the effectiveness of a pulsating jet is compared with a steady blowing jet, aiming to control the flow on a turbine airfoil trailing edge. Several injection frequencies were evaluated, and they demonstrated that pulsating forcing is more effective than steady actuation. A parametric optimization considering three AFC parameters on an NACA-8412 airfoil at a, Postprint (published version)

Recent research has shown the possibility to achieve microconfined turbulence utilizing supercritical fluids under high-pressure transcritical conditions. This has generated increasing interest in the hybrid nature of the fluid thermophysical properties near the pseudo-boiling region. However, obtaining thermophysical experimental data of microfluidic transcritical systems is challenging due to the difficulties associated with high-pressure experiments, and the focus on velocity-related data acquisition of traditional experimental techniques, such as particle image velocimetry and particle tracking velocimetry. To overcome these challenges, this work introduces a novel framework based on a physics-constrained neural network to recover thermophysical information from velocity data at high-pressure turbulent transcritical conditions. By incorporating thermophysical constraints and boundary conditions into the network architecture and the loss function, the proposed model surpasses non-physically informed approaches in terms of accuracy, satisfaction of thermophysical constraints and preservation of bulk quantities., This work is funded by the European Union (ERC, SCRAMBLE, 101040379)., Peer Reviewed, Postprint (published version)

We report a comparative study of three numerical solvers for the direct numerical simulation of the flow over a sphere at Re = 3700. A high-order spectral-element code (Nek5000), a general purpose, unstructured finite-volume solver (OpenFOAM) and an in-house Cartesian solver using the immersed-boundary method (IBM) are employed for the analysis; results are compared against previous numerical and experimental data. Numerical results show that Nek5000 and the IBM code operate within a similar computational performance range, in terms of cost-vs-accuracy analysis; on the other hand, OpenFOAM needed a significantly higher number of degrees of freedom (and,overall, a higher cost) to match some of the basic features of the flow. Overall, our results suggest that high-order methods and second-order, energy-conserving approaches based on the IBM may be both viable options for high-fidelity scale-resolving simulations of turbulent flows with separation., Postprint (published version)

Postprint (published version)

Accurate determination of modal parameters for disk-like structures submerged in heavy fluids is a topic of interest in many fields. For instance, some type of hydraulic turbomachinery can be modelled as disk structures due to the similarity in their mode shapes. Although in the recent years many studies have analyzed and correctly predicted the fluid added mass effect on the natural frequencies and mode shapes of these types of structures, it is particularly challenging to determine fluid damping and characterize its most influential effects. In this article, the influence of vibration amplitude on the fluid damping ratio of a submerged disk, when the disk gets close to a rigid wall, is analytically and experimentally studied. Particularly, the first diametrical modes of the disk have been analyzed at different vibration amplitudes and at very small distances away from a rigid surface. The disk is put in a resonant state by means of a piezoelectric patch generating a harmonic excitation and the response is measured by means of a Laser Doppler Vibrometer (LDV). The damping ratio is obtained directly from the vibrating free decay following a sudden stop of the excitation. It is shown that the fluid damping ratio values are related to the vibration amplitude: there is a trend of increasing damping ratio as the amplitude of the vibration increases. This effect is found to be much larger for small distances between the disk and the rigid surface., Peer Reviewed, Postprint (published version)

The effects of kinetic-energy preservation errors due to Runge-Kutta (RK) temporal integrators have been analyzed for the case of large-eddy simulations of incompressible turbulent channel flow. Simulations have been run using the open-source solver Xcompact3D with an implicit spectral vanishing viscosity model and a variety of temporal Runge-Kutta integrators. Explicit pseudo-symplectic schemes, with improved energy preservation properties, have been compared to standard RK methods. The results show a marked decrease in the temporal error for higher-order pseudo-symplectic methods, and suggest that these schemes could be used to attain results comparable to traditional methods at a reduced computational cost., Postprint (published version)

We report a comparative study of three numerical solvers for the direct numerical simulation of the flow over a sphere at Re = 3700. A high-order spectral-element code (Nek5000), a general purpose, unstructured finite-volume solver (OpenFOAM) and an in-house Cartesian solver using the immersed-boundary method (IBM) are employed for the analysis; results are compared against previous numerical and experimental data. Numerical results show that Nek5000 and the IBM code operate within a similar computational performance range, in terms of cost-vs-accuracy analysis, for both global parameters as well as local flow features. On the other hand, OpenFOAM needed a significantly higher number of degrees of freedom (and, overall, a higher cost) to match some of the basic features of the flow, such as the length of the recirculation bubble forming downstream the sphere. For the finest grid resolutions, the three codes are in good agreement for most of the analyzed flow metrics. Overall, our results suggest that high-order methods and second-order, energy-conserving approaches based on the IBM may be both viable options for high-fidelity scale-resolving simulations of turbulent flows with separation., Peer Reviewed, Postprint (author's final draft)

A large eddy simulation (LES) of the flow past a 30P30N high-lift three-element wing is performed at the angle of attack AoA = 9º and a Reynolds number of Re = 750,000, based on the nested chord. Results are validated against experimental data available in the literature, proving their reliability. The present work is focused on the boundary layer development along the main suction side, including the analysis of the evolution of the thickness, the friction Reynolds number, the shape factor and the Clauser pressure-gradient parameter, as well as the Reynolds stresses at given chord locations. Finally, a proper orthogonal decomposition (POD) is performed, which allows visualizing the main turbulent coherent structures present in a turbulent boundary layer., This work is partially financed by the Ministerio de Ciencia e Innovación of Spain (Ref. PID2020-116937RB-C21 / -C22). We also acknowledge the Red Española de Surpercomputación (RES) and the EuroHPC JU for granting us access to the MareNostrum IV machine based in Barcelona Supercomputing Center (Ref. IM-2022-3-0005) and to the HPC system Vega at the Institute of Information Science (Ref. EHPC-REG-2022R01-030), respectively. Finally, R. Montalá acknowledges the support of Departament de Recerca i Universitats de la Generalitat de Catalunya through a FI-SDUR graduate fellowship program., Peer Reviewed, Postprint (author's final draft)

The topology of multiscale turbulent motions in a high-pressure transcritical channel flow is studied using direct numerical simulations. In particular, the system analyzed corresponds to high-pressure (P/Pc=2) microconfined turbulence of N2 at transcritical conditions imposed by a temperature difference between the cold (T/Tc=0.75) and hot (T/Tc=1.5) walls for a friction Reynolds number of Ret=100 (cold wall). The invariants of the velocity-gradient, rate-of-strain, and rate-of-rotation tensors are computed at different length scales. The joint probability density functions of the invariants are calculated and conditioned based on the distance from the walls and additionally compared against an equivalent low-pressure case at isothermal conditions. The results indicate that the presence of higher levels of vorticity close to the hot wall significantly impacts the flow topologies in the viscous sublayer. As a result, topology distributions of the velocity-gradient tensor typical from the outer region are observed in the vicinity of the hot wall. Moreover, the second invariants of the rate-of-strain and rate-of-rotation tensors indicate a large probability of vortex sheet events in the hot (supercritical gaslike) region. Finally, at lengths comparable to the density gradient scale, the presence of vortex sheet structures is diminished with respect to the reference low-pressure case to favor more tubelike flow motions. As inferred from the vorticity transport equation, this behavior is connected to the appearance of a baroclinic-type instability generated from the combination of the external force driving the flow and the large variation of density across the pseudoboiling region., This work is funded by the European Union (ERC, SCRAMBLE, Grant No. 101040379). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. The authors also acknowledge support from the Beatriz Galindo program (Distinguished Researcher, Grant No. BGP18/00026) of the Ministerio de Ciencia, Innovación y Universidades (Spain), the SRG program (Grant No. 2021-SGR-01045) of the Generalitat de Catalunya (Spain), and the computer resources at FinisTerrae III and the technical support provided by CESGA (Grant No. RES-IM-2023-1-0005)., Peer Reviewed, Postprint (published version)

Stably stratified Taylor–Couette flow has attracted much attention due to its relevance as a canonical example of the interplay among rotation, stable stratification, shear and container boundaries, as well as its potential applications in geophysics and astrophysics. In this article, we review the current knowledge on this topic, highlight unanswered questions and propose directions for future research. This article is part of the theme issue ‘Taylor–Couette and related flows on the centennial of Taylor’s seminal Philosophical Transactions paper (part 2)’., Postprint (author's final draft)

El objetivo del presente trabajo es el pre-dimensionamiento de un embalse a escala física en los espacios del laboratorio Flumen-UPC. Las dimensiones predeterminadas en la escala adecuada permitirá reproducir la física de los eventos de Landslide Induced Tsunami (LIT) o también denominados Landslide Generated Wave (LGW). Con el propósito de validar la técnica numérica Particle Finite Element Method (PFEM), se contemplaron ejemplos sencillos en los que el comportamiento físico es bien conocido como: la caída libre, la presión hidrostática y la descarga de un fluido desde un contenedor. Posteriormente, se valida en problemas complejos como de generación, propagación y run up en olas solitarias puras y precisas. Además, se calculó la evolución de las contribuciones que gobiernan en cada uno de los modelos. Los resultados numéricos fueron validados mediante la comparación con resultados analíticos, experimentales y semi-empíricos. En todos los modelos, se obtuvieron indicadores estadísticos que describen su rendimiento como excelente. La validación permitió definir una longitud del embalse en la cual la onda se propaga sin perturbaciones, lo que a su vez facilitó establecer el dominio de la geometría y proceder a simular diversas configuraciones de escalas. Durante el análisis de estas configuraciones, se evaluaron las contribuciones y las relaciones que rigen su solución. Los resultados indicaron que, en la escala del decímetro, las contribuciones viscosas tienen un orden de magnitud despreciable y que, por lo tanto, es posible reproducir fenómenos de LIT de la escala real. Por último se aplica una estrategia de solución unificada en problemas de interacción fluido estructura Fluid Structure Interaction (FSI) en dos eventos de tsunami donde cada una interactúa con tres configuraciones estructurales, donde se valora la eficiencia hidráulica en contener, atenuar y reducir las presiones frente al impacto del tsunami., The objective of this work is the pre-dimensioning of a reservoir on a physical scale in the spaces of the Flumen - UPC laboratory. The pre-determined dimensions in the appropriate scale will allow to reproduce the physics of Landslide Induced Tsunami (LIT) events or also called Landslide Generated Wave (LGW). With the purpose of validating the Particle Finite Element Method (PFEM) numerical technique, simple examples were considered in which the physical behavior is well known, such as: free fall, hydrostatic pressure and the discharge of a fluid from a container. Subsequently, it is validated in complex problems such as generation, propagation and run up on pure and precise solo waves. In addition, the evolution of the contributions that govern in each of the models was calculated. The numerical results were validated by comparison with analytical, experimental and semi-empirical results. In all the models, statistical indicators were obtained that describe their performance as excellent. The validation made it possible to define a length of the reservoir in which the wave propagates without disturbances, which in turn made it easier to establish the domain of geometry and proceed to simulate various scale configurations. During the analysis of these configurations, the contributions and relationships were evaluated. that govern your solution. The results indicated that, at the decimeter scale, the viscous contributions have a negligible order of magnitude and that, therefore, it is possible to reproduce real-scale LIT phenomena. Finally, a unified solution strategy is applied to fluid structure interaction (FSI) problems in two tsunami events where each one interacts with three structural configurations, where the hydraulic efficiency in containing, attenuating and reducing the pressures against the impact of the tsunami.

The Boundary Layer (BL) dynamic performance greatly affects the forces acting on any Bluff body. Ideally, the boundary layer should be attached to the surface but when separation occurs, the vortical structures and the dynamic forces’ amplitude rapidly increase. In many aerodynamic applications, the Bluff bodies are shaped in such a way that the boundary separation is delayed as much as possible. Nevertheless, using novel technologies, it is possible to reattach the previously separated BL, or at least further delay its separation. One of the novel techniques which allows for the modification of the separation point of the BL is Active Flow Control (AFC). This consists of injecting/sucking fluid in pre-defined locations. In the vast majority of AFC applications, it is essential to perform an energy assessment in order to make sure that the energy saved by the reduction/increase of the forces due to the modification of the BL separation point is much larger than the energy employed for the actuation. In order to achieve this goal, it is essential to properly tune the five parameters associated to any AFC implementation, groove position, groove width, momentum coefficient, jet inclination angle and jet frequency. Such tuning can be carried out via a parametric optimization or using any optimizer. In other words, AFC is always associated with optimization methodologies; otherwise, the energy assessment cannot be successfully accomplished. The present book is based on a set of published articles that highlight some novel applications of flow control, Postprint (published version)

We investigate the flow inside a 2D square cavity driven by the motion of two mutually facing walls independently sliding at different speeds. The exploration, which employs the lattice Boltzmann method (LBM), extends on previous studies that had the two lids moving with the exact same speed in opposite directions. Unlike there, here the flow is governed by two Reynolds numbers (ReT, ReB) associated to the velocities of the two moving walls. For convenience, we define a bulk Reynolds number Re and quantify the driving velocity asymmetry by a parameter a. Parameter a has been defined in the range a¿[-p/4, 0] and a systematic sweep in Reynolds numbers has been undertaken to unfold the transitional dynamics path of the two-sided wall-driven cavity flow. In particular, the critical Reynolds numbers for Hopf and Neimark-Sacker bifurcations have been determined as a function of a. The eventual advent of chaotic dynamics and the symmetry properties of the intervening solutions are also analyzed and discussed. The study unfolds for the first time the full bifurcation scenario as a function of the two Reynolds numbers, and reveals the different flow topologies found along the transitional path, This work is supported by the projects of the North‐ western Polytechnical University (No. G2021KY05103), the Natioanl Key Laboratory of Science and Technology on Aero‐ dynamic Design and Research (No. 614220121030101), the Key Laboratory of Icing and Anti/De-icing of China Aerodynamics Research and Development Center (No. IADL20210302), the Spanish Government (Nos. FIS 2016-77849-R and PID2020- 114043GB-I00), the Catalan Government (No. 2017-2017-SGR00785), and the Barcelona Supercomputing Centre (Nos. FI2017-2-002, FI-2017-3-0009, and FI-2016-3-0038), Postprint (author's final draft)

Thermal plumes arising from localized buoyancy sources are very common in nature and industrial settings. They have been extensively studied experimentally, theoretically, and numerically, mostly focusing on entrainment and mixing processes in fully developed turbulent regimes. Here, we focus on the primary instabilities of laminar plumes and their transitions as the strength of the buoyancy source (quantified by a Rayleigh number) is increased. Numerous prior studies in this transitional regime have reported a myriad of disparate spatiotemporal plume characteristics. We show that this wide variety of behavior is tied to the various ways an O(2) axisymmetric system can undergo symmetry breaking. Our equivariant dynamical systems theory analyzing the breaking of O(2) symmetry accounts for the spatiotemporal characteristics of the plumes we compute solving the Navier–Stokes–Boussinesq equations. We find that the nature of the ambient stratification plays an important role in determining how O(2) symmetry is broken with increasing Rayleigh number, and this in turn determines the details of the plume spatiotemporal characteristics., Postprint (published version)

The study of complex multiscale flows (Groen et al., 2014), like for example the motion of small-scale turbulent eddies over large aerodynamic structures (Jofre & Doostan, 2022), microconfined high-pressure supercritical fluids for enhanced energy transfer (Bernades & Jofre, 2022), or hydrodynamic focusing of microorganisms in wall-bounded flows (Palacios et al., 2022), greatly benefits from the combination of interconnected theoretical, computational and experimental approaches. This manifold methodology provides a robust framework tocorroborate the phenomena observed, validate the modeling assumptions utilized, and facilitatesthe exploration of wider parameter spaces and extraction of more sophisticated insights. These analyses are typically encompassed within the field of Predictive Science & Engineering (Njam, 2009), which has attracted attention in the Fluid Mechanics community and is expected to exponentially grow as computational studies transition from (mostly) physics simulations to active vectors for scientific discovery and technological innovation with the advent of Exascale computing (Alowayyed et al., 2017). In this regard, the computational flow solver presented aims at bridging the gap between studying complex multiscale flow problems and utilizing present and future state-of-the-art supercomputing systems in academic environments.The solver presented is named RHEA, which stands for open-source Reproducible Hybrid-architecture flow solver Engineered for Academia, and is available as an open-source Git repository at https://gitlab.com/ProjectRHEA/flowsolverrhea, Peer Reviewed, Postprint (author's final draft)

In this work, consideration is given to thermal contraction of the boundary layer from the progressive cooling of the airfoil and its possible technological use as an active flow control technique. Here, it is thought that if the airfoil is progressively cooled from the leading-edge toward the trailing-edge the increase in density will translate into a progressive contraction of the boundary layer which eventually could be used as technique for either re-attachment or delay detachment of the boundary layer. Utilizing the von Karman momentum integral an analytical expression for the reduction of the thickness of the boundary layer by thermal contraction was derived which predicts contractions around 10 percent for temperatures drops around 50° K. Experimental measurements made on a heated flat plate with controlled thermal gradient agree with the theoretical prediction within a 10 per cent. The possibility of prompt contraction of the boundary layer by condensation (of the air or its moisture content) if the temperature drops below the subcooling temperature, is an open question which should be addressed in future research, Postprint (author's final draft)

Microfluidics technology has grown rapidly over the past decades due to its high surface-to-volume ratios, flow controllability, and length scales efficiently suited for interacting with microscopic elements.However, as a consequence of the small rates of mixing and transfer they achieve due to operating under laminar flow regimes, the utilization of microfluidics for energy applications has long been a key challenge.In this regard, as a result of the hydrodynamic and thermophysical properties they exhibit in the vicinity of the pseudo-boiling region, it has been recently proposed that microconfined turbulence could be achieved by operating at high-pressure transcritical fluid conditions.Nonetheless, the underlying flow mechanisms of such systems are still not well characterized, and, thus, need to be carefully investigated.This work, consequently, analyses supercritical microconfined turbulence by computing DNS of high-pressure ($P/P_c = 2$) N$_2$ at transcritical conditions imposed by a temperature difference between the bottom (${T/T_c}=0.75$) and top (${T/T_c}=1.5$) walls for a friction Reynolds number of $Re_\tau=100$ (bottom wall).The results obtained indicate that microconfined turbulence can be achieved under such conditions, leading to mixing and heat transfer increments up to $100\times$ and $20\times$, respectively, with respect to equivalent low-pressure systems.In addition, it is found that the near-wall flow physics deviates from single-phase boundary layer theory due to the presence of a baroclinic instability in the vicinity of the hot/top wall.This instability strongly modifies the flow behaviour in the vicinity of the wall and renders present 'law of the wall' transformation models not accurate., Peer Reviewed, Postprint (published version)

Wake characteristics of the flow past a circular cylinder are analysed in detail at Reynolds number Re = 1500 via direct numerical simulation. A periodic spanwise domain of length 1.5pD has been found to yield correct first- and second-order wake statistics in remarkable agreement with published results at the same and closeby Re. A KelvinHelmholtz instability with a frequency fKH ¿ 0.666 is observed to occur intermittently in the shear layers issued from the top and bottom of the cylinder. The three-dimensional patterns in the wake have an estimated spanwise length scale l1 z/D ¿ 0.70 (D is the cylinder diameter) in the near-wake at (x,y)/D = (3,0.5), downstream from the cylinder, when quantified by autocorrelation (global approach). When using the Hilbert-transform (local approach) instead, the predicted length scale of streamwise vortical structures is distributed around ¿z/D ¿ 0.33 at the same sampling location. Our results show that the two approaches measure different aspects of three-dimensionality: while the former informs of the typical spanwise spacing of streamwise vortices, the latter quantifies the local spanwise size of these same flow structures, This work has been financed by the Spanish and Catalan Governments under grants FIS2016-77849-R/PID2020-114043GB-I00 and 2017-SGR-00785, respectively. The authors also thankfully acknowledge the computer resources at MareNostrum and Calendula accessed through grants RES-FI-2017-2-0020 and RES-FI-2017-3-0009, respectively. F. Mellibovsky is a Serra-Húnter fellow, Postprint (author's final draft)

A numerical study of two dimensional lid-driven triangular and trapezoidal cavity flow is performed via using the lattice Boltzmann method (LBM) for steady solutions. The equilateral and right-angled isosceles triangular cavity flow at Reynolds numbers, respectively, 500 and 100 is employed as the benchmark case for code validation. The isosceles right-angled triangular cavity flow is studied for Reynolds numbers sweeping from 100 to 8100. Flow topologies are captured and analyzed. The critical Reynolds number of Hopf bifurcation is predicted by calculating the perturbation decay rate. Two different geometries of right-angled isosceles trapezoidal cavities, bowl-shaped and pyramid-shaped trapezoids, are studied at Reynolds numbers 1000 and 7000. For each type of the trapezoidal cavity, a geometric parameter l (top-line/base-line ratio) is presented to distinguish different geometries of trapezoidal cavities. The flow patterns regarding the streamlines, vortical structures, and velocity profiles are discussed. The impact of parameter l on the fluid characteristics are investigated, This work was sponsored by the foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research (No. 614220121030101) and Key Laboratory of Icing and Anti/De-icing of CARDC (Grant No. IADL20210302), Postprint (published version)

Genetic Algorithms (GA) are useful optimization methods for exploration of the search space, but they usually have slowness problems to exploit and converge to the minimum. On the other hand, gradient based methods converge faster to local minimums, although are not so robust (e.g., flat areas and discontinuities can cause problems) and they lack exploration capabilities. This article presents a hybrid optimization method trying to combine the virtues of genetic and gradient based algorithms, and to overcome their corresponding drawbacks. The performance of the Hybrid Method is compared against a gradient based method and a Genetic Algorithm, both used alone. The rate of convergence of the methods is used to compare their performance. To take into account the robustness of the methods, each one has been executed more than once, with different starting points for the gradient based method and different random seeds for the Genetic Algorithm and the Hybrid Method. The performance of the different methods is tested against an optimization Active Flow Control (AFC) problem over a 2D Selig–Donovan 7003 (SD7003) airfoil at Reynolds number 6×104 and a 14 degree angle of attack. Five design variables are considered: jet position, jet width, momentum coefficient, forcing frequency and jet inclination angle. The objective function is defined as minus the lift coefficient (-Cl), so it is defined as a minimization problem. The proposed Hybrid Method enables working with N optimization algorithms, multiple objective functions and design variables per optimization algorithm., This research has been partially supported through the Severo Ochoa Centre of Excellence (2019-2023) under the grant CEX2018-000797-S funded by MCIN/AEI/10.13039/501100011033. The third author, Jordi Pons-Prats, is a Serra Hunter Fellow., Peer Reviewed, Objectius de Desenvolupament Sostenible::13 - Acció per al Clima, Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructura, Postprint (published version)

Volume of fluid (VOF) method with its Piecewise Linear Interface Calculation (PLIC) reconstruction algorithm is one of the most popular approaches in numerical simulation of interfacial flows with a wide range of applications in different areas. In an effort to evaluate the similarity of the PLIC-generated planes in comparison with the exact interface, a point-cloud, based on the polygon centers of PLIC planes is extracted, which later is used to form a triangular grid that represents the estimated interface. The main objective of this article is to evaluate the interface geometrical properties based on the extracted triangular grid of the interface. The methods presented in this article, characterized by a higher spatially convergence ratio, are compared with the commonly used methods. The proposed methods are tested for two 3-dimensional general test cases, where an evident improvement is seen in calculation accuracy and spatial convergence of the errors of interface normal vector and curvature., This work has been financially supported by MCIN/AEI/10.13039/ 501100011033 Spain, project PID2020-115837RBI00. E. Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060)., Peer Reviewed, Postprint (author's final draft)

© 2022 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, An a priori reduced order method based on the proper generalised decomposition (PGD) is proposed to compute parametric solutions involving turbulent incompressible flows of interest in an industrial context, using OpenFOAM. The PGD framework is applied for the first time to the incompressible Navier-Stokes equations in the turbulent regime, to compute a generalised solution for velocity, pressure and turbulent viscosity, explicitly depending on the design parameters of the problem. In order to simulate flows of industrial interest, a minimally intrusive implementation based on OpenFOAM SIMPLE algorithm applied to the Reynolds-averaged Navier-Stokes equations with the Spalart-Allmaras turbulence model is devised. The resulting PGD strategy is applied to parametric flow control problems and achieves both qualitative and quantitative agreement with the full order OpenFOAM solution for convectiondominated fully-developed turbulent incompressible flows, with Reynolds number up to one million., This work was partially supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Sk lodowska-Curie Actions (Grant agreement No. 675919) that financed the PhD fellowship of V.T.. M.G. acknowledges the support of the Serra H´unter Programme of the Generalitat de Catalunya. M.G., R.S. and A.H. were supported by the Spanish Ministry of Economy and Competitiveness (Grant agreement No. DPI2017-85139-C2-2-R). M.G. and A.H. are also grateful for the financial support provided by the Spanish Ministry of Economy and Competitiveness through the Severo Ochoa programme for centres of excellence in RTD (Grant agreement No. CEX2018- 000797-S) and the Generalitat de Catalunya (Grant agreement No. 2017-SGR-1278). R.S. also acknowledges the support of the Engineering and Physical Sciences Research Council (Grant number: EP/P033997/1)., Peer Reviewed, Postprint (author's final draft)

L'objectiu d'aquest treball de final de màster és la implementació i validació, mitjançant exemples en la literatura, de la llei constitutiva de Herschel-Bulkley de la dinàmica de fluids mitjançant el mètode d'elements finits de partícules (PFEM). La base del treball és la formulació PFEM per a fluids Bingham implementada a la plataforma del codi obert Kratos Multiphysics. El model de Herschel-Bulkley relaciona el tensor de tensions tallant amb el tensor de velocitat de deformació tenint en compte el límit elàstic, que limita l'inici o el final del moviment del fluid; la viscositat del fluid, que és responsable de la resistència del fluid al moviment; i l'índex del fluid, un paràmetre que representa el nivell de no-linealitat del moviment. La llei de Herschel-Bulkley és validada amb problemes de referència de la literatura. Es duu a terme la convergència en l'espai i el temps, l'anàlisi de sensibilitat de les variables del model i les comparacions amb el model estàndard de Bingham. A més, s'aplica l'anomenada regularització de Papanastasiou per a evitar els inconvenients numèrics del model de Herschel-Bulkley. Tots els resultats son graficats al llarg del treball i es comparen amb referències numèriques i experimentals trobades en articles científics. Al llarg dels càlculs numèrics és possible calibrar una sèrie de paràmetres fonamentals per a les simulacions amb el model., El objetivo de este trabajo de final de máster es la implementación y validación, mediante ejemplos en la literatura, de la ley constitutiva de Herschel-Bulkley de la dinámica de fluidos mediante el método de elementos finitos de partículas (PFEM). La base del trabajo es la formulación PFEM para fluidos Bingham implementada en la plataforma de código abierto Kratos Multiphysics. El modelo de Herschel-Bulkley relaciona el tensor de tensiones cortante con el tensor de velocidad de deformación teniendo en cuenta el límite elástico, que limita el inicio o el final del movimiento del fluido; la viscosidad del fluido, que es responsable de la resistencia del fluido al movimiento; y el índice del fluido, un parámetro que representa el nivel de no linealidad del movimiento. La ley de Herschel-Bulkley se valida con problemas de referencia de la literatura. Se lleva a cabo la convergencia en el espacio y el tiempo, el análisis de sensibilidad de las variables del modelo y las comparaciones con el modelo estándar de Bingham. Además, se aplica la llamada regularización de Papanastasiou para evitar los inconvenientes numéricos del modelo de Herschel-Bulkley. Todos los resultados se grafican a lo largo del trabajo y se comparan con referencias numéricas y experimentales encontradas en artículos científicos. A lo largo de los cálculos numéricos es posible calibrar una serie de parámetros fundamentales para las simulaciones con el modelo., The objective of this master's thesis is the implementation and validation, by means of literature examples, of the Herschel-Bulkley constitutive law in fluid dynamics in a particle finite element method (PFEM). The basis of the work is the PFEM formulation for Bingham fluids implemented in the open-source platform Kratos Multiphysics. The Herschel-Bulkley model relates the shear stress tensor to the strain rate tensor taking into account the yield stress, which limits the beginning or the end of the fluid motion; the fluid viscosity, which is responsible for the fluid's resistance to motion; and the fluid index, a parameter representing the level of nonlinearity of the motion. The Herschel-Bulkley law is validated with benchmark problems from the literature. Convergence in space and time, sensitivity analysis of the model variables and comparisons with the standard Bingham model are carried out. In addition, the so-called Papanastasiou regularization is applied to avoid numerical drawbacks arisign from the Herschel-Bulkley model. All results are plotted throughout the paper and compared to both numerical and experimental references found in scientific articles. Throughout the numerical calculations it is possible to calibrate a series of fundamental parameters for simulations with the model.

In the quest to understand the basic universal features of compressible convection, one would like to disentangle genuine consequences of compression from spatial variations of transport properties. For instance, one may choose to consider a fluid with uniform dynamic viscosity, but, then, compressible effects will generate a density gradient and consequently the kinematic viscosity will not be uniform. In the present work, we consider a very peculiar equation of state, whereby entropy is solely dependent on density, so that a nearly isentropic fluid domain is nearly isochoric. Within this class of equations of state, there is a thermal adiabatic gradient and a key property of compressible convection is still present, namely its capacity to viscously dissipate a large fraction of the thermal energy involved, of the order of the well-named dissipation number. In the anelastic approximation, under the assumption of an infinite Prandtl number, the number of governing parameters can be brought down to two, the Rayleigh number and the dissipation number. This framework is proposed as a playground for compressible convection, an opportunity to extend the vast corpus of theoretical analyses on the Oberbeck–Boussinesq equations regarding stability, bifurcations or the determination of upper bounds for the turbulent heat transfer. Here, in a two-dimensional geometry, we concentrate on the structure of numerical solutions. For all Rayleigh numbers, a change in the vertical temperature profile is observed in the range of dissipation number between 0 and less than 0.4 , associated with the weakening of ascending plumes. For larger dissipation numbers, the heat flux dependence on this number is found to be well predicted by Malkus's model of critical layers. For dissipation numbers of order unity, and large Rayleigh numbers, dissipation becomes related to the entropy heat flux at each depth, so that the vertical dissipation profile can be predicted, and so does the total ratio o, Thanks are due to Daniel Lecoanet who provided help on how to use the Dedalus software. The authors are grateful to the LABEX Lyon Institute of Origins (ANR-10-LABX-0066) of the Université de Lyon for its financial support within the program “Investissements d’Avenir” (ANR-11-IDEX-0007) of the French government operated by the National Research Agency (ANR). Support was provided by the ICMAT Severo Ochoa Project No. SEV-2015-0554. J. Curbelo also acknowledges the support of the RyC project RYC2018-025169, the Spanish grant [PID2020-114043GB-I00] and the Catalan Grant [No. 2017SGR1049]., Peer Reviewed, Postprint (author's final draft)

We analyze numerically the effect of a slight inclination in the lowest part of the snaking branches of convectons that are present in negative separation ratio binary mixtures in two-dimensional elongated rectangular cells. The exploration reveals the existence of novel stationary localized solutions with striking spatial features different from those of convectons. The numerical continuation of these solutions with respect to the inclination of the cell unveils the existence of even further families of localized structures that can organize in closed branches. A variety of localized solutions coexist for the same heating and inclination, depicting a highly complex scenario for solutions in the lowest part of the snaking diagrams for moderate to high heating. The different localized solutions obtained in the horizontal cell are discussed in detail., Postprint (author's final draft)

En aquest projecte es realitza l’estudi d’un flux compressible a través d’una tovera convergentdivergent real i es simulen els resultats per CFD. S’han realitzat experiències amb diferents pressions a la sortida de la tovera amb l’equip de flux compressible del Laboratori de Mecànica de Fluids de la EEBE i s’han recollit les dades d’interès per tal de definir la simulació de la tovera per CFD i observar els diferents comportaments del flux. Seguidament es realitza el dimensionament de la tovera del laboratori mitjançant el programa CAD SolidWorks. Es defineixen dues geometries: una de la tovera real amb la sonda o sensor de toma de pressió estàtica utilitzat al seu interior i un altre de la tovera sense sonda amb la mateixa àrea de pas que el cas real. Aquestes geometries s’importen a l’ANSYS Fluent per dur a terme les simulacions per CFD. Posteriorment s’efectua el mallat de les geometries i es configuren les característiques del flux i de la tovera al software CFD. Un cop obtinguts els resultats de la simulació es defineixen gràfics de contorns de pressió, velocitat, temperatura i número de Mach per veure si el comportament del flux es correcte. Finalment es defineixen els perfils de les magnituds esmentades i es comparen els resultats obtinguts per CFD amb els experimentals. També es comparen els cabals màssics de les geometries per observar l’efecte de la sonda de mesurament a l’interior de la tovera., En este proyecto se realiza el estudio de un flujo compresible a través de una tobera convergentedivergente real y se simulan los resultados por CFD. Se han realizado experiencias con diferentes presiones en la salida de la tobera con el equipo de flujo compresible del Laboratorio de Mecánica de Fluidos de la EEBE y se han recogido los datos de interés para definir la simulación de la tobera por CFD y observar los distintos comportamientos del flujo. Seguidamente se realiza el dimensionado de la tobera del laboratorio mediante el programa CAD SolidWorks. Se definen dos geometrías: una de la tobera real con la sonda o sensor de toma de presión estática utilizado en su interior y otra de la tobera sin sonda con la misma área de paso que el caso real. Estas geometrías se importan a ANSYS Fluent para realizar las simulaciones por CFD. Posteriormente se efectúa el mallado de las geometrías y se configuran las características del flujo y de la tobera en el software CFD. Una vez obtenidos los resultados de la simulación, se definen gráficos de contornos de presión, velocidad, temperatura y número de Mach para ver si el comportamiento del flujo es correcto. Finalmente se definen los perfiles de las citadas magnitudes y se comparan los resultados obtenidos por CFD con los experimentales. También se comparan los caudales másicos de las geometrías para observar el efecto de la sonda de medición en el interior de la tobera., In this project, the study of a compressible flow through a real convergent-divergent nozzle is carried out and the results are simulated by CFD. Experiments have been done with different pressures at the nozzle outlet with the compressible flow unit of the EEBE, Fluid Mechanics Laboratory, the data of interest have been collected to define the simulation of the nozzle by CFD and to observe the different behaviours of the flow. Next, the sizing of the laboratory nozzle is performed using the SolidWorks CAD program. Two geometries are defined: one of the real nozzle with the probe or static pressure sensor used inside it and another of the nozzle without the sensor with the same passage area as the real case. These geometries are imported into ANSYS Fluent for CFD simulations. Subsequently, the meshing of the geometries is defined and the characteristics of the flow and the nozzle are configured in the CFD software. Once the simulation results are obtained, pressure, velocity, temperature and Mach number contour graphs are created to see if the flow behaviour is correct. Finally, the profiles of the aforementioned magnitudes are created and the results obtained by CFD are compared with the experimental ones. The mass flow rates of the geometries are also compared to observe the effect of the measurement sensor inside the nozzle.

The aim of this work is to elucidate the type of transitions that take place when the periodic axisymmetric flows, which can set up at the onset of thermal convection in rotating fluid spheres, lose stability and to study the behavior of the new stable velocity fields until the flows become temporally chaotic. The computations for Prandtl numbers Pr=0.715, 0.1, and 0.01 show that when it decreases, the range of stability of these flows becomes narrower because the kinetic energy of the axisymmetric periodic solutions increases very fast, favoring their instability. From the stability analysis and direct three-dimensional simulations it is found that the transition to stable quasiperiodic flows through Neimark-Sacker bifurcations is supercritical when Pr=0.01. For Pr=0.1 two branches of stable periodic flows emerging from the conduction state have been found due to the proximity to a double Hopf bifurcation. However, only the branches bifurcating from the azimuthal rotating waves are stable at large Rayleigh numbers. Far from this bifurcation the stable flows keep the influence of the axisymmetric dynamics up to large Rayleigh numbers. For small Pr they behave as repeated transients of mixed dynamics, controlled by the azimuthal wave numbers m=0, m=1, and m=2., Postprint (author's final draft)

The availability of high fidelity data due to the recent developments in high-performance computing(HPC), has opened new doors for wall modeling in Large Eddy Simulation(LES). By augmenting the physics-based model with information gained from data, new methodologies of model development are being developed. Towards this goal, a preliminary work on developing a non-equilibrium wall model is presented here briefly describing the method., Peer Reviewed, Postprint (published version)

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 th

A common approach in aerodynamic design is to optimize a performance function—provided some constraints—defined by a choice of an aerodynamic model at nominal operating conditions. Practical experience indicates that such a deterministic approach may result in considerably sub-optimal designs when the adopted aerodynamic model does not lead to accurate predictions, or when the actual operating conditions differ from those considered in the design. One approach to address this shortcoming is to consider an average or robust design, wherein the statistical moments of the performance function, given the uncertainty in the operating conditions and the aerodynamic model, is optimized. However, when the number of uncertain inputs is large or the performance function exhibits significant variability, an accurate evaluation of these moments may require a large number of function evaluations at each optimization iteration, rendering the problem significantly expensive. To tackle this difficulty, we consider a variant of the stochastic gradient descent method where in each iteration, a stochastic approximation of the objective, constraints, and their gradients is generated. This is done via a small number of forward/adjoint solutions corresponding to random selections of the uncertainties. The methodology is applied to the robust optimization of the NACA-0012 airfoil subject to operating condition and turbulence model uncertainty. With a cost that is only a small factor larger than that of the deterministic methodology, the stochastic gradient approach significantly improves the performance of the aerodynamic design for a wide range of operating conditions and turbulence models., Peer Reviewed, Postprint (published version)

The present work studies the hydro-gasdynamic analogy between a shock wave, occurring in supersonic internal or external compressible flows, and a hydraulic jump, a sort of normal shock occurring in open-channel flows. It consists of an extensive theoretical framework followed by a practical analysis, the aim of which was to experimentally trigger the hydraulic jump, both normal and oblique, while using a low-cost designed lab prototype. The assembly development, called ‘water table’, arises from the necessity of economical alternatives to expensive supersonic wind tunnels in the experimental study of compressible fluid dynamics. With this objective in mind, a hydraulic canal based on a Laval nozzle was constructed where water flow could accelerate from subcritical to critical to supercritical regime and then return to subcritical regime through a hydraulic jump. In addition, multiple design alternatives were evaluated considering environmental, economic, functional and aesthetics factors. A low-cost implementation was the critical criterion in the design process. The measurements have revealed that the geometry of the nozzle and the wedges designed as obstacles to cause obliquity are the most significant and influential elements in the formation of a hydraulic jump in the experimental set-up. Regarding the experimental variables, the experiments demonstrate the effect of the upstream and downstream heights of the hydraulic jump in the data collection. This experience is a step forward in supporting students in the understanding of compressible flow and its principles by providing an in-house experimental set-up that promotes active learning, motivation and interest in fluid mechanics, Postprint (author's final draft)