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- On the use of finite mixtures to improve the physical interpretation of a ground vortex flowPublication . Silva, A. R. R.; Panão, Miguel; Barata, Jorge M MLaser-Doppler measurements of the velocity characteristics of a ground vortex flow resulting from the collision of a wall jet with a boundary layer are analyzed using advanced statistical tools, namely finite mixtures of probability density functions. These are determined by the best fitting to experimental results using a Bayesian approach based on a Markov Chain Monte Carlo (MCMC) algorithm. This approach takes into account eventual multimodality and heterogeneities in velocity field distributions. Therefore, it provides a more complete information about heterogeneous velocity distributions and its corresponding characteristic velocities and turbulent fluctuations. The ground vortex flow investigated is generated by a wall jet-to-boundary layer velocity ratio of 2. The results evidence how finite mixtures are able to reconstruct the measured probability distribution in the form of a mathematical probability density function. This allows to improve the physical interpretation of the ground vortex flow through quantifying its complex structure, which is particularly relevant to VSTOL aircraft flows. Namely, identify the separation point oscillation region, and the enlargement of the region comprising the effect of collision between wall jet and boundary layer in planes moving away from the wall. Also, in the collision zone, following a conventional statistical analysis, the rms velocity fluctuation (u′) appears to be overestimated for the horizontal component due to the measured velocity range oscillating between positive and negative values. The results evidence how U‾ and u′ provide an idea of the flow dynamics, but their use is limited and an important amount of information associated with the highly curved flow complexity is lost. This prevents distinguish the magnitude of velocity fluctuations according to the flow direction, and the endorsement of anisotropy near the collision region, justifying the possibility of being numerically simulated.
- Multiple impinging jet air-assisted atomizationPublication . Pizziol, Bruno; Costa, Mário; Panão, Miguel; Silva, A. R. R.The growth of the aviation sector triggered the search for alternative fuels and continued improvements in the combustion process. This study addresses the technological challenges associated with spray systems and the concern of mixing biofuels with fossil fuels to produce alternative and more ecological fuels for aviation. This work proposes a new injector design based on sprays produced from the simultaneous impact of multiple jets, using an additional jet of air to assist the atomization process. The results evidence the ability to control the average droplet size through the air mass flow rate. Depending on the air mass flow rate there is a transition between atomization by a hydrodynamic breakup of the liquid sheet formed on the impact point, to an aerodynamic breakup mechanism, as found in the atomization of inclined jets under cross-flow conditions. The aerodynamic shear breakup deteriorates the atomization performance, but within the same order of magnitude of the atomization efficiency. Finally, despite using different configurations (2, 3 and 4-impinging jets), the outcome is similar is terms of the sizes of drops produced, although increasing the number of impinging jets also implies some deterioration of the atomization efficiency.
- Multiple Impinging Jet Air-Assisted AtomizationPublication . Costa, Mário; Pizziol, Bruno; Panão, Miguel; Silva, A. R. R.The growth of the aviation sector triggered the search for alternative fuels and continued improvements in the combustion process. This work addresses the technological challenges associated with spray systems and the concern of mixing biofuels with fossil fuels to produce alternative and more ecological fuels for aviation. This work proposes a new injector design based on sprays produced from the simultaneous impact of multiple jets, using an additional jet of air to assist the atomization process. The results evidence the ability to control the average drop size through the air-mass flow rate. Depending on the air-mass flow rate there is a transition between atomization by hydrodynamic breakup of the liquid sheet formed on the impact point, to an aerodynamic breakup mechanism, as found in the atomization of inclined jets under cross-flow conditions. The aerodynamic shear breakup deteriorates the atomization performance, but within the same order of magnitude. Finally, our experiments show that mixing a biofuel with a fossil fuel does not significantly alter the spray characteristics, regarded as a step further in developing alternative and more ecological fuels for aero-engines.
- Insights on Bubbling Formation after Drop Impact on Thin Liquid FilmsPublication . Ribeiro, Daniela; Panão, Miguel; Silva, A. R. R.; Barata, Jorge M MOver the years, the phenomena obtained when a drop impinges upon a dry, wetted or heated surface have been thoroughly studied. In previous works, the existence of splash was investigated by the authors with the goal of evaluating the possible implementation of biofuels in the civil aviation and it was found an episode of a phenomenon, seldom reported in the literature under specific pre-impingement conditions. The mechanism that leads to a bubble formation has two stages. After the drop impacts a steady liquid film, prompt splash occurs followed by crown splash. In the first stages of crown splash, the uprising sheet propagates almost normal to the liquid film, but its radius at the base continues to expand, eventually leading to the inward collapse of the crown bounding rim. Thus, the top of the crown closes in a bubble-like shape with the formation of two jets, one upwards and other downwards. The upward jet eventually disappears due to gravitational influence, while the downward jet continues to grow until it reaches the liquid film, attaching to it, stretching and detaching from the top at the hemispheric thin-sheet, forming a perfect bubble. Many secondary droplets fall on the bubble and one of them will eventually break the dome, leading to more secondary atomization. The few works reported in the literature referring to this phenomenon as “bubbling” or “floating bubble,” scarcely explore the hydrodynamic mechanism associated with this bubble formation and occurrence, mainly focusing on droplets impacting upon deep pools. However, in a previous study, the authors observed this event for a liquid film dimensionless thickness of 0:5 in a fluid mixture of Jet A-1 and biofuel NEXBTL. In this study, the impact conditions in the experiments performed allow to recreate the floating bubble with 100% of occurrence. After that, the authors present an extensive characterization of the bubbling phenomenon to understand better the mechanisms which lead to its formation, as well as its practical significance. A high-speed digital camera acquires several images of the floating bubble formation from different points of view (side and bottom). Namely, capturing the phenomenon from below, high-quality images allow retrieving essential data to describe the hydrodynamic mechanism accurately. The most relevant features include the bubble height and diameter, and the propagation velocity of the first perturbation imposed on the liquid film.
- Does liquid film temperature affects single drop impact dynamics?Publication . Mendes, André F. S. F.; Vasconcelos, Daniel; Ribeiro, Daniela; Panão, Miguel; Silva, A. R. R.The effect of liquid film dynamics in the hydrodynamics of an isolated drop impact is a complex phenomenon and not fully understood. Therefore, in this work, an experimental setup built to characterize the impact of an isolated droplet on heated and unheated liquid films consists of a heating element made of an aluminum block with resistances to produce several impact conditions. The parametric studies include the drop impact velocity and size for different fluids to evaluate their properties effect on the phenomena. The results were compared with existing thresholds in the literature to evaluate their validity and applicability range. This comparison allows us to assess if temperature causes the limits of the thresholds to change drastically or if its influence is negligible. Regarding IC engines, thresholds like splashing and bubble encapsulation are significant since they influence the atomization of the mixture and, consequently, the pollutant emissions.
- Insights on bubble encapsulation after drop impact on thin liquid filmsPublication . Ribeiro, Daniela; Panão, Miguel; Barata, Jorge M M; Silva, A. R. R.The accurate understanding of the phenomenology of drop impact onto dry/ wetted and cold/heated surfaces is increasingly relevant to implement biofuels in civil aviation. The outcome of drop impact depends on the pre-impact conditions and a seldom researched event is the encapsulation of a bubble when this impact occurs on thin liquid films. Therefore, the goal of the experimental work reported is to investigate the mechanism of this bubble encapsulation. Results show that the mechanism leading to a bubble formation has two stages. In the first stage, after the drop impacts a steady liquid film, a prompt splash occurs followed by a crown splash. The uprising sheet propagates in an almost normal direction relative to the liquid film, but its radius at the base continues to expand, eventually leading to the inward collapse of the crown-bounding rim encapsulating air inside the dome. In the second stage, three different phenomenologies of bubble encapsulation can occur. At the top of the closed crown, one jet (phenomenology 1) or two jets are formed (phenomenologies 2 and 3). For phenomenology 2, the upward jet eventually collapses due to gravitational influence, while the downward jet continues to grow until it reaches the liquid film, attaching to it, stretching and detaching from the top at the hemispheric thin sheet, forming a bubble. In phenomenology 3, the upward jet is high enough to allow its breakup and ejection of one large droplet before the collapse of the upward jet. Many secondary droplets fall on the bubble and one of them will eventually break the dome, leading to more secondary atomization. Additionally, the first perturbation imposed on the liquid film by the droplet impact is studied and an empirical correlation is proposed for its propagation velocity. Finally, bubble geometry is investigated.
- Morphology of Bubble Formation on Droplet Impact upon Thin Liquid LayersPublication . Ribeiro, Daniela; Panão, Miguel; Barata, Jorge M M; Silva, A. R. R.Within the wide possibilities for the resultant outcome of a single droplet impinging upon a liquid film, there is an unstudied phenomenon that seems to be underestimated or forgotten in time. The formation of a bubble by the crown sheet closure at the top after crown splash is the focus of this paper. The phenomenon was deeply studied as well as its occurrence, main features, and physical significance. It is important to understand completely the phenomenon to asses if it should be encouraged or avoided inside the combustion chamber of an IC engine, since it has higher production of secondary atomization due to the larger lifetime, and on the other hand, the bubble bursting increase the production of airborne particles that could affectthe pollutant emissions. Several impacts conditions were tested and it was found three different types of bubble formation depending on the jets formed at the closure point. The event was observed both from below and from the side, enhancing the knowledge about the droplet-film impact itself. The main features were studied including, the bubbling height and diameter, and the propagation velocity of the first perturbation imposed on the liquid film.
- Analysis And Visualization Of The Perturbations Imposed On The Liquid Film By Crown Sheet Collapse Or ClosurePublication . Ribeiro, Daniela; Panão, Miguel; Barata, Jorge M M; Silva, A. R. R.Bubble encapsulation is a phenomenon that results from droplet impact on a liquid film for very specific impact conditions. After splashing, the crown liquid sheet starts to bend inwards, and eventually, the jets at the top of the crown merge and form a perfect empty bubble. This bubble bursts due to the impingement of a secondary droplet that falls in the spherical dome or by reaching its critical thickness. However, bubble encapsulation is seldom reported in the literature. Due to that, this work focuses on understanding better its dynamics and formation mechanisms. By using a bottom perspective of the phenomenon, important information about its dynamics is disclosed. From the bottom shadowgraphs, the capillary waves and the perturbations imposed on the steady liquid film are clearly observed. From previous works, it was confirmed that its occurrence is systematic, so, one of the goals of this study is to realize how the impact conditions influence the phenomenon and if we can establish a criterion for its occurrence. Despite the scarce information about the phenomenon, there are some works about it that are focused on the cavity underneath the bubble. In this study, we observed the cavity and conclude that the cavity shape does not influence the bubble encapsulation phenomenon. Finally, the crown closure time was measured for a specific set of impact conditions and it was analyzed depending on the dimensionless thickness of the liquid film.
- Insights into Single Droplet Impact Models upon Liquid Films Using Alternative Fuels for Aero-EnginesPublication . Ribeiro, Daniela; Silva, André; Panão, MiguelIn aero-engines, the introduction of biofuels is among the best alternatives to fossil fuels, and this change is likely to affect the impact of droplets on interposed surfaces. Under this framework, this work reviews the main morphological hydrodynamic structures occurring upon the impact of a liquid droplet on a wetted surface, using jet fuel and biofuel mixtures as alternative fuels. The experiments performed allow investigating the effect of the liquid film thickness on the dynamic behavior of single drop impact, considering the relevancy of these phenomena to the optimization of engine operating parameters. Particular emphasis is given to the occurrence of crown splash, and the morphological differences in the outcomes of drop impact depending on the impact conditions and fluid properties. The four fluids tested included pure water (as reference), 100% Jet A-1, 75%/25%, and 50%/50% mixtures of Jet A-1 and NExBTL (Neste Renewable Diesel)—with the Weber impact number between 103 and 1625; Reynolds values 1411–16,889; and dimensionless film thicknesses of δ = 0.1, 0.5, and 1. The analysis on the secondary atomization for the different fluids evidences the predominance of prompt and crown splash, and jetting for alternative fuels. Finally, besides a systematic review of empirical correlations for the transition to splash, we investigate their universality by extrapolating the validation range to evaluate their ability to predict the outcome of impact accurately. One of the correlations studied show the highest degree of universality for the current experimental conditions, despite its limitation to thin liquid films (δ=0.1).
- Advanced statistical analysis of the collision of wall jet with a boundary layerPublication . Silva, André; Panão, Miguel; Barata, Jorge M MLaser-Doppler measurements of the velocity characteristics of a ground vortex flow resulting from the collision of a wall jet with a boundary layer are analyzed using advanced statistical tools. Namely, finite mixtures of probability density functions, which determine the best fitting using a Bayesian approach based on a Markov Chain Monte Carlo (MCMC) algorithm. This approach takes into account eventual multimodality and heterogeneities in velocity field distributions. Therefore, it provides more complete information about the probability density function of multimodal velocity distributions and allows the identification of characteristic velocities in the heterogeneous data. The experiments are performed for a wall jet-to-boundary layer velocity ratio of 2, and include mean and turbulent velocity characteristics along the two normal directions contained in planes parallel to the nozzle axis. The results, which have relevance to flows encountered by VSTOL aircraft, quantify the structure of the complex ground vortex flow. The results revealed that in the collision zone the rms velocity fluctuation appears to be overestimated for the horizontal component, probably due to the measured velocity range, oscillating between positive and negative values. The results revealed that finite mixture was able to accurately reconstruct a mathematical function describing the probability distribution obtained experimentally. The results shows that U and u'rms rms provide an idea of the flow dynamics, their use is limited and an important amount of information associated with the highly curved flow complexity is lost, preventing a more accurate description of turbulent structures emerging from the collision of wall jet with a boundary layer.