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For geophysical and environmental flows, diagnostic timescales (e.g., age, residence/exposure time) may be obtained at any time and position by solving the partial differential equations established in the framework of CART (Constituent-oriented Age and Residence time Theory, www.climate.be/cart). Such timescales are holistic in that they take into account in the whole domain of interest (advective and diffusive) transport as well as reactive processes. They may be instrumental in the development of reduced-complexity models such as a well-mixed box and various types of pipe flows (or a combination of such idealizations), aiming at interpreting the results of complex models. Illustrations are provided, focusing on the global water age distribution in the World Ocean, the top layer of Lake Tanganyika and the Scheldt Estuary. Clearly, diagnostic timescales paint a picture of the functioning of numerical models of reactive transport that is different from that obtained by analyzing primitive variables.

Global warming is killing corals; however, the effects of warming on population connectivity, a process fundamental to reef recovery, are largely unexplored. Using a high-resolution (as high as 200 m), empirically calibrated biophysical model of coral larval dispersal for the southern Great Barrier Reef, we show that the increased larval mortality and reduced competency duration under a 2 °C warming alter dispersal patterns, whereas projected changes in large-scale currents have limited effects. Overall, there was on average a 7% decrease in the distance larvae disperse (among-reef interquartile range (IQR), −10% to −4%), an 8% decrease in the number of connections into each reef (IQR, −11% to −3%) and a 20% increase in local retention (IQR, 0% to +49%). Collectively, these shifts imply that 2 °C of warming will reduce inter-reef connectivity, hampering recovery after disturbances and reducing the spread of warm-adapted genes. Such changes make protections more effective locally, but may require reducing spacing between protected areas.

This thesis consists of a detailed study of spray combustion of diesel and biodiesel fuels. Our study is focused on the complex physical and chemical phenomena that take place in such flows and consists of three parts. In the first part, we investigate atomization and droplet breakup of a non-reactive fuel spray via Large-Eddy Simulations (LES). To this end, we compare three popular droplet-breakup models, the Taylor Analogy Breakup (TAB), Reitz-Diwakar and Pilch-Erdman models. Further, we propose a modification of the original TAB model and assess its predictive capacity. According to our numerical tests, the proposed modification leads to improved accuracy in the computation of important properties of the spray, such as the liquid- and vapor-penetration distance. In the second part, we present a numerical study of n-dodecane spray flames via LES. With regard to combustion modelling and turbulence-chemistry interaction, we employ the Flamelet-Generated Manifolds (FGM). Herein, we propose a novel approach for the coupling of the energy equation with the FGM database that offers considerable computational savings. The efficiency of this approach is assessed via comparisons with experimental data and earlier numerical results for n-dodecane spray flames. In the last part, we present an LES study of spray combustion of Karanja Oil Methyl Ester. First, we examine the influence of the initial oxygen concentration level on the spray ignition and the flame topology. Further, we compare the combustion processes of the biodiesel surrogate with the corresponding diesel surrogate (n-dodecane). Finally, we assess the impact of the thermophysical properties of the biodiesel on the evolution of the spray and the overall combustion process., (SC - Sciences) -- UCL, 2022

Coral populations have declined dramatically worldwide under the combined effects of climate change and local anthropogenic stressors. In addition to ocean warming and acidification, threats to coral reefs in the Caribbean include frequent and virulent disease outbreaks, and intensifying hurricanes. In particular, Florida's Coral Reef (FCR) is currently facing a multi-year outbreak of the stony coral tissue loss disease (SCTLD). First observed in 2014 during the monitoring of the PortMiami Deep Dredge Project (PMDDP), the disease has now spread through the entire FCR and has been reported in numerous territories of the Caribbean. However, the propagation of the disease through FCR seemed to slow down when it reached its southwestern end, between the Marquesas and the Dry Tortugas (DRTO). Although the causative agent of the disease remains unknown, studies showed that its transmission was likely waterborne and that sediments could act as SCTLD vectors. The hydrodynamics should therefore be highly explanatory of its spread. Furthermore, Florida is a prime landfall target for hurricanes during the reproduction period of corals. In addition to causing wholesale destruction of the reefs, hurricanes might thus also impact larval dispersal through wind-wave-induced currents. Here, we use the high resolution coastal ocean model SLIM to capture transport processes in the FCR, and study the impact of SCTLD and hurricanes to coral reefs. First, a coupled epidemio-hydrodynamic model is developed to reproduce and understand the observed spread of SCTLD in Florida. A sediment transport model is then used to evaluate the impact of the PMDDP on the observed onset of the disease in 2014. Finally, a coupled wave-current model is implemented to study the impact of Hurricane Irma (2017) on transport processes over reefs in the Florida Keys. Assuming the dispersal of disease agents within neutrally buoyant material, our model successfully reproduced the observed spread of SCTLD through, (FSA - Sciences de l'ingénieur) -- UCL, 2022

The Great Barrier Reefs (GBR) is a treasure of biodiversity, with an immense ecological, economical and iconic value. However, this unique ecosystem is facing many threats, and is particularly suffering from global change. Management measures are therefore more than ever required to protect and restore the reefs. By reproducing both the ocean circulation and dispersal processes, biophysical models can help to inform and support such management measures. However, model outputs always come with a certain level of uncertainty, potentially impeding their reliability. Among others, the spatial resolution of ocean circulation models can affect the overall quality of their outputs. In this thesis, we use the 2D version of the multiscale coastal ocean model SLIM to assess the effect of spatial resolution on the whole modelling sequence. We hence compare the outputs of various model setups with five resolutions ranging from 250 m to 4 km. While the tidal signal is similar for all resolutions, strong discrepancies appear for simulated current velocities, especially over reefs and in their close vicinity. This in turn affects the dispersal pattern of virtual particles, leading to significant cumulative differences over multiple weeks. In a second stage, we also show that coral connectivity indicators inferred from larvae dispersal simulations vary with the resolution of the model. We hence underline the importance of using a resolution fine enough to represent the processes of interest when using models to support reef management. As a last step, we evaluate the footprint of sediments that would be released from a coal mine project adjacent to the GBR. We demonstrate that fine sediments could have far-reaching impacts on seagrass meadows, a dugong sanctuary, and important turtle nesting beaches. Throughout the thesis, we underline how processes happening at a small-scale influence in turn those happening at larger scales in coral reef environments., (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2022

Birds have always been a source of inspiration for humans. From the record distances crossed by the bar-tailed godwit during its migrations, to swifts staying aloft continuously for up to ten months, they are capable of achievements that artificial devices are still far from reproducing. We still have a lot to learn from the flight of birds, and the unveiling of the mechanisms behind their efficiency could open the way to new levels of performances for our man-made air vehicles. While many works have relied on observations and experimental measurements conducted on birds and have led us to a certain knowledge of their flight mechanisms, the present work builds upon the idea that reproducing bird flight in simulation can shed light on what makes them so efficient. Indeed, plain mimicry of the natural flight of birds does little toward the better understanding of its underlying principles whereas the numerical reproduction of the broad range of phenomena involved allows us to test various hypotheses. In this thesis, we report on the development of a model for bird flapping flight. This model includes a representation of the wing skeleton and the main flight feathers, controllers aimed at the stabilization of flight and a faithful modeling of the wake behind a bird. The model is applied to the study of the dynamics of a bird in controlled flight, the analysis of its wake and of the interactions between two birds flying in formation. In the results, we present predictions regarding the distribution of aerodynamic forces and the contributions to the flight power. We also analyze the structure of a bird wake, verifying hypotheses concerning the positioning of a bird flying behind another, and we simulate the formation flight of two birds, exploring the control strategies that stabilize the follower, and how its position affects the reduction in its flight power., Les oiseaux ont toujours été une source d’inspiration pour l’humanité. De la distance record parcourue par la barge rousse durant ses migrations annuelles, aux hirondelles capables de rester en vol ininterrompu pendant dix mois, ils sont capables de performances que nous sommes encore loin de pouvoir reproduire artificiellement. Nous avons encore beaucoup à apprendre du vol des oiseaux, et découvrir les mécanismes à l’origine de leur efficacité peut mener à de nouveaux niveaux de performances pour nos appareils volants. Malgré de nombreux travaux basés sur l’observation et des mesures expérimentales du vol des oiseaux, qui nous ont amené une certaine connaissance des mécanismes de leur vol, ce travail se base sur l’idée que la reproduction de ce vol en simulation peut nous éclairer sur ce qui le rend si efficace. En effet, une simple reproduction du vol des oiseaux tel qu’observé dans la nature ne suffit pas pour comprendre ses principes fondamentaux, tandis que la reproduction numérique de la large gamme de phénomènes impliqués dans le vol des oiseaux nous permet de tester des hypothèses. Dans cette thèse, nous présentons le développement d’un modèle du vol de l’oiseau à ailes battantes. Ce modèle comprend une représentation du squelette de l’aile et de ses principales plumes, des contrôleurs pour la stabilisation du vol, et un modèle fidèle du sillage derrière un oiseau. Le modèle est utilisé pour l’étude de la dynamique d’un oiseau en vol contrôlé, l’analyse de son sillage et les interaction entre deux oiseaux volant en formation. Dans les résultats nous présentons des prédictions concernant la distribution des forces aérodynamiques pendant le vol, et les différentes contributions à la consommation d’énergie. Nous analysons aussi la structure du sillage et vérifions des hypothèses concernant le positionnement d’un oiseau volant derrière un autre. Finalement, nous simulons le vol en formation de deux oiseaux, explorant ainsi les stratégies de contrôle disponibles, (FSA - Sciences de l'ingénieur) -- UCL, 2022

La version actuelle du syllabus du cours LBIR1121 (Mécanique générale) affirme que les forces de frottement tendent à faire diminuer l'énergie mécanique d'un point matériel. Cette assertion est correcte dans les cas spécifiques considérés, mais elle ignore le fait qu'en réalité le frottement implique au moins deux corps — au sens le plus général. On propose de ne pas modifier significativement la cinquième leçon (Forces de frottement) et la sixième (Travail, énergie, puissance). Par contre, dans le cadre de huitième leçon (Dynamique des systèmes I), on pourrait étudier de manière succincte l'impact des forces intérieures de frottement sur la diminution de l'énergie mécanique d'un système composé de plusieurs points matériels. Dans cette perspective, on traite d'abord un problème admettant une solution analytique simple qui porte sur le mouvement relatif de deux corps dont la surface de contact est rugueuse. On met ainsi en lumière la relation qui existe entre la vitesse relative et la diminution de l'énergie mécanique. On propose ensuite une généralisation de ce résultat à un système composé d'un nombre arbitraire de corps — traités comme des points matériels.