Numerical analysis of multiphase flows through the lattice boltzmann method

Questa tesi analizza il comportamento dei flussi multifase, nei quali ad esempio c’è la contemporanea presenza di diverse fasi fluide o di fase solida e fluida. Più specificatamente, in questo lavoro, i fluidi considerati sono fasi diverse della stessa sostanza, come ad esempio liquido e vapore. Una precisa definizione delle interazioni tra le molecole è di difficile formulazione. Ci sono molteplici fattori che sono sorgente di complessità nei flussi multifase, non solo l’interazione tra bolle/gocce/particelle immerse in un fluido, ma anche problemi fisici, come la transizione tra diversi regimi di flusso liguido-gas, o la presenza di interfacce perturbate, così come la simultanea presenza di fenomeni che si manifestano a diverse scale caratteristiche. Questa complessità rappresenta un limite piuttosto stringente all’utilizzo di metodi analitici sviluppati al fine di risolvere in forma chiusa questo tipo di problemi. Per le applicazioni più interessanti, riferiti a moderati numeri di Reynolds, manca una soluzione matematica chiusa. Dopo aver evidenziato il grande vantaggio dell’analisi numerica di flussi multifase, è utile introdurre alcune applicazioni ingegneristiche. Alcuni esempi di flussi multifase sono spray (ad esempio nei Motori a Combustione Interna), oppure flussi nei tubi, letti fluidi, colonne di distillazione, etc. In aggiunta, ci sono diversi fenomeni “naturali” che coinvolgono flussi multifase come nuvole e pioggia, impatto di gocce liquide, onde, fiumi e cascate. Ancora, come anticipato in precedenza, le scale coinvolte nei flussi multifase coprono un ampio intervallo, partendo dai micrometri (spray) fino a raggiungere scale macroscopiche (kilometri). Questo enorme intervallo non permette la definizione di un modello universale per risolvere tutte queste applicazioni. Partendo dalla semplicità dell’implementazione e maneggevolezza del metodo un sempre maggior numero di utilizzatori è stato incoraggiato ad utilizzare il Metodo Lattice Boltzmann al fine di ricostruire le equazioni fluidodinamiche. La grande diffusione è stata raggiunta soprattutto per modelli monofase. Ad ogni modo, diversi modelli sono stati definiti per questo approccio innovativo al fine di modellare le interazioni tra le diverse fasi. Il grande vantaggio nell’utilizzo di modelli multifase accoppiati con il Metodo Lattice Boltzmann è la possibilità di risolvere le equazioni di stato in ogni punto della griglia, con evidenti vantaggi intermini di accuratezza. Quindi, lo scopo di questa tesi è di analizzare ed evidenziare quali sono le potenzialità di un innovativo modello multifase per LBM. Il modello proposto, basato sulla modellazione dell’energia libera, permette di raggiungere elevati livelli del rapporto tra le densità in relazione alle formulazioni tradizionalmente adottate. I risultati ottenuti in termini di break up primario e secondario, così come di coalescenza tra due gocce saranno approfonditamente analizzate. Nella parte finale del lavoro alcuni risultati per la formulazione tri-dimensionale saranno mostrati.This thesis deals with multiphase flows, i.e. systems in which different fluid phases, or fluid and solid phases, are simultaneously present. More specifically, in this work, the fluids are different phases of the same substance, such as a liquid and its vapor. A precise definition of interparticles’ interactions is difficult to formulate as, often, whether a certain situation should be considered as a multiphase flow problem depends more on the point of view of the investigator than on its intrinsic nature. There are a lot of factors which are source of complexity in the multiphase flow phenomena; not only the interaction between bubble/droplets/particles immersed in a fluid, but also physical problems, like the transition between different liquid-gas flow regimes, or the presence of a perturbed interface, as well the simultaneous presence of phenomena occurring at different scales. This complexity represents a tough limit in the use of fully analytical methods designed in order to solve this kind of problems. For the most interesting applications, referred to moderate Reynolds numbers, a closed analytical solution is missing. After having pointed out the great advantage of multiphase flow numerical analysis, introducing some interesting engineering applications is useful. The multiphase problems, in fact, some examples are sprays (e.g. in Internal Combustion Engine -ICE-), or pipelines, fluidized bed, distillation columns, etc. Moreover there are many “Natural” phenomena which involve multiphase flows like clouds and rain, liquid droplets impingement, waves, rivers and water-falls. Again, as pointed out above, the scales involved in multiphase flows cover a complete range, starting from micro-meters (sprays) until reaching kilo-meters. This wide range does not allow defining a universal model capable to solve all these applications. Due to the simplicity of implementation and managing a lot of users have been encouraged in using the Lattice Boltzmann Method –LBM in order to recast fluid-dynamic equations. The great diffusion has been reached concerning single-phase approach. However, many techniques have been defined for this innovative approach in order to model interactions between phases. The main advantage in using multiphase models coupled with the LBM is the possibility to solve the Equation of State -EOS- in every grid-point, with apparent advantages in terms of accuracy. Thus, the aim of this work is to analyze and point out which are the capabilities for an innovative multiphase LBM approach. The proposed model, based on free energy modelling, allows to reach higher value for density ratio in comparison with standard formulations. The results obtained in terms of primary and secondary break-up, as well as coalescence between two droplets will be deeply described. In the final part of the work some results of the three-dimensional fully parallelized code will be shown

Hydrodynamic behavior of the Pseudo-Potential lattice Boltzmann method for interfacial flows

The lattice Boltzmann method (LBM) is routinely employed in the simulation of complex multiphase flows comprising bulk phases separated by non-ideal interfaces. LBM is intrinsically mesoscale with an hydro-dynamic equivalence popularly set by the Chapman-Enskog analysis, requiring that fields slowly vary in space and time. The latter assumptions become questionable close to interfaces, where the method is also known to be affected by spurious non hydrodynamical contributions. This calls for quantitative hydrodynamical checks. In this paper we analyze the hydrodynamic behaviour of LBM pseudo-potential models for the problem of break-up of a liquid ligament triggered by the Plateau-Rayleigh instability. Simulations are performed at fixed interface thickness, while increasing the ligament radius, i.e. in the "sharp interface" limit. Influence of different LBM collision operators is also assessed. We find that different distributions of spurious currents along the interface may change the outcome of the pseudo-potential model simulations quite sensibly, which suggests that a proper fine-tuning of pseudo-potential models in time-dependent problems is needed before the utilization in concrete applications. Taken all together, we argue that the results of the proposed study provide a valuable insight for engineering pseudo-potential model applications involving the hydrodynamics of liquid jets

Ligament break-up simulation through pseudo-potential Lattice Boltzmann Method

The Plateau-Rayleigh instability causes the fragmentation of a liquid ligament into smaller droplets. In this study a numerical study of this phenomenon based on a single relaxation time (SRT) pseudo-potential lattice Boltzmann method (LBM) is proposed. If systematically analysed, this test case allows to design appropriate parameters sets to deal with engineering applications involving the hydrodynamics of a jet. Grid convergence simulations are performed in the limit where the interface thickness is asymptotically smaller than the characteristic size of the ligament. These simulations show a neat asymptotic behaviour, possibly related to the convergence of LBM diffuse-interface physics to sharp interface hydrodynamics

Climate change, biodiversity loss and mental health: a global perspective

Climate change can have various psychopathological manifestations which have been more actively addressed by scientific research only in recent years. Indeed, extreme weather events and environmental changes have been shown to be associated with a range of mental health problems. Following the destruction of ecosystems, biodiversity loss can cause mental distress and emotional responses, including so-called 'psychoterratic' syndromes arising from negatively felt and perceived environmental change. Studies investigating relationships between biodiversity and mental health reveal a complex landscape of scientific evidence, calling for a better understanding of this challenging issue

Multicentre Italian study of SARS-CoV-2 infection in children and adolescents, preliminary data as at 10 April 2020

Data on features of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in children and adolescents are scarce. We report preliminary results of an Italian multicentre study comprising 168 laboratory-confirmed paediatric cases (median: 2.3 years, range: 1 day-17.7 years, 55.9% males), of which 67.9% were hospitalised and 19.6% had comorbidities. Fever was the most common symptom, gastrointestinal manifestations were frequent; two children required intensive care, five had seizures, 49 received experimental treatments and all recovered