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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