6,820 research outputs found

    The terminator region of tidally locked M-dwarf exoplanets in 3-d general circulation models

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    The impressive sensitivity of the James Webb Space Telescope has made it possible to study the atmospheres of planets beyond the solar system. It will soon be followed by space missions aiming specifically at this goal, such as the Ariel mission, Twinkle, and the Habitable Worlds Observatory. One category of exoplanet has drawn interest because of its potential to harbour temperate climates with liquid surface water—and therefore potentially life. These are rocky planets orbiting cool M-class stars, or "M-Earths." Stellar population trends and observing biases lead to a high proportion of potentially habitable, terrestrial planets falling into this category. Because of the low temperatures of their host stars, however, habitable worlds of this type are found in close orbits where they are likely to be tidally locked. As the solar system has no tidally locked planets, our knowledge of their atmospheric circulation is currently limited to theoretical modelling. Past modelling work has shown that the asymmetrical irradiation of tidally locked planets results in characteristic circulation regimes which have profound consequences for observations. Atmospheric retrievals, which use statistical methods to fit 1-D atmospheric models to observational data and quantify the confidence of the fit, are not yet able to account for the 3-D nature of this circulation. For planets with large spatial variation in environmental conditions caused by tidal locking, 1-D models are not able to capture the differences and interconnections between planetary regions such as the dayside, nightside, and planetary limb or terminator. In addition, planetary atmospheres exhibit variation over time, potentially resulting in differences in retrieved properties between observing visits or even between different phases of a planet’s orbit. Accounting for 4-D circulation effects in atmospheric retrievals first requires a theoretical understanding of the impact of global-scale phenomena such as atmospheric waves and horizontal transport on conditions at the planetary limb, and then requires incorporation of this knowledge into the retrieval pipeline in the form of, for example, parameterisations. In this thesis, I address the first requirement: the theoretical understanding of the effects of fully modelled 4-D atmospheric circulation on the planetary limb, the region probed by transmission spectroscopy, on tidally locked planets. I focus in particular on effects caused by the global propagation of atmospheric waves and by horizontal transport of clouds and hazes. In Chapter 2, I show that that the atmospheric dynamics on the tidally locked Proxima Centauri b support a longitudinally asymmetric stratospheric wind oscillation (LASO), analogous to Earth’s quasi-biennial oscillation (QBO). The LASO has a vertical extent of 35–55 km, a period of 5–6.5 months, and a peak-to-peak wind speed amplitude of -70 to +130 ms−1 with a maximum at an altitude of 41 km. Unlike the QBO, the LASO displays longitudinal asymmetries related to the asymmetric thermal forcing of the planet and to interactions with the resulting stationary Rossby waves. The equatorial gravity wave sources driving the LASO are localised in the deep convection region at the substellar point and in a jet exit region near the western terminator, unlike the QBO, for which these sources are distributed uniformly around the planet. Longitudinally, the western terminator experiences the highest wind speeds and undergoes reversals earlier than other longitudes. The antistellar point only experiences a weak oscillation with a very brief, low-speed westward phase. The QBO on Earth is associated with fluctuations in the abundances of water vapour and trace gases such as ozone which are also likely to occur on exoplanets if these gases are present. Strong fluctuations in temperature and the abundance of atmospheric species at the terminators will need to be considered when interpreting atmospheric observations of tidally locked exoplanets. In Chapter 3, I investigate the presence of cloud cover at the planetary limb of water-rich Earth-like planets, which is likely to weaken chemical signatures in transmission spectra and impede attempts to characterise these atmospheres. Based on observations of Earth and solar system worlds, exoplanets with atmospheres should have both short-term weather and long-term climate variability, implying that cloud cover may be less during some observing periods. I identify and describe a mechanism driving periodic clear sky events at the terminators in simulations of tidally locked Earth-like planets. A feedback between dayside cloud radiative effects, incoming stellar radiation and heating, and the dynamical state of the atmosphere, especially the zonal wavenumber-1 Rossby wave identified in past work on tidally locked planets, leads to oscillations in Rossby wave phase speeds and in the position of Rossby gyres and results in advection of clouds to or away from the planet’s eastern terminator. I study this oscillation in simulations of Proxima Centauri b, TRAPPIST 1-e, and rapidly rotating versions of these worlds located at the inner edge of their stars’ habitable zones. I simulate time series of the transit depths of the 1.4 ”m water feature and 2.7 ”m carbon dioxide feature. The impact of atmospheric variability on the transmission spectra is sensitive to the structure of the dayside cloud cover and the location of the Rossby gyres, but none of my simulations have variability significant enough to be detectable with current methods. In Chapter 4, I study the interaction between the atmospheric circulation and photochemical hazes and describe the resulting haze abundances at the terminator. Transmission spectroscopy supports the presence of unknown, light-scattering aerosols in the atmospheres of many exoplanets. The complexity of factors influencing the formation, 3-D transport, radiative impact, and removal of aerosols makes it challenging to match theoretical models to the existing data. My study simplifies these factors to focus on the interaction between planetary general circulation and haze distribution at the planetary limb. I use an intermediate complexity general circulation model, ExoPlaSim, to simulate idealised organic haze particles as radiatively active tracers in the atmospheres of tidally locked terrestrial planets for a range of rotation rates. I find three distinct 3-D spatial haze distributions, corresponding to three circulation regimes, each with a different haze profile at the limb. All regimes display significant terminator asymmetry. In my parameter space, super-Earth-sized planets with rotation periods greater than 13 days have the lowest haze optical depths at the terminator, supporting the choice of slower rotators as observing targets. My thesis supports the existence of characteristic forms of temporal and spatial variability on tidally locked planets which will undoubtedly impact observations and inform our understanding of climate conditions on the surface. Overall, the effects of purely dynamical variability may be too small to be detected for Earth-like planets (but potentially detectable for larger ones). The impact of the atmospheric circulation on the distribution of clouds and hazes, on the other hand, is likely to affect even observations of terrestrial planets due to the highly scattering nature of these aerosols and will need to be accounted for in atmospheric retrievals

    Mineral snowflakes on exoplanets and brown dwarfs

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    The diversity of exoplanets and brown dwarfs provides ideal atmospheric laboratories to investigate novel physico-chemical regimes. Furthermore, the atmospheres of exoplanets act as the history books of planetary system. However, as observational data improves, the contributions of cloud particles in exoplanet and brown dwarf atmospheres must be adequately accounted for. Microphysical modelling of cloud formation provides the best method to investigate the potentially observable properties of clouds in these atmospheres. Most observed gas-giant exoplanets have been suggested to host mineral clouds which could form `snowflake-like' structures through condensation and constructive collisions. Cloud particle porosity, size and number density are influenced by constructive and destructive collisions. In this thesis, we expand our kinetic non-equilibrium cloud formation model to explore the effects of non-compact, non-spherical cloud particles on cloud structure and their spectroscopic properties. Additionally, we investigate the effects on clouds of collisional growth and fragmentation. The impact of these affects is assessed on prescribed 1D (Tgas-Pgas) profiles in DRIFT-PHOENIX model atmospheres of brown dwarfs and exoplanets. We utilise Mie theory and effective medium theory to study cloud optical depths, where we additionally represent non-spherical cloud particles with a statistical distribution of hollow spheres. We find that micro-porosity can affect the distribution of cloud particles in an exoplanet atmosphere, and that irregular particle shape impacts the optical depth in the near- and mid-infrared. However, we also find that cloud particle collisions driven by turbulence result in fragmentation of cloud particles for exoplanet atmospheres, which also impacts optical depths in the optical and mid-infrared regions. The global distribution and properties of clouds is also important as observations begin to allow for treating exoplanets in their full 3D nature. We therefore apply a hierarchical approach to global cloud formation modelling. We also apply our 1D cloud formation model to profiles extracted from results of 3D General Circulation Models (GCM) for the gas-giant exoplanet WASP-43b and the ultra-hot Jupiter HAT-P-7b, revealing a dramatic difference in the distribution of clouds between these types of exoplanets as a result of stellar radiation heating the day-side of the ultra-hot planets. This results in an asymmetry in cloud structures for the terminators of WASP-43b and more significantly for HAT-P-7b, observable in the optical depth of the clouds at these points, further complicating retrieval of cloud properties from spectra."This work was supported by the Science and Technology Facilities Council (STFC), UK [grant number 2093954]; and the Österreichische Akademie der Wissenschaften."--Fundin

    Development of Small-Angle X-Ray Scattering on a Nanometer and Femtosecond Scale for the Investigation of Laser-Driven Matter

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    Laser-Plasma-Beschleunigung mittels ultraintensiver Laserstrahlung ist eine vielversprechende Technologie fĂŒr die Entwicklung kompakter Strahlungsquellen. Diese werden in einem breiten Spektrum technischer AnwendungsfĂ€lle genutzt, zum Beispiel zur Krebstherapie, in der Laborastrophysik und fĂŒr die TrĂ€gheitsfusion, weshalb viele interdisziplinĂ€ren Forschungsfelder ein großes Interesse an ihrer Entwicklung haben. Die ersten Machbarkeitsstudien zur Nutzung gepulster Protonenstrahlung zur Tumorbehandlung haben bereits erfreuliche Ergebnisse geliefert. Dennoch lagen die erzielten Parameter des Protonenstrahls weit unter den erwarteten Werten. Die bekannten Faktoren, die diese Performance einschrĂ€nken, wurden fast ausschließlich durch Simulationen identifiziert. Der experimentelle Zugang zur Laser-Plasma-Wechselwirkung ist bisher auf die Auswertung der resultierenden Strahlung und auf makroskopische OberflĂ€cheneffekte beschrĂ€nkt, die mit optischen Messtechniken untersucht werden können. Diese Diagnostiken liefern allerdings keinerlei Informationen ĂŒber die VorgĂ€nge im Inneren des Plasmas, die letztlich die Parameter der beschleunigten Protonen bestimmen. Diese Prozesse werden in ihrer GrĂ¶ĂŸe und Zeitskala durch die Plasmaoszillation bzw. deren Frequenz und WellenlĂ€nge bestimmt. Das Ziel dieses Forschungsprojekts war es, diese LĂŒcke in der Auflösung bestehender Messmethoden zu schließen und eine Diagnostik zu entwickeln, die in der Lage ist, nanoskopische Plasma-PhĂ€nomene im Inneren der lasergetriebenen Probe zu untersuchen. Dieses Ziel konnten wir durch die EinfĂŒhrung von Röntgenkleinwinkelstreuung (SAXS) in Laserexperimenten an Röntgen-Freie-Elektronen-Lasern (XFELs) erreichen. In dieser Arbeit erlĂ€utere ich das technische Design und die methodische Auswertung des ersten dedizierten SAXS Experiments, das an der Matter in Extreme Conditions Messstation (auch MEC, Materie unter extremen Bedingungen) der Linac Coherent Light Source (auch LCLS, Linearbeschleuniger als kohĂ€rente Lichtquelle) durchgefĂŒhrt wurde. Dieses Experiment war vorrangig eine Machbarkeitsstudie, die als Basis fĂŒr die weitere Verwendung von SAXS in Laserexperimenten dienen soll. Meine Arbeit wird ausfĂŒhrlich die dafĂŒr nötigen experimentellen Techniken, den Aufbau, die Reinigung des gemessenen Beugungsbilds, das Probendesign und den Auswerteprozess erlĂ€utern. Um die experimentelle DurchfĂŒhrbarkeit dieser Methode zu testen, nutzten wir SAXS, um die Ausbreitung einer nanostrukturierten Probe in der Zeit kurz vor und wĂ€hrend des Beginns des Laserpulses zu messen. Der Ausbreitungsparameter, den wir so aus den experimentellen Daten gewinnen konnten, liegt im einstelligen Nanometer- und teilweise im Subnanometer-Bereich und stimmte gut mit den Ergebnissen einer Particle In Cell (PIC) Simulation zur frĂŒhen Ausbreitungsphase ĂŒberein. Dies zeigt, dass SAXS in der Lage ist, Plasma Prozesse zu messen, die fĂŒr andere Diagnostiken bisher nicht zugĂ€nglich waren. Außerdem beobachteten wir eine Abweichung der experimentellen Daten von dem von uns entwickelten Modell zur Beschreibung der ungehinderten Ausbreitung des Plasmas ins Vakuum. Dies veranlasste uns zu einer genaueren Untersuchung der Ausbreitung mittels PIC Simulation und tatsĂ€chlich sahen wir darin die Bildung von Plasma-Strömen, die auch in der SAXS-Auswertung qualitativ bestĂ€tigt werden konnten. Die KomplexitĂ€t des Ausbreitungsprozesses, die wir in diesem Forschungsprojekt aufdecken konnten, zeigt, dass weitere Studien dazu durchgefĂŒhrt werden sollten. Wenn wir die Ergebnisse der hier prĂ€sentierten SAXS Modelle nutzen, um unser VerstĂ€ndnis des Effekts von Vorpulsen und IntensitĂ€ts-Plateaus auf die Protonenbeschleunigung mit nanostrukturierten Proben zu verbessern, werden wir zukĂŒnftig in der Lage sein, die damit erzielten Strahlparameter zu verbessern. Der entwickelte SAXS Aufbau wurde auch an die Gegebenheiten von Experimenten zur Schockwellenverdichtung mittels Hochenergielasern angepasst und angewendet. Es gibt großes wissenschaftliches Interesse an der Entmischung von Kohlenwasserstoffen im Zustand warmer dichter Materie (WDM). Viele Laborastrophysikexperimente untersuchen das Innere von Eisriesen wie Uranus und Neptun, insbesondere den Verlauf der Phasentrennung von leichten Elementen wie Kohlenstoff und Wasserstoff, die zu Diamantregen fĂŒhrt. Bisher war es bei diesen Messungen nicht möglich, nanoskopische DichteĂ€nderungen im Inneren einer dichten Probe unter extremen Bedingungen zu untersuchen. Im Rahmen dieser Forschungsarbeit wurde SAXS als ergĂ€nzende Diagnostik in Hochenergiedichte-Experimenten mit Lasern an Einrichtungen wie an der MEC Messstation und an anderen XFELs etabliert. Ich wendete bekannte SAXS Auswerteroutinen auf den besonderen Fall eines sich von Schuss zu Schuss Ă€ndernden Dichtekontrasts an. Die verschiedenen Komponenten der SAXS Daten wurden mit den Informationen korreliert, die aus anderen Diagnostiken wie Beugung und VISAR gewonnen wurden. So konnte ich durch die Auswertung der Nanodiamant-Komponente eine SchĂ€tzung der DiamantgrĂ¶ĂŸe und des Diamant-Volumenanteils ableiten, indem ich spezifische Modelle fittete, die auf hydrodynamischen Simulationen basieren. ZukĂŒnftig möchten wir diese experimentellen Grundlagen auch auf die Untersuchung von FlĂŒssig-FlĂŒssig-Entmischung leichter Elemente im WDM Zustand anwenden. In dieser Arbeit erlĂ€utere ich die von mir entwickelten Auswerteprozesse, die auf weitere Messungen angewendet werden können, sobald deren Messbereich und SensitivitĂ€t so verbessert wurde, dass die Parameter von Interesse bestimmbar sind. Dieses Projekt half dabei, SAXS als Standarddiagnostik in Forschungseinrichtungen zu etablieren, die XFELs mit Hochleistungslaserexperimenten verbinden. Es bereitet sowohl die technische als auch die methodische Grundlage fĂŒr weitere Experimente.Laser plasma acceleration with ultra-high intensity (UHI) lasers is a promising technology for building compact radiation sources. These hold immense potential for a wide array of applications including cancer therapy, laboratory astrophysics and inertial confinement fusion and there is great interest in their development in many interdisciplinary fields of research. But while proof of concept experiments using proton pulses for tumor irradiation have delivered encouraging results, the achieved proton beam parameters fell short of the originally expected values. The limiting factors to this performance have mostly been identified in simulation only. Experimental access to the interaction between the drive laser and the dense plasma is so far limited to the analysis of the emitted radiation and the macroscopic surface effects that can be probed by visible light. These diagnostics cannot provide information about the processes in the bulk of the plasma that eventually determine the properties of the accelerated particles. Their spatial and temporal domain is dominated by the plasma oscillation frequency and wavelength. The aim of this project was to bridge this resolution gap with a diagnostic that is capable of investigating nanoscopic plasma features in the bulk of a laser-driven sample on a femtosecond scale. This was achieved by establishing the use of Small Angle X-Ray Scattering (SAXS) at UHI laser experiments at X-Ray Free Electron Lasers. My thesis will outline the technical design and scientific analysis of the first dedicated SAXS experiment at the Matter in Extreme Conditions (MEC) instrument of the Linac Coherent Light Source. The primary goal of the experiment was proof of concept as a foundation for regular use of SAXS in UHI experiments in the future. I will discuss the experimental procedures, the setup, the cleaning of the diffraction pattern, the target design and the analysis process that were developed for this new diagnostic in detail. To test the feasibility of this method, we used SAXS to measure the expansion of a nanostructured target in the femtosecond time span before and around the onset of a low intensity drive laser pulse. The expansion parameter that was extracted from the experimental data is in the in the sub- to single nanometer range and was in good agreement with the results of a particle-in-cell (PIC) simulation describing the early expansion phase. This demonstrates that SAXS is capable of measuring plasma processes on scales that were previously unobtainable by other diagnostics. We also identified a deviation of the experimental data from the simple model that we developed to describe an unobstructed expansion of plasma into vacuum. This lead us to examine the expansion in more detail via PIC simulation and indeed we discovered the formation of plasma jets at a later phase of the plasma expansion in simulation for a grating target. This additional effect was confirmed qualitatively by the SAXS analysis. The complexity of the plasma expansion process for a structured target we found in this project demonstrates the need for further studies. If we use the SAXS models presented here to improve our understanding of the effect of prepulses and pedestals on proton acceleration using nanostructured targets, we can apply this knowledge to the improvement of the proton beam parameters in future developments. %Additionally the technical implementation of SAXS for UHI laser experiments was developed in the framework of this thesis and established as a useful tool for the investigation of other nanoscopic plasma features. The developed experimental setup for SAXS was also adapted and applied to laser shock compression experiments using high energy drive lasers. There is great research interest in the demixing of hydrocarbons in the Warm Dense Matter (WDM) state. Many laboratory astrophysics experiments investigate the internal structure of ice giants like Uranus and Neptune, specifically the dynamics of the phase separation of light elements like carbon and hydrogen which can result in diamond rain. So far these measurements lacked a diagnostic that is capable of probing nanoscopic density modulations in the bulk of a dense target in an extreme state of matter. SAXS allowed us to gain access to the parameters of the demixing process. In the framework of this project SAXS was established as a complementary diagnostic to the standard setup for high energy density laser experiments at the MEC instrument and at other XFELs. I applied existing SAXS analysis procedures to the special case of a density contrast that changes on every shot. The different components of the SAXS data were correlated to information from other standard diagnostics including diffraction and VISAR. I was able to quantitatively analyze the component caused by nanodiamonds and retrieved an estimate of the diamond size and volume fraction from fits to custom models that are based on hydrodynamic simulations. In the future, we would like to extend this experimental basis to the investigation of liquid-liquid demixing of light elements in the WDM state. In this thesis I will discuss the SAXS analysis procedures that I dweveloped so that they can be applied to future measurements, once the experimental range and sensitivity has been improved to retrieve the parameters of interest. This project helped to establish SAXS as a standard diagnostic at facilities combining XFELs with high power laser experiments. It is supposed to lay both the technical and methodical groundwork for further experiments

    Observational Evidence for Primordial Black Holes: A Positivist Perspective

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    We review numerous arguments for primordial black holes (PBHs) based on observational evidence from a variety of lensing, dynamical, accretion and gravitational-wave effects. This represents a shift from the usual emphasis on PBH constraints and provides what we term a positivist perspective. Microlensing observations of stars and quasars suggest that PBHs of around 1 M⊙1\,M_{\odot} could provide much of the dark matter in galactic halos, this being allowed by the Large Magellanic Cloud observations if the PBHs have an extended mass function. More generally, providing the mass and dark matter fraction of the PBHs is large enough, the associated Poisson fluctuations could generate the first bound objects at a much earlier epoch than in the standard cosmological scenario. This simultaneously explains the recent detection of high-redshift dwarf galaxies, puzzling correlations of the source-subtracted infrared and X-ray cosmic backgrounds, the size and the mass-to-light ratios of ultra-faint-dwarf galaxies, the dynamical heating of the Galactic disk, and the binary coalescences observed by LIGO/Virgo/KAGRA in a mass range not usually associated with stellar remnants. Even if PBHs provide only a small fraction of the dark matter, they could explain various other observational conundra, and sufficiently large ones could seed the supermassive black holes in galactic nuclei or even early galaxies themselves. We argue that PBHs would naturally have formed around the electroweak, quantum chromodynamics and electron-positron annihilation epochs, when the sound-speed inevitably dips. This leads to an extended PBH mass function with a number of distinct bumps, the most prominent one being at around 1 M⊙1\,M_{\odot}, and this would allow PBHs to explain much of the evidence in a unified way.Comment: 107 pages, 39 figures, 2 tables, 380 references; invited review for Physics Report

    Precision Studies of QCD in the Low Energy Domain of the EIC

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    The manuscript focuses on the high impact science of the EIC with objective to identify a portion of the science program for QCD precision studies that requires or greatly benefits from high luminosity and low center-of-mass energies. The science topics include (1) Generalized Parton Distributions, 3D imagining and mechanical properties of the nucleon (2) mass and spin of the nucleon (3) Momentum dependence of the nucleon in semi-inclusive deep inelastic scattering (4) Exotic meson spectroscopy (5) Science highlights of nuclei (6) Precision studies of Lattice QCD in the EIC era (7) Science of far-forward particle detection (8) Radiative effects and corrections (9) Artificial Intelligence (10) EIC interaction regions for high impact science program with discovery potential. This paper documents the scientific basis for supporting such a program and helps to define the path toward the realization of the second EIC interaction region.Comment: 103 pages,47 figure

    Dynamics of bubbles across scales

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    This thesis presents numerical investigations of bubbly flow phenomena across a wide range of relevant spatial and temporal scales. The aim is to increase our understanding of a great variety of underlying phenomena and to facilitate improved predictions of bubbly flows at all relevant scales. The investigations start at small spatial scales (size of individual bubbles and below). We focus on the evolution of vapour bubbles by formulating a multiphase Direct Numerical Simulation (DNS) framework and a computationally inexpensive 1D framework, which both consider phase change- and thermal effects. These frameworks are used to study laser-induced thermocavitation bubbles that are a part of a promising technology to achieve good control of the properties of the formed crystals in the crystallisation process. Our findings identify plausible mechanisms that induce crystallisation and give guidelines for selecting suitable system parameters to maintain and control the crystallisation process. We continue to larger scales by focusing on the dynamics of individual rising bubbles. An efficient multiscale methodology is developed in an Eulerian-Lagrangian framework that predicts the liquid-phase fluctuations experienced by a bubble rising in a turbulent flow field. The dynamics and deformation of the bubble due to the liquid-phase fluctuations are resolved using a multiphase DNS framework together with a formulated Moving Reference Frame (MRF) technique. This multiscale approach is useful for studying numerous small-scale processes where bubbles are smaller than the Kolmogorov scales and can be used for bubbles, droplets or particles in both laminar and turbulent flows. We use the developed DNS framework with the MRF to study the lift force acting on deformable bubbles in steady shear flows. We formulate a theoretical framework and support it with DNS to provide a comprehensive explanation for the several identified mechanisms behind the lift force. The findings also elucidate the influence of the shear rate and governing parameters on the lift force. Finally, we study, using DNS, the dynamics and mixing properties of bubbly flows at large spatial scales (size of the entire system). We extract and analyse the dynamics and statistics of passive scalars involving O(10-100) bubbles in periodic domains. The results show a significant influence of the bubble-induced turbulence on the scalar spectra and elucidate the influence of the governing parameters on the scalar dynamics and mixing properties

    The Forward Physics Facility at the High-Luminosity LHC

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    Determination of the strong coupling αs from transverse energy-energy correlations in multi-jet events at √ s = 13 TeV with the ATLAS detector

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    Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica. Fecha de Lectura: 24-02-202

    A CFD methodology for mass transfer of soluble species in incompressible two-phase flows: modelling and applications

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    Continuous flow chemistry is an interesting technology that allows to overcome many of the limitations in terms of scalability of classical batch reactor designs. This approach is particularly relevant for both photochemistry and electrochemistry as new optimal solutions can be designed to limit, for example, the issues related to light penetration, reactor fouling, excessive distance between electrodes and management of hazardous compounds, whilst keeping the productivity high. Such devices operate often in a two-phase regime, where the appearance of a gas in the form of a disperse bubbly flow can be either a desirable feature (e.g. when the gas is needed for the reaction) or the result of a spontaneous reaction (e.g. electrochemistry). Such systems are very complicated flows where many bubbles populate the reactor at the same time and deform under the effect of several forces, such as surface tension, buoyancy and pressure and viscosity terms. Due to the solubility of gas in the liquid solvent, the disperse phase exchanges mass with the liquid (where the reactions generally occur) and the volume of the bubbles changes accordingly. Such physics is mainly a convection-dominated process that occurs at very small length scales (within the concentration boundary layer, which is generally thinner than the hydrodynamic one) and numerical tools for routine design are based on simplifying assumptions (reduced order methods) for the modelling of this region. However, such approaches often lead to errors in the prediction of the mass transfer rate and a fully-resolved method is generally needed to capture the physics at the interface. This last approach comes with a high computational cost (which makes it non suitable for common design processes) but can be employed in simplified scenarios to explore fundamental physics and derive correlation formulae to be used in reduced order models. For the above reasons, this work aims at developing a high-fidelity numerical simulation framework for the study of mass transfer of soluble species in two-phase systems. The numerical modelling of these processes has several challenges, such as the small characteristic spatial scales and the discontinuities in both concentration and velocity profiles at the interface. All these points need to be properly taken into account to obtain an accurate solution at the gas-liquid interface. In this thesis, a new methodology, based on a two scalar approach for the transport of species, is combined with a geometric Volume of Fluid method in the open source software Basilisk (http://basilisk.fr/). A new algorithm is proposed for the treatment of the interfacial velocity jump, which consists of the redistribution of the mass transfer term from the interfacial cells to the neighbouring pure gas ones, in order to ensure the conservation of mass during the advection of the interface. This step is a crucial point of the methodology, since it allows to accurately describe the velocity field near the interface and, consequently, to capture the distribution of species within the concentration boundary layer. The solver is extensively validated against analytical, experimental and numerical benchmarks, which include suspended bubbles in both super- and under-saturated solutions, the Stefan problem for a planar interface, dissolving rising bubbles and competing mass transfer of mixtures in mixed super- and under-saturated liquids. Finally, the methodology is used for the study of real applications, namely the growth of electrochemically generated bubbles on a planar electrode and the mass transfer of a single bubble in a Taylor-Couette device. The effects of the main parameters that characterise the systems (e.g. contact angle, current density and rotor speed) on the growth/dissolution rate of bubbles are investigated. Although these systems need to be necessarily simplified to allow for direct numerical simulations, these examples show that the insight gained into the fundamental physics is valuable information that can be used to develop reduced order models

    Enhancing Jet Velocity and Power Conversion Efficiency of Piezoelectric Synthetic Jet Actuators

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    The present work discusses an experimental investigation into the effect of piezoceramic employed to drive a synthetic jet actuator into a quiescent flow. The electromechanical coupling ratio of polycrystalline piezoceramics, lead zirconate–titanate 5A/5H (PZT-5A/5H), conventionally used in synthetic jet actuators, is inherently low. Therefore, this study aims to investigate using more electromechanically efficient piezoceramics, such as single-crystal, lead magnesium niobate–lead titanate (PMN-PT). In addition, two different orifice-diaphragm configurations of synthetic jet actuators, opposite and adjacent, are tested. It is identified that PMN-PT piezoceramic promotes three times higher transverse diaphragm displacement and two times more peak jet velocity compared to the PZT-5A piezoelectric actuator for the same input diaphragm voltage. A peak exit jet velocity of 99.5  m⋅s−1 was obtained at 40 V of peak supply voltage, which can be classified as a low voltage supply compared to other studies in the literature that obtained similar exit jet velocity. Also, a power conversion efficiency of 72% was achieved, corresponding to the Helmholtz resonance frequency. A new figure-of-merit, momentum coefficient per power consumption, is defined to evaluate the potential impact for full-scale implementation. A state-of-the-art value of 0.09  MW−1 is achieved
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